Improve notation of BuiltinTrancheNames

[pgsql.git] / src / backend / storage / lmgr / lwlock.c

/*-------------------------------------------------------------------------
 *
 * lwlock.c
 *        Lightweight lock manager
 *
 * Lightweight locks are intended primarily to provide mutual exclusion of
 * access to shared-memory data structures.  Therefore, they offer both
 * exclusive and shared lock modes (to support read/write and read-only
 * access to a shared object).  There are few other frammishes.  User-level
 * locking should be done with the full lock manager --- which depends on
 * LWLocks to protect its shared state.
 *
 * In addition to exclusive and shared modes, lightweight locks can be used to
 * wait until a variable changes value.  The variable is initially not set
 * when the lock is acquired with LWLockAcquire, i.e. it remains set to the
 * value it was set to when the lock was released last, and can be updated
 * without releasing the lock by calling LWLockUpdateVar.  LWLockWaitForVar
 * waits for the variable to be updated, or until the lock is free.  When
 * releasing the lock with LWLockReleaseClearVar() the value can be set to an
 * appropriate value for a free lock.  The meaning of the variable is up to
 * the caller, the lightweight lock code just assigns and compares it.
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *        src/backend/storage/lmgr/lwlock.c
 *
 * NOTES:
 *
 * This used to be a pretty straight forward reader-writer lock
 * implementation, in which the internal state was protected by a
 * spinlock. Unfortunately the overhead of taking the spinlock proved to be
 * too high for workloads/locks that were taken in shared mode very
 * frequently. Often we were spinning in the (obviously exclusive) spinlock,
 * while trying to acquire a shared lock that was actually free.
 *
 * Thus a new implementation was devised that provides wait-free shared lock
 * acquisition for locks that aren't exclusively locked.
 *
 * The basic idea is to have a single atomic variable 'lockcount' instead of
 * the formerly separate shared and exclusive counters and to use atomic
 * operations to acquire the lock. That's fairly easy to do for plain
 * rw-spinlocks, but a lot harder for something like LWLocks that want to wait
 * in the OS.
 *
 * For lock acquisition we use an atomic compare-and-exchange on the lockcount
 * variable. For exclusive lock we swap in a sentinel value
 * (LW_VAL_EXCLUSIVE), for shared locks we count the number of holders.
 *
 * To release the lock we use an atomic decrement to release the lock. If the
 * new value is zero (we get that atomically), we know we can/have to release
 * waiters.
 *
 * Obviously it is important that the sentinel value for exclusive locks
 * doesn't conflict with the maximum number of possible share lockers -
 * luckily MAX_BACKENDS makes that easily possible.
 *
 *
 * The attentive reader might have noticed that naively doing the above has a
 * glaring race condition: We try to lock using the atomic operations and
 * notice that we have to wait. Unfortunately by the time we have finished
 * queuing, the former locker very well might have already finished it's
 * work. That's problematic because we're now stuck waiting inside the OS.

 * To mitigate those races we use a two phased attempt at locking:
 *       Phase 1: Try to do it atomically, if we succeed, nice
 *       Phase 2: Add ourselves to the waitqueue of the lock
 *       Phase 3: Try to grab the lock again, if we succeed, remove ourselves from
 *                        the queue
 *       Phase 4: Sleep till wake-up, goto Phase 1
 *
 * This protects us against the problem from above as nobody can release too
 *        quick, before we're queued, since after Phase 2 we're already queued.
 * -------------------------------------------------------------------------
 */
#include "postgres.h"

#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "port/pg_bitutils.h"
#include "postmaster/postmaster.h"
#include "replication/slot.h"
#include "storage/ipc.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/proclist.h"
#include "storage/spin.h"
#include "utils/memutils.h"

#ifdef LWLOCK_STATS
#include "utils/hsearch.h"
#endif


/* We use the ShmemLock spinlock to protect LWLockCounter */
extern slock_t *ShmemLock;

#define LW_FLAG_HAS_WAITERS                     ((uint32) 1 << 30)
#define LW_FLAG_RELEASE_OK                      ((uint32) 1 << 29)
#define LW_FLAG_LOCKED                          ((uint32) 1 << 28)

#define LW_VAL_EXCLUSIVE                        ((uint32) 1 << 24)
#define LW_VAL_SHARED                           1

#define LW_LOCK_MASK                            ((uint32) ((1 << 25)-1))
/* Must be greater than MAX_BACKENDS - which is 2^23-1, so we're fine. */
#define LW_SHARED_MASK                          ((uint32) ((1 << 24)-1))

StaticAssertDecl(LW_VAL_EXCLUSIVE > (uint32) MAX_BACKENDS,
                                 "MAX_BACKENDS too big for lwlock.c");

/*
 * There are three sorts of LWLock "tranches":
 *
 * 1. The individually-named locks defined in lwlocknames.h each have their
 * own tranche.  The names of these tranches appear in IndividualLWLockNames[]
 * in lwlocknames.c.
 *
 * 2. There are some predefined tranches for built-in groups of locks.
 * These are listed in enum BuiltinTrancheIds in lwlock.h, and their names
 * appear in BuiltinTrancheNames[] below.
 *
 * 3. Extensions can create new tranches, via either RequestNamedLWLockTranche
 * or LWLockRegisterTranche.  The names of these that are known in the current
 * process appear in LWLockTrancheNames[].
 *
 * All these names are user-visible as wait event names, so choose with care
 * ... and do not forget to update the documentation's list of wait events.
 */
extern const char *const IndividualLWLockNames[];       /* in lwlocknames.c */

static const char *const BuiltinTrancheNames[] = {
        [LWTRANCHE_XACT_BUFFER] = "XactBuffer",
        [LWTRANCHE_COMMITTS_BUFFER] = "CommitTsBuffer",
        [LWTRANCHE_SUBTRANS_BUFFER] = "SubtransBuffer",
        [LWTRANCHE_MULTIXACTOFFSET_BUFFER] = "MultiXactOffsetBuffer",
        [LWTRANCHE_MULTIXACTMEMBER_BUFFER] = "MultiXactMemberBuffer",
        [LWTRANCHE_NOTIFY_BUFFER] = "NotifyBuffer",
        [LWTRANCHE_SERIAL_BUFFER] = "SerialBuffer",
        [LWTRANCHE_WAL_INSERT] = "WALInsert",
        [LWTRANCHE_BUFFER_CONTENT] = "BufferContent",
        [LWTRANCHE_REPLICATION_ORIGIN_STATE] = "ReplicationOriginState",
        [LWTRANCHE_REPLICATION_SLOT_IO] = "ReplicationSlotIO",
        [LWTRANCHE_LOCK_FASTPATH] = "LockFastPath",
        [LWTRANCHE_BUFFER_MAPPING] = "BufferMapping",
        [LWTRANCHE_LOCK_MANAGER] = "LockManager",
        [LWTRANCHE_PREDICATE_LOCK_MANAGER] = "PredicateLockManager",
        [LWTRANCHE_PARALLEL_HASH_JOIN] = "ParallelHashJoin",
        [LWTRANCHE_PARALLEL_QUERY_DSA] = "ParallelQueryDSA",
        [LWTRANCHE_PER_SESSION_DSA] = "PerSessionDSA",
        [LWTRANCHE_PER_SESSION_RECORD_TYPE] = "PerSessionRecordType",
        [LWTRANCHE_PER_SESSION_RECORD_TYPMOD] = "PerSessionRecordTypmod",
        [LWTRANCHE_SHARED_TUPLESTORE] = "SharedTupleStore",
        [LWTRANCHE_SHARED_TIDBITMAP] = "SharedTidBitmap",
        [LWTRANCHE_PARALLEL_APPEND] = "ParallelAppend",
        [LWTRANCHE_PER_XACT_PREDICATE_LIST] = "PerXactPredicateList",
        [LWTRANCHE_PGSTATS_DSA] = "PgStatsDSA",
        [LWTRANCHE_PGSTATS_HASH] = "PgStatsHash",
        [LWTRANCHE_PGSTATS_DATA] = "PgStatsData",
        [LWTRANCHE_LAUNCHER_DSA] = "LogicalRepLauncherDSA",
        [LWTRANCHE_LAUNCHER_HASH] = "LogicalRepLauncherHash",
        [LWTRANCHE_DSM_REGISTRY_DSA] = "DSMRegistryDSA",
        [LWTRANCHE_DSM_REGISTRY_HASH] = "DSMRegistryHash",
};

StaticAssertDecl(lengthof(BuiltinTrancheNames) ==
                                 LWTRANCHE_FIRST_USER_DEFINED,
                                 "missing entries in BuiltinTrancheNames[]");

/*
 * This is indexed by tranche ID minus LWTRANCHE_FIRST_USER_DEFINED, and
 * stores the names of all dynamically-created tranches known to the current
 * process.  Any unused entries in the array will contain NULL.
 */
static const char **LWLockTrancheNames = NULL;
static int      LWLockTrancheNamesAllocated = 0;

/*
 * This points to the main array of LWLocks in shared memory.  Backends inherit
 * the pointer by fork from the postmaster (except in the EXEC_BACKEND case,
 * where we have special measures to pass it down).
 */
LWLockPadded *MainLWLockArray = NULL;

/*
 * We use this structure to keep track of locked LWLocks for release
 * during error recovery.  Normally, only a few will be held at once, but
 * occasionally the number can be much higher; for example, the pg_buffercache
 * extension locks all buffer partitions simultaneously.
 */
#define MAX_SIMUL_LWLOCKS       200

/* struct representing the LWLocks we're holding */
typedef struct LWLockHandle
{
        LWLock     *lock;
        LWLockMode      mode;
} LWLockHandle;

static int      num_held_lwlocks = 0;
static LWLockHandle held_lwlocks[MAX_SIMUL_LWLOCKS];

/* struct representing the LWLock tranche request for named tranche */
typedef struct NamedLWLockTrancheRequest
{
        char            tranche_name[NAMEDATALEN];
        int                     num_lwlocks;
} NamedLWLockTrancheRequest;

static NamedLWLockTrancheRequest *NamedLWLockTrancheRequestArray = NULL;
static int      NamedLWLockTrancheRequestsAllocated = 0;

/*
 * NamedLWLockTrancheRequests is both the valid length of the request array,
 * and the length of the shared-memory NamedLWLockTrancheArray later on.
 * This variable and NamedLWLockTrancheArray are non-static so that
 * postmaster.c can copy them to child processes in EXEC_BACKEND builds.
 */
int                     NamedLWLockTrancheRequests = 0;

/* points to data in shared memory: */
NamedLWLockTranche *NamedLWLockTrancheArray = NULL;

static void InitializeLWLocks(void);
static inline void LWLockReportWaitStart(LWLock *lock);
static inline void LWLockReportWaitEnd(void);
static const char *GetLWTrancheName(uint16 trancheId);

#define T_NAME(lock) \
        GetLWTrancheName((lock)->tranche)

#ifdef LWLOCK_STATS
typedef struct lwlock_stats_key
{
        int                     tranche;
        void       *instance;
}                       lwlock_stats_key;

typedef struct lwlock_stats
{
        lwlock_stats_key key;
        int                     sh_acquire_count;
        int                     ex_acquire_count;
        int                     block_count;
        int                     dequeue_self_count;
        int                     spin_delay_count;
}                       lwlock_stats;

static HTAB *lwlock_stats_htab;
static lwlock_stats lwlock_stats_dummy;
#endif

#ifdef LOCK_DEBUG
bool            Trace_lwlocks = false;

inline static void
PRINT_LWDEBUG(const char *where, LWLock *lock, LWLockMode mode)
{
        /* hide statement & context here, otherwise the log is just too verbose */
        if (Trace_lwlocks)
        {
                uint32          state = pg_atomic_read_u32(&lock->state);

                ereport(LOG,
                                (errhidestmt(true),
                                 errhidecontext(true),
                                 errmsg_internal("%d: %s(%s %p): excl %u shared %u haswaiters %u waiters %u rOK %d",
                                                                 MyProcPid,
                                                                 where, T_NAME(lock), lock,
                                                                 (state & LW_VAL_EXCLUSIVE) != 0,
                                                                 state & LW_SHARED_MASK,
                                                                 (state & LW_FLAG_HAS_WAITERS) != 0,
                                                                 pg_atomic_read_u32(&lock->nwaiters),
                                                                 (state & LW_FLAG_RELEASE_OK) != 0)));
        }
}

inline static void
LOG_LWDEBUG(const char *where, LWLock *lock, const char *msg)
{
        /* hide statement & context here, otherwise the log is just too verbose */
        if (Trace_lwlocks)
        {
                ereport(LOG,
                                (errhidestmt(true),
                                 errhidecontext(true),
                                 errmsg_internal("%s(%s %p): %s", where,
                                                                 T_NAME(lock), lock, msg)));
        }
}

#else                                                   /* not LOCK_DEBUG */
#define PRINT_LWDEBUG(a,b,c) ((void)0)
#define LOG_LWDEBUG(a,b,c) ((void)0)
#endif                                                  /* LOCK_DEBUG */

#ifdef LWLOCK_STATS

static void init_lwlock_stats(void);
static void print_lwlock_stats(int code, Datum arg);
static lwlock_stats * get_lwlock_stats_entry(LWLock *lock);

static void
init_lwlock_stats(void)
{
        HASHCTL         ctl;
        static MemoryContext lwlock_stats_cxt = NULL;
        static bool exit_registered = false;

        if (lwlock_stats_cxt != NULL)
                MemoryContextDelete(lwlock_stats_cxt);

        /*
         * The LWLock stats will be updated within a critical section, which
         * requires allocating new hash entries. Allocations within a critical
         * section are normally not allowed because running out of memory would
         * lead to a PANIC, but LWLOCK_STATS is debugging code that's not normally
         * turned on in production, so that's an acceptable risk. The hash entries
         * are small, so the risk of running out of memory is minimal in practice.
         */
        lwlock_stats_cxt = AllocSetContextCreate(TopMemoryContext,
                                                                                         "LWLock stats",
                                                                                         ALLOCSET_DEFAULT_SIZES);
        MemoryContextAllowInCriticalSection(lwlock_stats_cxt, true);

        ctl.keysize = sizeof(lwlock_stats_key);
        ctl.entrysize = sizeof(lwlock_stats);
        ctl.hcxt = lwlock_stats_cxt;
        lwlock_stats_htab = hash_create("lwlock stats", 16384, &ctl,
                                                                        HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
        if (!exit_registered)
        {
                on_shmem_exit(print_lwlock_stats, 0);
                exit_registered = true;
        }
}

static void
print_lwlock_stats(int code, Datum arg)
{
        HASH_SEQ_STATUS scan;
        lwlock_stats *lwstats;

        hash_seq_init(&scan, lwlock_stats_htab);

        /* Grab an LWLock to keep different backends from mixing reports */
        LWLockAcquire(&MainLWLockArray[0].lock, LW_EXCLUSIVE);

        while ((lwstats = (lwlock_stats *) hash_seq_search(&scan)) != NULL)
        {
                fprintf(stderr,
                                "PID %d lwlock %s %p: shacq %u exacq %u blk %u spindelay %u dequeue self %u\n",
                                MyProcPid, GetLWTrancheName(lwstats->key.tranche),
                                lwstats->key.instance, lwstats->sh_acquire_count,
                                lwstats->ex_acquire_count, lwstats->block_count,
                                lwstats->spin_delay_count, lwstats->dequeue_self_count);
        }

        LWLockRelease(&MainLWLockArray[0].lock);
}

static lwlock_stats *
get_lwlock_stats_entry(LWLock *lock)
{
        lwlock_stats_key key;
        lwlock_stats *lwstats;
        bool            found;

        /*
         * During shared memory initialization, the hash table doesn't exist yet.
         * Stats of that phase aren't very interesting, so just collect operations
         * on all locks in a single dummy entry.
         */
        if (lwlock_stats_htab == NULL)
                return &lwlock_stats_dummy;

        /* Fetch or create the entry. */
        MemSet(&key, 0, sizeof(key));
        key.tranche = lock->tranche;
        key.instance = lock;
        lwstats = hash_search(lwlock_stats_htab, &key, HASH_ENTER, &found);
        if (!found)
        {
                lwstats->sh_acquire_count = 0;
                lwstats->ex_acquire_count = 0;
                lwstats->block_count = 0;
                lwstats->dequeue_self_count = 0;
                lwstats->spin_delay_count = 0;
        }
        return lwstats;
}
#endif                                                  /* LWLOCK_STATS */


/*
 * Compute number of LWLocks required by named tranches.  These will be
 * allocated in the main array.
 */
static int
NumLWLocksForNamedTranches(void)
{
        int                     numLocks = 0;
        int                     i;

        for (i = 0; i < NamedLWLockTrancheRequests; i++)
                numLocks += NamedLWLockTrancheRequestArray[i].num_lwlocks;

        return numLocks;
}

/*
 * Compute shmem space needed for LWLocks and named tranches.
 */
Size
LWLockShmemSize(void)
{
        Size            size;
        int                     i;
        int                     numLocks = NUM_FIXED_LWLOCKS;

        /* Calculate total number of locks needed in the main array. */
        numLocks += NumLWLocksForNamedTranches();

        /* Space for the LWLock array. */
        size = mul_size(numLocks, sizeof(LWLockPadded));

        /* Space for dynamic allocation counter, plus room for alignment. */
        size = add_size(size, sizeof(int) + LWLOCK_PADDED_SIZE);

        /* space for named tranches. */
        size = add_size(size, mul_size(NamedLWLockTrancheRequests, sizeof(NamedLWLockTranche)));

        /* space for name of each tranche. */
        for (i = 0; i < NamedLWLockTrancheRequests; i++)
                size = add_size(size, strlen(NamedLWLockTrancheRequestArray[i].tranche_name) + 1);

        return size;
}

/*
 * Allocate shmem space for the main LWLock array and all tranches and
 * initialize it.  We also register extension LWLock tranches here.
 */
void
CreateLWLocks(void)
{
        if (!IsUnderPostmaster)
        {
                Size            spaceLocks = LWLockShmemSize();
                int                *LWLockCounter;
                char       *ptr;

                /* Allocate space */
                ptr = (char *) ShmemAlloc(spaceLocks);

                /* Leave room for dynamic allocation of tranches */
                ptr += sizeof(int);

                /* Ensure desired alignment of LWLock array */
                ptr += LWLOCK_PADDED_SIZE - ((uintptr_t) ptr) % LWLOCK_PADDED_SIZE;

                MainLWLockArray = (LWLockPadded *) ptr;

                /*
                 * Initialize the dynamic-allocation counter for tranches, which is
                 * stored just before the first LWLock.
                 */
                LWLockCounter = (int *) ((char *) MainLWLockArray - sizeof(int));
                *LWLockCounter = LWTRANCHE_FIRST_USER_DEFINED;

                /* Initialize all LWLocks */
                InitializeLWLocks();
        }

        /* Register named extension LWLock tranches in the current process. */
        for (int i = 0; i < NamedLWLockTrancheRequests; i++)
                LWLockRegisterTranche(NamedLWLockTrancheArray[i].trancheId,
                                                          NamedLWLockTrancheArray[i].trancheName);
}

/*
 * Initialize LWLocks that are fixed and those belonging to named tranches.
 */
static void
InitializeLWLocks(void)
{
        int                     numNamedLocks = NumLWLocksForNamedTranches();
        int                     id;
        int                     i;
        int                     j;
        LWLockPadded *lock;

        /* Initialize all individual LWLocks in main array */
        for (id = 0, lock = MainLWLockArray; id < NUM_INDIVIDUAL_LWLOCKS; id++, lock++)
                LWLockInitialize(&lock->lock, id);

        /* Initialize buffer mapping LWLocks in main array */
        lock = MainLWLockArray + BUFFER_MAPPING_LWLOCK_OFFSET;
        for (id = 0; id < NUM_BUFFER_PARTITIONS; id++, lock++)
                LWLockInitialize(&lock->lock, LWTRANCHE_BUFFER_MAPPING);

        /* Initialize lmgrs' LWLocks in main array */
        lock = MainLWLockArray + LOCK_MANAGER_LWLOCK_OFFSET;
        for (id = 0; id < NUM_LOCK_PARTITIONS; id++, lock++)
                LWLockInitialize(&lock->lock, LWTRANCHE_LOCK_MANAGER);

        /* Initialize predicate lmgrs' LWLocks in main array */
        lock = MainLWLockArray + PREDICATELOCK_MANAGER_LWLOCK_OFFSET;
        for (id = 0; id < NUM_PREDICATELOCK_PARTITIONS; id++, lock++)
                LWLockInitialize(&lock->lock, LWTRANCHE_PREDICATE_LOCK_MANAGER);

        /*
         * Copy the info about any named tranches into shared memory (so that
         * other processes can see it), and initialize the requested LWLocks.
         */
        if (NamedLWLockTrancheRequests > 0)
        {
                char       *trancheNames;

                NamedLWLockTrancheArray = (NamedLWLockTranche *)
                        &MainLWLockArray[NUM_FIXED_LWLOCKS + numNamedLocks];

                trancheNames = (char *) NamedLWLockTrancheArray +
                        (NamedLWLockTrancheRequests * sizeof(NamedLWLockTranche));
                lock = &MainLWLockArray[NUM_FIXED_LWLOCKS];

                for (i = 0; i < NamedLWLockTrancheRequests; i++)
                {
                        NamedLWLockTrancheRequest *request;
                        NamedLWLockTranche *tranche;
                        char       *name;

                        request = &NamedLWLockTrancheRequestArray[i];
                        tranche = &NamedLWLockTrancheArray[i];

                        name = trancheNames;
                        trancheNames += strlen(request->tranche_name) + 1;
                        strcpy(name, request->tranche_name);
                        tranche->trancheId = LWLockNewTrancheId();
                        tranche->trancheName = name;

                        for (j = 0; j < request->num_lwlocks; j++, lock++)
                                LWLockInitialize(&lock->lock, tranche->trancheId);
                }
        }
}

/*
 * InitLWLockAccess - initialize backend-local state needed to hold LWLocks
 */
void
InitLWLockAccess(void)
{
#ifdef LWLOCK_STATS
        init_lwlock_stats();
#endif
}

/*
 * GetNamedLWLockTranche - returns the base address of LWLock from the
 *              specified tranche.
 *
 * Caller needs to retrieve the requested number of LWLocks starting from
 * the base lock address returned by this API.  This can be used for
 * tranches that are requested by using RequestNamedLWLockTranche() API.
 */
LWLockPadded *
GetNamedLWLockTranche(const char *tranche_name)
{
        int                     lock_pos;
        int                     i;

        /*
         * Obtain the position of base address of LWLock belonging to requested
         * tranche_name in MainLWLockArray.  LWLocks for named tranches are placed
         * in MainLWLockArray after fixed locks.
         */
        lock_pos = NUM_FIXED_LWLOCKS;
        for (i = 0; i < NamedLWLockTrancheRequests; i++)
        {
                if (strcmp(NamedLWLockTrancheRequestArray[i].tranche_name,
                                   tranche_name) == 0)
                        return &MainLWLockArray[lock_pos];

                lock_pos += NamedLWLockTrancheRequestArray[i].num_lwlocks;
        }

        elog(ERROR, "requested tranche is not registered");

        /* just to keep compiler quiet */
        return NULL;
}

/*
 * Allocate a new tranche ID.
 */
int
LWLockNewTrancheId(void)
{
        int                     result;
        int                *LWLockCounter;

        LWLockCounter = (int *) ((char *) MainLWLockArray - sizeof(int));
        SpinLockAcquire(ShmemLock);
        result = (*LWLockCounter)++;
        SpinLockRelease(ShmemLock);

        return result;
}

/*
 * Register a dynamic tranche name in the lookup table of the current process.
 *
 * This routine will save a pointer to the tranche name passed as an argument,
 * so the name should be allocated in a backend-lifetime context
 * (shared memory, TopMemoryContext, static constant, or similar).
 *
 * The tranche name will be user-visible as a wait event name, so try to
 * use a name that fits the style for those.
 */
void
LWLockRegisterTranche(int tranche_id, const char *tranche_name)
{
        /* This should only be called for user-defined tranches. */
        if (tranche_id < LWTRANCHE_FIRST_USER_DEFINED)
                return;

        /* Convert to array index. */
        tranche_id -= LWTRANCHE_FIRST_USER_DEFINED;

        /* If necessary, create or enlarge array. */
        if (tranche_id >= LWLockTrancheNamesAllocated)
        {
                int                     newalloc;

                newalloc = pg_nextpower2_32(Max(8, tranche_id + 1));

                if (LWLockTrancheNames == NULL)
                        LWLockTrancheNames = (const char **)
                                MemoryContextAllocZero(TopMemoryContext,
                                                                           newalloc * sizeof(char *));
                else
                        LWLockTrancheNames =
                                repalloc0_array(LWLockTrancheNames, const char *, LWLockTrancheNamesAllocated, newalloc);
                LWLockTrancheNamesAllocated = newalloc;
        }

        LWLockTrancheNames[tranche_id] = tranche_name;
}

/*
 * RequestNamedLWLockTranche
 *              Request that extra LWLocks be allocated during postmaster
 *              startup.
 *
 * This may only be called via the shmem_request_hook of a library that is
 * loaded into the postmaster via shared_preload_libraries.  Calls from
 * elsewhere will fail.
 *
 * The tranche name will be user-visible as a wait event name, so try to
 * use a name that fits the style for those.
 */
void
RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
{
        NamedLWLockTrancheRequest *request;

        if (!process_shmem_requests_in_progress)
                elog(FATAL, "cannot request additional LWLocks outside shmem_request_hook");

        if (NamedLWLockTrancheRequestArray == NULL)
        {
                NamedLWLockTrancheRequestsAllocated = 16;
                NamedLWLockTrancheRequestArray = (NamedLWLockTrancheRequest *)
                        MemoryContextAlloc(TopMemoryContext,
                                                           NamedLWLockTrancheRequestsAllocated
                                                           * sizeof(NamedLWLockTrancheRequest));
        }

        if (NamedLWLockTrancheRequests >= NamedLWLockTrancheRequestsAllocated)
        {
                int                     i = pg_nextpower2_32(NamedLWLockTrancheRequests + 1);

                NamedLWLockTrancheRequestArray = (NamedLWLockTrancheRequest *)
                        repalloc(NamedLWLockTrancheRequestArray,
                                         i * sizeof(NamedLWLockTrancheRequest));
                NamedLWLockTrancheRequestsAllocated = i;
        }

        request = &NamedLWLockTrancheRequestArray[NamedLWLockTrancheRequests];
        Assert(strlen(tranche_name) + 1 <= NAMEDATALEN);
        strlcpy(request->tranche_name, tranche_name, NAMEDATALEN);
        request->num_lwlocks = num_lwlocks;
        NamedLWLockTrancheRequests++;
}

/*
 * LWLockInitialize - initialize a new lwlock; it's initially unlocked
 */
void
LWLockInitialize(LWLock *lock, int tranche_id)
{
        pg_atomic_init_u32(&lock->state, LW_FLAG_RELEASE_OK);
#ifdef LOCK_DEBUG
        pg_atomic_init_u32(&lock->nwaiters, 0);
#endif
        lock->tranche = tranche_id;
        proclist_init(&lock->waiters);
}

/*
 * Report start of wait event for light-weight locks.
 *
 * This function will be used by all the light-weight lock calls which
 * needs to wait to acquire the lock.  This function distinguishes wait
 * event based on tranche and lock id.
 */
static inline void
LWLockReportWaitStart(LWLock *lock)
{
        pgstat_report_wait_start(PG_WAIT_LWLOCK | lock->tranche);
}

/*
 * Report end of wait event for light-weight locks.
 */
static inline void
LWLockReportWaitEnd(void)
{
        pgstat_report_wait_end();
}

/*
 * Return the name of an LWLock tranche.
 */
static const char *
GetLWTrancheName(uint16 trancheId)
{
        /* Individual LWLock? */
        if (trancheId < NUM_INDIVIDUAL_LWLOCKS)
                return IndividualLWLockNames[trancheId];

        /* Built-in tranche? */
        if (trancheId < LWTRANCHE_FIRST_USER_DEFINED)
                return BuiltinTrancheNames[trancheId];

        /*
         * It's an extension tranche, so look in LWLockTrancheNames[].  However,
         * it's possible that the tranche has never been registered in the current
         * process, in which case give up and return "extension".
         */
        trancheId -= LWTRANCHE_FIRST_USER_DEFINED;

        if (trancheId >= LWLockTrancheNamesAllocated ||
                LWLockTrancheNames[trancheId] == NULL)
                return "extension";

        return LWLockTrancheNames[trancheId];
}

/*
 * Return an identifier for an LWLock based on the wait class and event.
 */
const char *
GetLWLockIdentifier(uint32 classId, uint16 eventId)
{
        Assert(classId == PG_WAIT_LWLOCK);
        /* The event IDs are just tranche numbers. */
        return GetLWTrancheName(eventId);
}

/*
 * Internal function that tries to atomically acquire the lwlock in the passed
 * in mode.
 *
 * This function will not block waiting for a lock to become free - that's the
 * callers job.
 *
 * Returns true if the lock isn't free and we need to wait.
 */
static bool
LWLockAttemptLock(LWLock *lock, LWLockMode mode)
{
        uint32          old_state;

        Assert(mode == LW_EXCLUSIVE || mode == LW_SHARED);

        /*
         * Read once outside the loop, later iterations will get the newer value
         * via compare & exchange.
         */
        old_state = pg_atomic_read_u32(&lock->state);

        /* loop until we've determined whether we could acquire the lock or not */
        while (true)
        {
                uint32          desired_state;
                bool            lock_free;

                desired_state = old_state;

                if (mode == LW_EXCLUSIVE)
                {
                        lock_free = (old_state & LW_LOCK_MASK) == 0;
                        if (lock_free)
                                desired_state += LW_VAL_EXCLUSIVE;
                }
                else
                {
                        lock_free = (old_state & LW_VAL_EXCLUSIVE) == 0;
                        if (lock_free)
                                desired_state += LW_VAL_SHARED;
                }

                /*
                 * Attempt to swap in the state we are expecting. If we didn't see
                 * lock to be free, that's just the old value. If we saw it as free,
                 * we'll attempt to mark it acquired. The reason that we always swap
                 * in the value is that this doubles as a memory barrier. We could try
                 * to be smarter and only swap in values if we saw the lock as free,
                 * but benchmark haven't shown it as beneficial so far.
                 *
                 * Retry if the value changed since we last looked at it.
                 */
                if (pg_atomic_compare_exchange_u32(&lock->state,
                                                                                   &old_state, desired_state))
                {
                        if (lock_free)
                        {
                                /* Great! Got the lock. */
#ifdef LOCK_DEBUG
                                if (mode == LW_EXCLUSIVE)
                                        lock->owner = MyProc;
#endif
                                return false;
                        }
                        else
                                return true;    /* somebody else has the lock */
                }
        }
        pg_unreachable();
}

/*
 * Lock the LWLock's wait list against concurrent activity.
 *
 * NB: even though the wait list is locked, non-conflicting lock operations
 * may still happen concurrently.
 *
 * Time spent holding mutex should be short!
 */
static void
LWLockWaitListLock(LWLock *lock)
{
        uint32          old_state;
#ifdef LWLOCK_STATS
        lwlock_stats *lwstats;
        uint32          delays = 0;

        lwstats = get_lwlock_stats_entry(lock);
#endif

        while (true)
        {
                /* always try once to acquire lock directly */
                old_state = pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_LOCKED);
                if (!(old_state & LW_FLAG_LOCKED))
                        break;                          /* got lock */

                /* and then spin without atomic operations until lock is released */
                {
                        SpinDelayStatus delayStatus;

                        init_local_spin_delay(&delayStatus);

                        while (old_state & LW_FLAG_LOCKED)
                        {
                                perform_spin_delay(&delayStatus);
                                old_state = pg_atomic_read_u32(&lock->state);
                        }
#ifdef LWLOCK_STATS
                        delays += delayStatus.delays;
#endif
                        finish_spin_delay(&delayStatus);
                }

                /*
                 * Retry. The lock might obviously already be re-acquired by the time
                 * we're attempting to get it again.
                 */
        }

#ifdef LWLOCK_STATS
        lwstats->spin_delay_count += delays;
#endif
}

/*
 * Unlock the LWLock's wait list.
 *
 * Note that it can be more efficient to manipulate flags and release the
 * locks in a single atomic operation.
 */
static void
LWLockWaitListUnlock(LWLock *lock)
{
        uint32          old_state PG_USED_FOR_ASSERTS_ONLY;

        old_state = pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_LOCKED);

        Assert(old_state & LW_FLAG_LOCKED);
}

/*
 * Wakeup all the lockers that currently have a chance to acquire the lock.
 */
static void
LWLockWakeup(LWLock *lock)
{
        bool            new_release_ok;
        bool            wokeup_somebody = false;
        proclist_head wakeup;
        proclist_mutable_iter iter;

        proclist_init(&wakeup);

        new_release_ok = true;

        /* lock wait list while collecting backends to wake up */
        LWLockWaitListLock(lock);

        proclist_foreach_modify(iter, &lock->waiters, lwWaitLink)
        {
                PGPROC     *waiter = GetPGProcByNumber(iter.cur);

                if (wokeup_somebody && waiter->lwWaitMode == LW_EXCLUSIVE)
                        continue;

                proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
                proclist_push_tail(&wakeup, iter.cur, lwWaitLink);

                if (waiter->lwWaitMode != LW_WAIT_UNTIL_FREE)
                {
                        /*
                         * Prevent additional wakeups until retryer gets to run. Backends
                         * that are just waiting for the lock to become free don't retry
                         * automatically.
                         */
                        new_release_ok = false;

                        /*
                         * Don't wakeup (further) exclusive locks.
                         */
                        wokeup_somebody = true;
                }

                /*
                 * Signal that the process isn't on the wait list anymore. This allows
                 * LWLockDequeueSelf() to remove itself of the waitlist with a
                 * proclist_delete(), rather than having to check if it has been
                 * removed from the list.
                 */
                Assert(waiter->lwWaiting == LW_WS_WAITING);
                waiter->lwWaiting = LW_WS_PENDING_WAKEUP;

                /*
                 * Once we've woken up an exclusive lock, there's no point in waking
                 * up anybody else.
                 */
                if (waiter->lwWaitMode == LW_EXCLUSIVE)
                        break;
        }

        Assert(proclist_is_empty(&wakeup) || pg_atomic_read_u32(&lock->state) & LW_FLAG_HAS_WAITERS);

        /* unset required flags, and release lock, in one fell swoop */
        {
                uint32          old_state;
                uint32          desired_state;

                old_state = pg_atomic_read_u32(&lock->state);
                while (true)
                {
                        desired_state = old_state;

                        /* compute desired flags */

                        if (new_release_ok)
                                desired_state |= LW_FLAG_RELEASE_OK;
                        else
                                desired_state &= ~LW_FLAG_RELEASE_OK;

                        if (proclist_is_empty(&wakeup))
                                desired_state &= ~LW_FLAG_HAS_WAITERS;

                        desired_state &= ~LW_FLAG_LOCKED;       /* release lock */

                        if (pg_atomic_compare_exchange_u32(&lock->state, &old_state,
                                                                                           desired_state))
                                break;
                }
        }

        /* Awaken any waiters I removed from the queue. */
        proclist_foreach_modify(iter, &wakeup, lwWaitLink)
        {
                PGPROC     *waiter = GetPGProcByNumber(iter.cur);

                LOG_LWDEBUG("LWLockRelease", lock, "release waiter");
                proclist_delete(&wakeup, iter.cur, lwWaitLink);

                /*
                 * Guarantee that lwWaiting being unset only becomes visible once the
                 * unlink from the link has completed. Otherwise the target backend
                 * could be woken up for other reason and enqueue for a new lock - if
                 * that happens before the list unlink happens, the list would end up
                 * being corrupted.
                 *
                 * The barrier pairs with the LWLockWaitListLock() when enqueuing for
                 * another lock.
                 */
                pg_write_barrier();
                waiter->lwWaiting = LW_WS_NOT_WAITING;
                PGSemaphoreUnlock(waiter->sem);
        }
}

/*
 * Add ourselves to the end of the queue.
 *
 * NB: Mode can be LW_WAIT_UNTIL_FREE here!
 */
static void
LWLockQueueSelf(LWLock *lock, LWLockMode mode)
{
        /*
         * If we don't have a PGPROC structure, there's no way to wait. This
         * should never occur, since MyProc should only be null during shared
         * memory initialization.
         */
        if (MyProc == NULL)
                elog(PANIC, "cannot wait without a PGPROC structure");

        if (MyProc->lwWaiting != LW_WS_NOT_WAITING)
                elog(PANIC, "queueing for lock while waiting on another one");

        LWLockWaitListLock(lock);

        /* setting the flag is protected by the spinlock */
        pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_HAS_WAITERS);

        MyProc->lwWaiting = LW_WS_WAITING;
        MyProc->lwWaitMode = mode;

        /* LW_WAIT_UNTIL_FREE waiters are always at the front of the queue */
        if (mode == LW_WAIT_UNTIL_FREE)
                proclist_push_head(&lock->waiters, MyProc->pgprocno, lwWaitLink);
        else
                proclist_push_tail(&lock->waiters, MyProc->pgprocno, lwWaitLink);

        /* Can release the mutex now */
        LWLockWaitListUnlock(lock);

#ifdef LOCK_DEBUG
        pg_atomic_fetch_add_u32(&lock->nwaiters, 1);
#endif
}

/*
 * Remove ourselves from the waitlist.
 *
 * This is used if we queued ourselves because we thought we needed to sleep
 * but, after further checking, we discovered that we don't actually need to
 * do so.
 */
static void
LWLockDequeueSelf(LWLock *lock)
{
        bool            on_waitlist;

#ifdef LWLOCK_STATS
        lwlock_stats *lwstats;

        lwstats = get_lwlock_stats_entry(lock);

        lwstats->dequeue_self_count++;
#endif

        LWLockWaitListLock(lock);

        /*
         * Remove ourselves from the waitlist, unless we've already been removed.
         * The removal happens with the wait list lock held, so there's no race in
         * this check.
         */
        on_waitlist = MyProc->lwWaiting == LW_WS_WAITING;
        if (on_waitlist)
                proclist_delete(&lock->waiters, MyProc->pgprocno, lwWaitLink);

        if (proclist_is_empty(&lock->waiters) &&
                (pg_atomic_read_u32(&lock->state) & LW_FLAG_HAS_WAITERS) != 0)
        {
                pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_HAS_WAITERS);
        }

        /* XXX: combine with fetch_and above? */
        LWLockWaitListUnlock(lock);

        /* clear waiting state again, nice for debugging */
        if (on_waitlist)
                MyProc->lwWaiting = LW_WS_NOT_WAITING;
        else
        {
                int                     extraWaits = 0;

                /*
                 * Somebody else dequeued us and has or will wake us up. Deal with the
                 * superfluous absorption of a wakeup.
                 */

                /*
                 * Reset RELEASE_OK flag if somebody woke us before we removed
                 * ourselves - they'll have set it to false.
                 */
                pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);

                /*
                 * Now wait for the scheduled wakeup, otherwise our ->lwWaiting would
                 * get reset at some inconvenient point later. Most of the time this
                 * will immediately return.
                 */
                for (;;)
                {
                        PGSemaphoreLock(MyProc->sem);
                        if (MyProc->lwWaiting == LW_WS_NOT_WAITING)
                                break;
                        extraWaits++;
                }

                /*
                 * Fix the process wait semaphore's count for any absorbed wakeups.
                 */
                while (extraWaits-- > 0)
                        PGSemaphoreUnlock(MyProc->sem);
        }

#ifdef LOCK_DEBUG
        {
                /* not waiting anymore */
                uint32          nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);

                Assert(nwaiters < MAX_BACKENDS);
        }
#endif
}

/*
 * LWLockAcquire - acquire a lightweight lock in the specified mode
 *
 * If the lock is not available, sleep until it is.  Returns true if the lock
 * was available immediately, false if we had to sleep.
 *
 * Side effect: cancel/die interrupts are held off until lock release.
 */
bool
LWLockAcquire(LWLock *lock, LWLockMode mode)
{
        PGPROC     *proc = MyProc;
        bool            result = true;
        int                     extraWaits = 0;
#ifdef LWLOCK_STATS
        lwlock_stats *lwstats;

        lwstats = get_lwlock_stats_entry(lock);
#endif

        Assert(mode == LW_SHARED || mode == LW_EXCLUSIVE);

        PRINT_LWDEBUG("LWLockAcquire", lock, mode);

#ifdef LWLOCK_STATS
        /* Count lock acquisition attempts */
        if (mode == LW_EXCLUSIVE)
                lwstats->ex_acquire_count++;
        else
                lwstats->sh_acquire_count++;
#endif                                                  /* LWLOCK_STATS */

        /*
         * We can't wait if we haven't got a PGPROC.  This should only occur
         * during bootstrap or shared memory initialization.  Put an Assert here
         * to catch unsafe coding practices.
         */
        Assert(!(proc == NULL && IsUnderPostmaster));

        /* Ensure we will have room to remember the lock */
        if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
                elog(ERROR, "too many LWLocks taken");

        /*
         * Lock out cancel/die interrupts until we exit the code section protected
         * by the LWLock.  This ensures that interrupts will not interfere with
         * manipulations of data structures in shared memory.
         */
        HOLD_INTERRUPTS();

        /*
         * Loop here to try to acquire lock after each time we are signaled by
         * LWLockRelease.
         *
         * NOTE: it might seem better to have LWLockRelease actually grant us the
         * lock, rather than retrying and possibly having to go back to sleep. But
         * in practice that is no good because it means a process swap for every
         * lock acquisition when two or more processes are contending for the same
         * lock.  Since LWLocks are normally used to protect not-very-long
         * sections of computation, a process needs to be able to acquire and
         * release the same lock many times during a single CPU time slice, even
         * in the presence of contention.  The efficiency of being able to do that
         * outweighs the inefficiency of sometimes wasting a process dispatch
         * cycle because the lock is not free when a released waiter finally gets
         * to run.  See pgsql-hackers archives for 29-Dec-01.
         */
        for (;;)
        {
                bool            mustwait;

                /*
                 * Try to grab the lock the first time, we're not in the waitqueue
                 * yet/anymore.
                 */
                mustwait = LWLockAttemptLock(lock, mode);

                if (!mustwait)
                {
                        LOG_LWDEBUG("LWLockAcquire", lock, "immediately acquired lock");
                        break;                          /* got the lock */
                }

                /*
                 * Ok, at this point we couldn't grab the lock on the first try. We
                 * cannot simply queue ourselves to the end of the list and wait to be
                 * woken up because by now the lock could long have been released.
                 * Instead add us to the queue and try to grab the lock again. If we
                 * succeed we need to revert the queuing and be happy, otherwise we
                 * recheck the lock. If we still couldn't grab it, we know that the
                 * other locker will see our queue entries when releasing since they
                 * existed before we checked for the lock.
                 */

                /* add to the queue */
                LWLockQueueSelf(lock, mode);

                /* we're now guaranteed to be woken up if necessary */
                mustwait = LWLockAttemptLock(lock, mode);

                /* ok, grabbed the lock the second time round, need to undo queueing */
                if (!mustwait)
                {
                        LOG_LWDEBUG("LWLockAcquire", lock, "acquired, undoing queue");

                        LWLockDequeueSelf(lock);
                        break;
                }

                /*
                 * Wait until awakened.
                 *
                 * It is possible that we get awakened for a reason other than being
                 * signaled by LWLockRelease.  If so, loop back and wait again.  Once
                 * we've gotten the LWLock, re-increment the sema by the number of
                 * additional signals received.
                 */
                LOG_LWDEBUG("LWLockAcquire", lock, "waiting");

#ifdef LWLOCK_STATS
                lwstats->block_count++;
#endif

                LWLockReportWaitStart(lock);
                if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
                        TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), mode);

                for (;;)
                {
                        PGSemaphoreLock(proc->sem);
                        if (proc->lwWaiting == LW_WS_NOT_WAITING)
                                break;
                        extraWaits++;
                }

                /* Retrying, allow LWLockRelease to release waiters again. */
                pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);

#ifdef LOCK_DEBUG
                {
                        /* not waiting anymore */
                        uint32          nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);

                        Assert(nwaiters < MAX_BACKENDS);
                }
#endif

                if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
                        TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
                LWLockReportWaitEnd();

                LOG_LWDEBUG("LWLockAcquire", lock, "awakened");

                /* Now loop back and try to acquire lock again. */
                result = false;
        }

        if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_ENABLED())
                TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), mode);

        /* Add lock to list of locks held by this backend */
        held_lwlocks[num_held_lwlocks].lock = lock;
        held_lwlocks[num_held_lwlocks++].mode = mode;

        /*
         * Fix the process wait semaphore's count for any absorbed wakeups.
         */
        while (extraWaits-- > 0)
                PGSemaphoreUnlock(proc->sem);

        return result;
}

/*
 * LWLockConditionalAcquire - acquire a lightweight lock in the specified mode
 *
 * If the lock is not available, return false with no side-effects.
 *
 * If successful, cancel/die interrupts are held off until lock release.
 */
bool
LWLockConditionalAcquire(LWLock *lock, LWLockMode mode)
{
        bool            mustwait;

        Assert(mode == LW_SHARED || mode == LW_EXCLUSIVE);

        PRINT_LWDEBUG("LWLockConditionalAcquire", lock, mode);

        /* Ensure we will have room to remember the lock */
        if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
                elog(ERROR, "too many LWLocks taken");

        /*
         * Lock out cancel/die interrupts until we exit the code section protected
         * by the LWLock.  This ensures that interrupts will not interfere with
         * manipulations of data structures in shared memory.
         */
        HOLD_INTERRUPTS();

        /* Check for the lock */
        mustwait = LWLockAttemptLock(lock, mode);

        if (mustwait)
        {
                /* Failed to get lock, so release interrupt holdoff */
                RESUME_INTERRUPTS();

                LOG_LWDEBUG("LWLockConditionalAcquire", lock, "failed");
                if (TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL_ENABLED())
                        TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL(T_NAME(lock), mode);
        }
        else
        {
                /* Add lock to list of locks held by this backend */
                held_lwlocks[num_held_lwlocks].lock = lock;
                held_lwlocks[num_held_lwlocks++].mode = mode;
                if (TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_ENABLED())
                        TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE(T_NAME(lock), mode);
        }
        return !mustwait;
}

/*
 * LWLockAcquireOrWait - Acquire lock, or wait until it's free
 *
 * The semantics of this function are a bit funky.  If the lock is currently
 * free, it is acquired in the given mode, and the function returns true.  If
 * the lock isn't immediately free, the function waits until it is released
 * and returns false, but does not acquire the lock.
 *
 * This is currently used for WALWriteLock: when a backend flushes the WAL,
 * holding WALWriteLock, it can flush the commit records of many other
 * backends as a side-effect.  Those other backends need to wait until the
 * flush finishes, but don't need to acquire the lock anymore.  They can just
 * wake up, observe that their records have already been flushed, and return.
 */
bool
LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
{
        PGPROC     *proc = MyProc;
        bool            mustwait;
        int                     extraWaits = 0;
#ifdef LWLOCK_STATS
        lwlock_stats *lwstats;

        lwstats = get_lwlock_stats_entry(lock);
#endif

        Assert(mode == LW_SHARED || mode == LW_EXCLUSIVE);

        PRINT_LWDEBUG("LWLockAcquireOrWait", lock, mode);

        /* Ensure we will have room to remember the lock */
        if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
                elog(ERROR, "too many LWLocks taken");

        /*
         * Lock out cancel/die interrupts until we exit the code section protected
         * by the LWLock.  This ensures that interrupts will not interfere with
         * manipulations of data structures in shared memory.
         */
        HOLD_INTERRUPTS();

        /*
         * NB: We're using nearly the same twice-in-a-row lock acquisition
         * protocol as LWLockAcquire(). Check its comments for details.
         */
        mustwait = LWLockAttemptLock(lock, mode);

        if (mustwait)
        {
                LWLockQueueSelf(lock, LW_WAIT_UNTIL_FREE);

                mustwait = LWLockAttemptLock(lock, mode);

                if (mustwait)
                {
                        /*
                         * Wait until awakened.  Like in LWLockAcquire, be prepared for
                         * bogus wakeups.
                         */
                        LOG_LWDEBUG("LWLockAcquireOrWait", lock, "waiting");

#ifdef LWLOCK_STATS
                        lwstats->block_count++;
#endif

                        LWLockReportWaitStart(lock);
                        if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
                                TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), mode);

                        for (;;)
                        {
                                PGSemaphoreLock(proc->sem);
                                if (proc->lwWaiting == LW_WS_NOT_WAITING)
                                        break;
                                extraWaits++;
                        }

#ifdef LOCK_DEBUG
                        {
                                /* not waiting anymore */
                                uint32          nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);

                                Assert(nwaiters < MAX_BACKENDS);
                        }
#endif
                        if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
                                TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
                        LWLockReportWaitEnd();

                        LOG_LWDEBUG("LWLockAcquireOrWait", lock, "awakened");
                }
                else
                {
                        LOG_LWDEBUG("LWLockAcquireOrWait", lock, "acquired, undoing queue");

                        /*
                         * Got lock in the second attempt, undo queueing. We need to treat
                         * this as having successfully acquired the lock, otherwise we'd
                         * not necessarily wake up people we've prevented from acquiring
                         * the lock.
                         */
                        LWLockDequeueSelf(lock);
                }
        }

        /*
         * Fix the process wait semaphore's count for any absorbed wakeups.
         */
        while (extraWaits-- > 0)
                PGSemaphoreUnlock(proc->sem);

        if (mustwait)
        {
                /* Failed to get lock, so release interrupt holdoff */
                RESUME_INTERRUPTS();
                LOG_LWDEBUG("LWLockAcquireOrWait", lock, "failed");
                if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_FAIL_ENABLED())
                        TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_FAIL(T_NAME(lock), mode);
        }
        else
        {
                LOG_LWDEBUG("LWLockAcquireOrWait", lock, "succeeded");
                /* Add lock to list of locks held by this backend */
                held_lwlocks[num_held_lwlocks].lock = lock;
                held_lwlocks[num_held_lwlocks++].mode = mode;
                if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_ENABLED())
                        TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT(T_NAME(lock), mode);
        }

        return !mustwait;
}

/*
 * Does the lwlock in its current state need to wait for the variable value to
 * change?
 *
 * If we don't need to wait, and it's because the value of the variable has
 * changed, store the current value in newval.
 *
 * *result is set to true if the lock was free, and false otherwise.
 */
static bool
LWLockConflictsWithVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 oldval,
                                           uint64 *newval, bool *result)
{
        bool            mustwait;
        uint64          value;

        /*
         * Test first to see if it the slot is free right now.
         *
         * XXX: the unique caller of this routine, WaitXLogInsertionsToFinish()
         * via LWLockWaitForVar(), uses an implied barrier with a spinlock before
         * this, so we don't need a memory barrier here as far as the current
         * usage is concerned.  But that might not be safe in general.
         */
        mustwait = (pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE) != 0;

        if (!mustwait)
        {
                *result = true;
                return false;
        }

        *result = false;

        /*
         * Reading this value atomically is safe even on platforms where uint64
         * cannot be read without observing a torn value.
         */
        value = pg_atomic_read_u64(valptr);

        if (value != oldval)
        {
                mustwait = false;
                *newval = value;
        }
        else
        {
                mustwait = true;
        }

        return mustwait;
}

/*
 * LWLockWaitForVar - Wait until lock is free, or a variable is updated.
 *
 * If the lock is held and *valptr equals oldval, waits until the lock is
 * either freed, or the lock holder updates *valptr by calling
 * LWLockUpdateVar.  If the lock is free on exit (immediately or after
 * waiting), returns true.  If the lock is still held, but *valptr no longer
 * matches oldval, returns false and sets *newval to the current value in
 * *valptr.
 *
 * Note: this function ignores shared lock holders; if the lock is held
 * in shared mode, returns 'true'.
 *
 * Be aware that LWLockConflictsWithVar() does not include a memory barrier,
 * hence the caller of this function may want to rely on an explicit barrier or
 * an implied barrier via spinlock or LWLock to avoid memory ordering issues.
 */
bool
LWLockWaitForVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 oldval,
                                 uint64 *newval)
{
        PGPROC     *proc = MyProc;
        int                     extraWaits = 0;
        bool            result = false;
#ifdef LWLOCK_STATS
        lwlock_stats *lwstats;

        lwstats = get_lwlock_stats_entry(lock);
#endif

        PRINT_LWDEBUG("LWLockWaitForVar", lock, LW_WAIT_UNTIL_FREE);

        /*
         * Lock out cancel/die interrupts while we sleep on the lock.  There is no
         * cleanup mechanism to remove us from the wait queue if we got
         * interrupted.
         */
        HOLD_INTERRUPTS();

        /*
         * Loop here to check the lock's status after each time we are signaled.
         */
        for (;;)
        {
                bool            mustwait;

                mustwait = LWLockConflictsWithVar(lock, valptr, oldval, newval,
                                                                                  &result);

                if (!mustwait)
                        break;                          /* the lock was free or value didn't match */

                /*
                 * Add myself to wait queue. Note that this is racy, somebody else
                 * could wakeup before we're finished queuing. NB: We're using nearly
                 * the same twice-in-a-row lock acquisition protocol as
                 * LWLockAcquire(). Check its comments for details. The only
                 * difference is that we also have to check the variable's values when
                 * checking the state of the lock.
                 */
                LWLockQueueSelf(lock, LW_WAIT_UNTIL_FREE);

                /*
                 * Set RELEASE_OK flag, to make sure we get woken up as soon as the
                 * lock is released.
                 */
                pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);

                /*
                 * We're now guaranteed to be woken up if necessary. Recheck the lock
                 * and variables state.
                 */
                mustwait = LWLockConflictsWithVar(lock, valptr, oldval, newval,
                                                                                  &result);

                /* Ok, no conflict after we queued ourselves. Undo queueing. */
                if (!mustwait)
                {
                        LOG_LWDEBUG("LWLockWaitForVar", lock, "free, undoing queue");

                        LWLockDequeueSelf(lock);
                        break;
                }

                /*
                 * Wait until awakened.
                 *
                 * It is possible that we get awakened for a reason other than being
                 * signaled by LWLockRelease.  If so, loop back and wait again.  Once
                 * we've gotten the LWLock, re-increment the sema by the number of
                 * additional signals received.
                 */
                LOG_LWDEBUG("LWLockWaitForVar", lock, "waiting");

#ifdef LWLOCK_STATS
                lwstats->block_count++;
#endif

                LWLockReportWaitStart(lock);
                if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
                        TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), LW_EXCLUSIVE);

                for (;;)
                {
                        PGSemaphoreLock(proc->sem);
                        if (proc->lwWaiting == LW_WS_NOT_WAITING)
                                break;
                        extraWaits++;
                }

#ifdef LOCK_DEBUG
                {
                        /* not waiting anymore */
                        uint32          nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);

                        Assert(nwaiters < MAX_BACKENDS);
                }
#endif

                if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
                        TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), LW_EXCLUSIVE);
                LWLockReportWaitEnd();

                LOG_LWDEBUG("LWLockWaitForVar", lock, "awakened");

                /* Now loop back and check the status of the lock again. */
        }

        /*
         * Fix the process wait semaphore's count for any absorbed wakeups.
         */
        while (extraWaits-- > 0)
                PGSemaphoreUnlock(proc->sem);

        /*
         * Now okay to allow cancel/die interrupts.
         */
        RESUME_INTERRUPTS();

        return result;
}


/*
 * LWLockUpdateVar - Update a variable and wake up waiters atomically
 *
 * Sets *valptr to 'val', and wakes up all processes waiting for us with
 * LWLockWaitForVar().  It first sets the value atomically and then wakes up
 * waiting processes so that any process calling LWLockWaitForVar() on the same
 * lock is guaranteed to see the new value, and act accordingly.
 *
 * The caller must be holding the lock in exclusive mode.
 */
void
LWLockUpdateVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 val)
{
        proclist_head wakeup;
        proclist_mutable_iter iter;

        PRINT_LWDEBUG("LWLockUpdateVar", lock, LW_EXCLUSIVE);

        /*
         * Note that pg_atomic_exchange_u64 is a full barrier, so we're guaranteed
         * that the variable is updated before waking up waiters.
         */
        pg_atomic_exchange_u64(valptr, val);

        proclist_init(&wakeup);

        LWLockWaitListLock(lock);

        Assert(pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE);

        /*
         * See if there are any LW_WAIT_UNTIL_FREE waiters that need to be woken
         * up. They are always in the front of the queue.
         */
        proclist_foreach_modify(iter, &lock->waiters, lwWaitLink)
        {
                PGPROC     *waiter = GetPGProcByNumber(iter.cur);

                if (waiter->lwWaitMode != LW_WAIT_UNTIL_FREE)
                        break;

                proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
                proclist_push_tail(&wakeup, iter.cur, lwWaitLink);

                /* see LWLockWakeup() */
                Assert(waiter->lwWaiting == LW_WS_WAITING);
                waiter->lwWaiting = LW_WS_PENDING_WAKEUP;
        }

        /* We are done updating shared state of the lock itself. */
        LWLockWaitListUnlock(lock);

        /*
         * Awaken any waiters I removed from the queue.
         */
        proclist_foreach_modify(iter, &wakeup, lwWaitLink)
        {
                PGPROC     *waiter = GetPGProcByNumber(iter.cur);

                proclist_delete(&wakeup, iter.cur, lwWaitLink);
                /* check comment in LWLockWakeup() about this barrier */
                pg_write_barrier();
                waiter->lwWaiting = LW_WS_NOT_WAITING;
                PGSemaphoreUnlock(waiter->sem);
        }
}


/*
 * LWLockRelease - release a previously acquired lock
 */
void
LWLockRelease(LWLock *lock)
{
        LWLockMode      mode;
        uint32          oldstate;
        bool            check_waiters;
        int                     i;

        /*
         * Remove lock from list of locks held.  Usually, but not always, it will
         * be the latest-acquired lock; so search array backwards.
         */
        for (i = num_held_lwlocks; --i >= 0;)
                if (lock == held_lwlocks[i].lock)
                        break;

        if (i < 0)
                elog(ERROR, "lock %s is not held", T_NAME(lock));

        mode = held_lwlocks[i].mode;

        num_held_lwlocks--;
        for (; i < num_held_lwlocks; i++)
                held_lwlocks[i] = held_lwlocks[i + 1];

        PRINT_LWDEBUG("LWLockRelease", lock, mode);

        /*
         * Release my hold on lock, after that it can immediately be acquired by
         * others, even if we still have to wakeup other waiters.
         */
        if (mode == LW_EXCLUSIVE)
                oldstate = pg_atomic_sub_fetch_u32(&lock->state, LW_VAL_EXCLUSIVE);
        else
                oldstate = pg_atomic_sub_fetch_u32(&lock->state, LW_VAL_SHARED);

        /* nobody else can have that kind of lock */
        Assert(!(oldstate & LW_VAL_EXCLUSIVE));

        if (TRACE_POSTGRESQL_LWLOCK_RELEASE_ENABLED())
                TRACE_POSTGRESQL_LWLOCK_RELEASE(T_NAME(lock));

        /*
         * We're still waiting for backends to get scheduled, don't wake them up
         * again.
         */
        if ((oldstate & (LW_FLAG_HAS_WAITERS | LW_FLAG_RELEASE_OK)) ==
                (LW_FLAG_HAS_WAITERS | LW_FLAG_RELEASE_OK) &&
                (oldstate & LW_LOCK_MASK) == 0)
                check_waiters = true;
        else
                check_waiters = false;

        /*
         * As waking up waiters requires the spinlock to be acquired, only do so
         * if necessary.
         */
        if (check_waiters)
        {
                /* XXX: remove before commit? */
                LOG_LWDEBUG("LWLockRelease", lock, "releasing waiters");
                LWLockWakeup(lock);
        }

        /*
         * Now okay to allow cancel/die interrupts.
         */
        RESUME_INTERRUPTS();
}

/*
 * LWLockReleaseClearVar - release a previously acquired lock, reset variable
 */
void
LWLockReleaseClearVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 val)
{
        /*
         * Note that pg_atomic_exchange_u64 is a full barrier, so we're guaranteed
         * that the variable is updated before releasing the lock.
         */
        pg_atomic_exchange_u64(valptr, val);

        LWLockRelease(lock);
}


/*
 * LWLockReleaseAll - release all currently-held locks
 *
 * Used to clean up after ereport(ERROR). An important difference between this
 * function and retail LWLockRelease calls is that InterruptHoldoffCount is
 * unchanged by this operation.  This is necessary since InterruptHoldoffCount
 * has been set to an appropriate level earlier in error recovery. We could
 * decrement it below zero if we allow it to drop for each released lock!
 */
void
LWLockReleaseAll(void)
{
        while (num_held_lwlocks > 0)
        {
                HOLD_INTERRUPTS();              /* match the upcoming RESUME_INTERRUPTS */

                LWLockRelease(held_lwlocks[num_held_lwlocks - 1].lock);
        }
}


/*
 * LWLockHeldByMe - test whether my process holds a lock in any mode
 *
 * This is meant as debug support only.
 */
bool
LWLockHeldByMe(LWLock *lock)
{
        int                     i;

        for (i = 0; i < num_held_lwlocks; i++)
        {
                if (held_lwlocks[i].lock == lock)
                        return true;
        }
        return false;
}

/*
 * LWLockHeldByMe - test whether my process holds any of an array of locks
 *
 * This is meant as debug support only.
 */
bool
LWLockAnyHeldByMe(LWLock *lock, int nlocks, size_t stride)
{
        char       *held_lock_addr;
        char       *begin;
        char       *end;
        int                     i;

        begin = (char *) lock;
        end = begin + nlocks * stride;
        for (i = 0; i < num_held_lwlocks; i++)
        {
                held_lock_addr = (char *) held_lwlocks[i].lock;
                if (held_lock_addr >= begin &&
                        held_lock_addr < end &&
                        (held_lock_addr - begin) % stride == 0)
                        return true;
        }
        return false;
}

/*
 * LWLockHeldByMeInMode - test whether my process holds a lock in given mode
 *
 * This is meant as debug support only.
 */
bool
LWLockHeldByMeInMode(LWLock *lock, LWLockMode mode)
{
        int                     i;

        for (i = 0; i < num_held_lwlocks; i++)
        {
                if (held_lwlocks[i].lock == lock && held_lwlocks[i].mode == mode)
                        return true;
        }
        return false;
}