libc/citrus: Catch a NULL pointer earlier, check for empty string later.

[dragonfly.git] / sys / kern / kern_spinlock.c

/*
 * Copyright (c) 2005 Jeffrey M. Hsu.  All rights reserved.
 *
 * This code is derived from software contributed to The DragonFly Project
 * by Jeffrey M. Hsu. and Matthew Dillon
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * The implementation is designed to avoid looping when compatible operations
 * are executed.
 *
 * To acquire a spinlock we first increment counta.  Then we check if counta
 * meets our requirements.  For an exclusive spinlock it must be 1, of a
 * shared spinlock it must either be 1 or the SHARED_SPINLOCK bit must be set.
 *
 * Shared spinlock failure case: Decrement the count, loop until we can
 * transition from 0 to SHARED_SPINLOCK|1, or until we find SHARED_SPINLOCK
 * is set and increment the count.
 *
 * Exclusive spinlock failure case: While maintaining the count, clear the
 * SHARED_SPINLOCK flag unconditionally.  Then use an atomic add to transfer
 * the count from the low bits to the high bits of counta.  Then loop until
 * all low bits are 0.  Once the low bits drop to 0 we can transfer the
 * count back with an atomic_cmpset_int(), atomically, and return.
 */
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/types.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#ifdef INVARIANTS
#include <sys/proc.h>
#endif
#include <sys/priv.h>
#include <machine/atomic.h>
#include <machine/cpu.h>
#include <machine/cpufunc.h>
#include <machine/specialreg.h>
#include <machine/clock.h>
#include <sys/indefinite2.h>
#include <sys/spinlock.h>
#include <sys/spinlock2.h>
#include <sys/ktr.h>

#ifdef _KERNEL_VIRTUAL
#include <pthread.h>
#endif

struct spinlock pmap_spin = SPINLOCK_INITIALIZER(pmap_spin, "pmap_spin");

/*
 * Kernal Trace
 */
#if !defined(KTR_SPIN_CONTENTION)
#define KTR_SPIN_CONTENTION     KTR_ALL
#endif
#define SPIN_STRING     "spin=%p type=%c"
#define SPIN_ARG_SIZE   (sizeof(void *) + sizeof(int))

KTR_INFO_MASTER(spin);
#if 0
KTR_INFO(KTR_SPIN_CONTENTION, spin, beg, 0, SPIN_STRING, SPIN_ARG_SIZE);
KTR_INFO(KTR_SPIN_CONTENTION, spin, end, 1, SPIN_STRING, SPIN_ARG_SIZE);
#endif

#define logspin(name, spin, type)                       \
        KTR_LOG(spin_ ## name, spin, type)

#ifdef INVARIANTS
static int spin_lock_test_mode;
#endif

#ifdef DEBUG_LOCKS_LATENCY

static long spinlocks_add_latency;
SYSCTL_LONG(_debug, OID_AUTO, spinlocks_add_latency, CTLFLAG_RW,
    &spinlocks_add_latency, 0,
    "Add spinlock latency");

#endif

/*
 * We contested due to another exclusive lock holder.  We lose.
 *
 * We have to unwind the attempt and may acquire the spinlock
 * anyway while doing so.
 */
int
spin_trylock_contested(struct spinlock *spin)
{
        globaldata_t gd = mycpu;

        /*
         * Handle degenerate case, else fail.
         */
        if (atomic_cmpset_int(&spin->counta, SPINLOCK_SHARED|0, 1))
                return TRUE;
        /*atomic_add_int(&spin->counta, -1);*/
        --gd->gd_spinlocks;
        crit_exit_raw(gd->gd_curthread);

        return (FALSE);
}

/*
 * The spin_lock() inline was unable to acquire the lock and calls this
 * function with spin->counta already incremented, passing (spin->counta - 1)
 * to the function (the result of the inline's fetchadd).
 *
 * Note that we implement both exclusive and shared spinlocks, so we cannot
 * use atomic_swap_int().  Instead, we try to use atomic_fetchadd_int()
 * to put most of the burden on the cpu.  Atomic_cmpset_int() (cmpxchg)
 * can cause a lot of unnecessary looping in situations where it is just
 * trying to increment the count.
 *
 * Similarly, we leave the SHARED flag intact and incur slightly more
 * overhead when switching from shared to exclusive.  This allows us to
 * use atomic_fetchadd_int() for both spinlock types in the critical
 * path.
 *
 * Backoff algorithms can create even worse starvation problems, particularly
 * on multi-socket cpus, and don't really improve performance when a lot
 * of cores are contending.  However, if we are contested on an exclusive
 * lock due to a large number of shared locks being present, we throw in
 * extra cpu_pause()'s to account for the necessary time it will take other
 * cores to contend among themselves and release their shared locks.
 */
void
_spin_lock_contested(struct spinlock *spin, const char *ident, int value)
{
        indefinite_info_t info;

        /*
         * WARNING! Caller has already incremented the lock.  We must
         *          increment the count value (from the inline's fetch-add)
         *          to match.
         *
         * Handle the degenerate case where the spinlock is flagged SHARED
         * with only our reference.  We can convert it to EXCLUSIVE.
         */
        ++value;
        if (value == (SPINLOCK_SHARED | 1)) {
                if (atomic_cmpset_int(&spin->counta, SPINLOCK_SHARED | 1, 1))
                        return;
        }
        indefinite_init(&info, ident, 0, 'S');

        /*
         * Transfer our exclusive request to the high bits and clear the
         * SPINLOCK_SHARED bit if it was set.  This makes the spinlock
         * appear exclusive, preventing any NEW shared or exclusive
         * spinlocks from being obtained while we wait for existing
         * shared or exclusive holders to unlock.
         *
         * Don't tread on earlier exclusive waiters by stealing the lock
         * away early if the low bits happen to now be 1.
         *
         * The shared unlock understands that this may occur.
         */
        atomic_add_int(&spin->counta, SPINLOCK_EXCLWAIT - 1);
        if (value & SPINLOCK_SHARED)
                atomic_clear_int(&spin->counta, SPINLOCK_SHARED);

        /*
         * Spin until we can acquire a low-count of 1.
         */
        for (;;) {
                /*
                 * If the low bits are zero, try to acquire the exclusive lock
                 * by transfering our high bit reservation to the low bits.
                 *
                 * NOTE: Reading spin->counta prior to the swap is extremely
                 *       important on multi-chip/many-core boxes.  On 48-core
                 *       this one change improves fully concurrent all-cores
                 *       compiles by 100% or better.
                 *
                 *       I can't emphasize enough how important the pre-read
                 *       is in preventing hw cache bus armageddon on
                 *       multi-chip systems.  And on single-chip/multi-core
                 *       systems it just doesn't hurt.
                 */
                uint32_t ovalue = spin->counta;
                cpu_ccfence();
                if ((ovalue & (SPINLOCK_EXCLWAIT - 1)) == 0 &&
                    atomic_cmpset_int(&spin->counta, ovalue,
                                      (ovalue - SPINLOCK_EXCLWAIT) | 1)) {
                        break;
                }

                /*
                 * Throw in extra cpu_pause()'s when we are waiting on
                 * multiple other shared lock holders to release (the
                 * indefinite_check() also throws one in).
                 *
                 * We know these are shared lock holders when the count
                 * is larger than 1, because an exclusive lock holder can
                 * only have one count.  Do this optimization only when
                 * the number of shared lock holders is 3 or greater.
                 */
                ovalue &= SPINLOCK_EXCLWAIT - 1;
                while (ovalue > 2) {
                        cpu_pause();
                        cpu_pause();
                        --ovalue;
                }

                if (indefinite_check(&info))
                        break;
        }
        indefinite_done(&info);
}

/*
 * The spin_lock_shared() inline was unable to acquire the lock and calls
 * this function with spin->counta already incremented.
 *
 * This is not in the critical path unless there is contention between
 * shared and exclusive holders.
 */
void
_spin_lock_shared_contested(struct spinlock *spin, const char *ident)
{
        indefinite_info_t info;

        indefinite_init(&info, ident, 0, 's');

        /*
         * Undo the inline's increment.
         */
        atomic_add_int(&spin->counta, -1);

#ifdef DEBUG_LOCKS_LATENCY
        long j;
        for (j = spinlocks_add_latency; j > 0; --j)
                cpu_ccfence();
#endif

        for (;;) {
                /*
                 * Loop until we can acquire the shared spinlock.  Note that
                 * the low bits can be zero while the high EXCLWAIT bits are
                 * non-zero.  In this situation exclusive requesters have
                 * priority (otherwise shared users on multiple cpus can hog
                 * the spinlnock).
                 *
                 * NOTE: Reading spin->counta prior to the swap is extremely
                 *       important on multi-chip/many-core boxes.  On 48-core
                 *       this one change improves fully concurrent all-cores
                 *       compiles by 100% or better.
                 *
                 *       I can't emphasize enough how important the pre-read
                 *       is in preventing hw cache bus armageddon on
                 *       multi-chip systems.  And on single-chip/multi-core
                 *       systems it just doesn't hurt.
                 */
                uint32_t ovalue = spin->counta;

                cpu_ccfence();
                if (ovalue == 0) {
                        if (atomic_cmpset_int(&spin->counta, 0,
                                              SPINLOCK_SHARED | 1))
                                break;
                } else if (ovalue & SPINLOCK_SHARED) {
                        if (atomic_cmpset_int(&spin->counta, ovalue,
                                              ovalue + 1))
                                break;
                }
                if (indefinite_check(&info))
                        break;
        }
        indefinite_done(&info);
}

/*
 * If INVARIANTS is enabled various spinlock timing tests can be run
 * by setting debug.spin_lock_test:
 *
 *      1       Test the indefinite wait code
 *      2       Time the best-case exclusive lock overhead (spin_test_count)
 *      3       Time the best-case shared lock overhead (spin_test_count)
 */

#ifdef INVARIANTS

static int spin_test_count = 10000000;
SYSCTL_INT(_debug, OID_AUTO, spin_test_count, CTLFLAG_RW, &spin_test_count, 0,
    "Number of iterations to use for spinlock wait code test");

static int
sysctl_spin_lock_test(SYSCTL_HANDLER_ARGS)
{
        struct spinlock spin;
        int error;
        int value = 0;
        int i;

        if ((error = priv_check(curthread, PRIV_ROOT)) != 0)
                return (error);
        if ((error = SYSCTL_IN(req, &value, sizeof(value))) != 0)
                return (error);

        /*
         * Indefinite wait test
         */
        if (value == 1) {
                spin_init(&spin, "sysctllock");
                spin_lock(&spin);       /* force an indefinite wait */
                spin_lock_test_mode = 1;
                spin_lock(&spin);
                spin_unlock(&spin);     /* Clean up the spinlock count */
                spin_unlock(&spin);
                spin_lock_test_mode = 0;
        }

        /*
         * Time best-case exclusive spinlocks
         */
        if (value == 2) {
                globaldata_t gd = mycpu;

                spin_init(&spin, "sysctllocktest");
                for (i = spin_test_count; i > 0; --i) {
                    _spin_lock_quick(gd, &spin, "test");
                    spin_unlock_quick(gd, &spin);
                }
        }

        return (0);
}

SYSCTL_PROC(_debug, KERN_PROC_ALL, spin_lock_test, CTLFLAG_RW|CTLTYPE_INT,
        0, 0, sysctl_spin_lock_test, "I", "Test spinlock wait code");

#endif  /* INVARIANTS */