include/qemu/coroutine.h

   1 /*
   2  * QEMU coroutine implementation
   3  *
   4  * Copyright IBM, Corp. 2011
   5  *
   6  * Authors:
   7  *  Stefan Hajnoczi    <stefanha@linux.vnet.ibm.com>
   8  *  Kevin Wolf         <kwolf@redhat.com>
   9  *
  10  * This work is licensed under the terms of the GNU LGPL, version 2 or later.
  11  * See the COPYING.LIB file in the top-level directory.
  12  *
  13  */
  14
  15 #ifndef QEMU_COROUTINE_H
  16 #define QEMU_COROUTINE_H
  17
  18 #include "qemu/queue.h"
  19 #include "qemu/timer.h"
  20
  21 /**
  22  * Coroutines are a mechanism for stack switching and can be used for
  23  * cooperative userspace threading.  These functions provide a simple but
  24  * useful flavor of coroutines that is suitable for writing sequential code,
  25  * rather than callbacks, for operations that need to give up control while
  26  * waiting for events to complete.
  27  *
  28  * These functions are re-entrant and may be used outside the global mutex.
  29  */
  30
  31 /**
  32  * Mark a function that executes in coroutine context
  33  *
  34  * Functions that execute in coroutine context cannot be called directly from
  35  * normal functions.  In the future it would be nice to enable compiler or
  36  * static checker support for catching such errors.  This annotation might make
  37  * it possible and in the meantime it serves as documentation.
  38  *
  39  * For example:
  40  *
  41  *   static void coroutine_fn foo(void) {
  42  *       ....
  43  *   }
  44  */
  45 #define coroutine_fn
  46
  47 typedef struct Coroutine Coroutine;
  48
  49 /**
  50  * Coroutine entry point
  51  *
  52  * When the coroutine is entered for the first time, opaque is passed in as an
  53  * argument.
  54  *
  55  * When this function returns, the coroutine is destroyed automatically and
  56  * execution continues in the caller who last entered the coroutine.
  57  */
  58 typedef void coroutine_fn CoroutineEntry(void *opaque);
  59
  60 /**
  61  * Create a new coroutine
  62  *
  63  * Use qemu_coroutine_enter() to actually transfer control to the coroutine.
  64  * The opaque argument is passed as the argument to the entry point.
  65  */
  66 Coroutine *qemu_coroutine_create(CoroutineEntry *entry, void *opaque);
  67
  68 /**
  69  * Transfer control to a coroutine
  70  */
  71 void qemu_coroutine_enter(Coroutine *coroutine);
  72
  73 /**
  74  * Transfer control to a coroutine if it's not active (i.e. part of the call
  75  * stack of the running coroutine). Otherwise, do nothing.
  76  */
  77 void qemu_coroutine_enter_if_inactive(Coroutine *co);
  78
  79 /**
  80  * Transfer control to a coroutine and associate it with ctx
  81  */
  82 void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co);
  83
  84 /**
  85  * Transfer control back to a coroutine's caller
  86  *
  87  * This function does not return until the coroutine is re-entered using
  88  * qemu_coroutine_enter().
  89  */
  90 void coroutine_fn qemu_coroutine_yield(void);
  91
  92 /**
  93  * Get the currently executing coroutine
  94  */
  95 Coroutine *coroutine_fn qemu_coroutine_self(void);
  96
  97 /**
  98  * Return whether or not currently inside a coroutine
  99  *
 100  * This can be used to write functions that work both when in coroutine context
 101  * and when not in coroutine context.  Note that such functions cannot use the
 102  * coroutine_fn annotation since they work outside coroutine context.
 103  */
 104 bool qemu_in_coroutine(void);
 105
 106 /**
 107  * Return true if the coroutine is currently entered
 108  *
 109  * A coroutine is "entered" if it has not yielded from the current
 110  * qemu_coroutine_enter() call used to run it.  This does not mean that the
 111  * coroutine is currently executing code since it may have transferred control
 112  * to another coroutine using qemu_coroutine_enter().
 113  *
 114  * When several coroutines enter each other there may be no way to know which
 115  * ones have already been entered.  In such situations this function can be
 116  * used to avoid recursively entering coroutines.
 117  */
 118 bool qemu_coroutine_entered(Coroutine *co);
 119
 120 /**
 121  * Provides a mutex that can be used to synchronise coroutines
 122  */
 123 struct CoWaitRecord;
 124 typedef struct CoMutex {
 125     /* Count of pending lockers; 0 for a free mutex, 1 for an
 126      * uncontended mutex.
 127      */
 128     unsigned locked;
 129
 130     /* Context that is holding the lock.  Useful to avoid spinning
 131      * when two coroutines on the same AioContext try to get the lock. :)
 132      */
 133     AioContext *ctx;
 134
 135     /* A queue of waiters.  Elements are added atomically in front of
 136      * from_push.  to_pop is only populated, and popped from, by whoever
 137      * is in charge of the next wakeup.  This can be an unlocker or,
 138      * through the handoff protocol, a locker that is about to go to sleep.
 139      */
 140     QSLIST_HEAD(, CoWaitRecord) from_push, to_pop;
 141
 142     unsigned handoff, sequence;
 143
 144     Coroutine *holder;
 145 } CoMutex;
 146
 147 /**
 148  * Initialises a CoMutex. This must be called before any other operation is used
 149  * on the CoMutex.
 150  */
 151 void qemu_co_mutex_init(CoMutex *mutex);
 152
 153 /**
 154  * Locks the mutex. If the lock cannot be taken immediately, control is
 155  * transferred to the caller of the current coroutine.
 156  */
 157 void coroutine_fn qemu_co_mutex_lock(CoMutex *mutex);
 158
 159 /**
 160  * Unlocks the mutex and schedules the next coroutine that was waiting for this
 161  * lock to be run.
 162  */
 163 void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex);
 164
 165
 166 /**
 167  * CoQueues are a mechanism to queue coroutines in order to continue executing
 168  * them later.  They are similar to condition variables, but they need help
 169  * from an external mutex in order to maintain thread-safety.
 170  */
 171 typedef struct CoQueue {
 172     QSIMPLEQ_HEAD(, Coroutine) entries;
 173 } CoQueue;
 174
 175 /**
 176  * Initialise a CoQueue. This must be called before any other operation is used
 177  * on the CoQueue.
 178  */
 179 void qemu_co_queue_init(CoQueue *queue);
 180
 181 /**
 182  * Adds the current coroutine to the CoQueue and transfers control to the
 183  * caller of the coroutine.  The mutex is unlocked during the wait and
 184  * locked again afterwards.
 185  */
 186 void coroutine_fn qemu_co_queue_wait(CoQueue *queue, CoMutex *mutex);
 187
 188 /**
 189  * Restarts the next coroutine in the CoQueue and removes it from the queue.
 190  *
 191  * Returns true if a coroutine was restarted, false if the queue is empty.
 192  */
 193 bool coroutine_fn qemu_co_queue_next(CoQueue *queue);
 194
 195 /**
 196  * Restarts all coroutines in the CoQueue and leaves the queue empty.
 197  */
 198 void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue);
 199
 200 /**
 201  * Enter the next coroutine in the queue
 202  */
 203 bool qemu_co_enter_next(CoQueue *queue);
 204
 205 /**
 206  * Checks if the CoQueue is empty.
 207  */
 208 bool qemu_co_queue_empty(CoQueue *queue);
 209
 210
 211 typedef struct CoRwlock {
 212     int pending_writer;
 213     int reader;
 214     CoMutex mutex;
 215     CoQueue queue;
 216 } CoRwlock;
 217
 218 /**
 219  * Initialises a CoRwlock. This must be called before any other operation
 220  * is used on the CoRwlock
 221  */
 222 void qemu_co_rwlock_init(CoRwlock *lock);
 223
 224 /**
 225  * Read locks the CoRwlock. If the lock cannot be taken immediately because
 226  * of a parallel writer, control is transferred to the caller of the current
 227  * coroutine.
 228  */
 229 void qemu_co_rwlock_rdlock(CoRwlock *lock);
 230
 231 /**
 232  * Write Locks the CoRwlock from a reader.  This is a bit more efficient than
 233  * @qemu_co_rwlock_unlock followed by a separate @qemu_co_rwlock_wrlock.
 234  * However, if the lock cannot be upgraded immediately, control is transferred
 235  * to the caller of the current coroutine.  Also, @qemu_co_rwlock_upgrade
 236  * only overrides CoRwlock fairness if there are no concurrent readers, so
 237  * another writer might run while @qemu_co_rwlock_upgrade blocks.
 238  */
 239 void qemu_co_rwlock_upgrade(CoRwlock *lock);
 240
 241 /**
 242  * Downgrades a write-side critical section to a reader.  Downgrading with
 243  * @qemu_co_rwlock_downgrade never blocks, unlike @qemu_co_rwlock_unlock
 244  * followed by @qemu_co_rwlock_rdlock.  This makes it more efficient, but
 245  * may also sometimes be necessary for correctness.
 246  */
 247 void qemu_co_rwlock_downgrade(CoRwlock *lock);
 248
 249 /**
 250  * Write Locks the mutex. If the lock cannot be taken immediately because
 251  * of a parallel reader, control is transferred to the caller of the current
 252  * coroutine.
 253  */
 254 void qemu_co_rwlock_wrlock(CoRwlock *lock);
 255
 256 /**
 257  * Unlocks the read/write lock and schedules the next coroutine that was
 258  * waiting for this lock to be run.
 259  */
 260 void qemu_co_rwlock_unlock(CoRwlock *lock);
 261
 262 /**
 263  * Yield the coroutine for a given duration
 264  *
 265  * Behaves similarly to co_sleep_ns(), but the sleeping coroutine will be
 266  * resumed when using aio_poll().
 267  */
 268 void coroutine_fn co_aio_sleep_ns(AioContext *ctx, QEMUClockType type,
 269                                   int64_t ns);
 270
 271 /**
 272  * Yield until a file descriptor becomes readable
 273  *
 274  * Note that this function clobbers the handlers for the file descriptor.
 275  */
 276 void coroutine_fn yield_until_fd_readable(int fd);
 277
 278 #endif /* QEMU_COROUTINE_H */