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 AioContext of the given coroutine
  94  */
  95 AioContext *coroutine_fn qemu_coroutine_get_aio_context(Coroutine *co);
  96
  97 /**
  98  * Get the currently executing coroutine
  99  */
 100 Coroutine *coroutine_fn qemu_coroutine_self(void);
 101
 102 /**
 103  * Return whether or not currently inside a coroutine
 104  *
 105  * This can be used to write functions that work both when in coroutine context
 106  * and when not in coroutine context.  Note that such functions cannot use the
 107  * coroutine_fn annotation since they work outside coroutine context.
 108  */
 109 bool qemu_in_coroutine(void);
 110
 111 /**
 112  * Return true if the coroutine is currently entered
 113  *
 114  * A coroutine is "entered" if it has not yielded from the current
 115  * qemu_coroutine_enter() call used to run it.  This does not mean that the
 116  * coroutine is currently executing code since it may have transferred control
 117  * to another coroutine using qemu_coroutine_enter().
 118  *
 119  * When several coroutines enter each other there may be no way to know which
 120  * ones have already been entered.  In such situations this function can be
 121  * used to avoid recursively entering coroutines.
 122  */
 123 bool qemu_coroutine_entered(Coroutine *co);
 124
 125 /**
 126  * Provides a mutex that can be used to synchronise coroutines
 127  */
 128 struct CoWaitRecord;
 129 struct CoMutex {
 130     /* Count of pending lockers; 0 for a free mutex, 1 for an
 131      * uncontended mutex.
 132      */
 133     unsigned locked;
 134
 135     /* Context that is holding the lock.  Useful to avoid spinning
 136      * when two coroutines on the same AioContext try to get the lock. :)
 137      */
 138     AioContext *ctx;
 139
 140     /* A queue of waiters.  Elements are added atomically in front of
 141      * from_push.  to_pop is only populated, and popped from, by whoever
 142      * is in charge of the next wakeup.  This can be an unlocker or,
 143      * through the handoff protocol, a locker that is about to go to sleep.
 144      */
 145     QSLIST_HEAD(, CoWaitRecord) from_push, to_pop;
 146
 147     unsigned handoff, sequence;
 148
 149     Coroutine *holder;
 150 };
 151
 152 /**
 153  * Initialises a CoMutex. This must be called before any other operation is used
 154  * on the CoMutex.
 155  */
 156 void qemu_co_mutex_init(CoMutex *mutex);
 157
 158 /**
 159  * Locks the mutex. If the lock cannot be taken immediately, control is
 160  * transferred to the caller of the current coroutine.
 161  */
 162 void coroutine_fn qemu_co_mutex_lock(CoMutex *mutex);
 163
 164 /**
 165  * Unlocks the mutex and schedules the next coroutine that was waiting for this
 166  * lock to be run.
 167  */
 168 void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex);
 169
 170 /**
 171  * Assert that the current coroutine holds @mutex.
 172  */
 173 static inline coroutine_fn void qemu_co_mutex_assert_locked(CoMutex *mutex)
 174 {
 175     /*
 176      * mutex->holder doesn't need any synchronisation if the assertion holds
 177      * true because the mutex protects it. If it doesn't hold true, we still
 178      * don't mind if another thread takes or releases mutex behind our back,
 179      * because the condition will be false no matter whether we read NULL or
 180      * the pointer for any other coroutine.
 181      */
 182     assert(qatomic_read(&mutex->locked) &&
 183            mutex->holder == qemu_coroutine_self());
 184 }
 185
 186 /**
 187  * CoQueues are a mechanism to queue coroutines in order to continue executing
 188  * them later.  They are similar to condition variables, but they need help
 189  * from an external mutex in order to maintain thread-safety.
 190  */
 191 typedef struct CoQueue {
 192     QSIMPLEQ_HEAD(, Coroutine) entries;
 193 } CoQueue;
 194
 195 /**
 196  * Initialise a CoQueue. This must be called before any other operation is used
 197  * on the CoQueue.
 198  */
 199 void qemu_co_queue_init(CoQueue *queue);
 200
 201 /**
 202  * Adds the current coroutine to the CoQueue and transfers control to the
 203  * caller of the coroutine.  The mutex is unlocked during the wait and
 204  * locked again afterwards.
 205  */
 206 #define qemu_co_queue_wait(queue, lock) \
 207     qemu_co_queue_wait_impl(queue, QEMU_MAKE_LOCKABLE(lock))
 208 void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock);
 209
 210 /**
 211  * Removes the next coroutine from the CoQueue, and wake it up.
 212  * Returns true if a coroutine was removed, false if the queue is empty.
 213  * OK to run from coroutine and non-coroutine context.
 214  */
 215 bool qemu_co_queue_next(CoQueue *queue);
 216
 217 /**
 218  * Empties the CoQueue; all coroutines are woken up.
 219  * OK to run from coroutine and non-coroutine context.
 220  */
 221 void qemu_co_queue_restart_all(CoQueue *queue);
 222
 223 /**
 224  * Removes the next coroutine from the CoQueue, and wake it up.  Unlike
 225  * qemu_co_queue_next, this function releases the lock during aio_co_wake
 226  * because it is meant to be used outside coroutine context; in that case, the
 227  * coroutine is entered immediately, before qemu_co_enter_next returns.
 228  *
 229  * If used in coroutine context, qemu_co_enter_next is equivalent to
 230  * qemu_co_queue_next.
 231  */
 232 #define qemu_co_enter_next(queue, lock) \
 233     qemu_co_enter_next_impl(queue, QEMU_MAKE_LOCKABLE(lock))
 234 bool qemu_co_enter_next_impl(CoQueue *queue, QemuLockable *lock);
 235
 236 /**
 237  * Checks if the CoQueue is empty.
 238  */
 239 bool qemu_co_queue_empty(CoQueue *queue);
 240
 241
 242 typedef struct CoRwTicket CoRwTicket;
 243 typedef struct CoRwlock {
 244     CoMutex mutex;
 245
 246     /* Number of readers, or -1 if owned for writing.  */
 247     int owners;
 248
 249     /* Waiting coroutines.  */
 250     QSIMPLEQ_HEAD(, CoRwTicket) tickets;
 251 } CoRwlock;
 252
 253 /**
 254  * Initialises a CoRwlock. This must be called before any other operation
 255  * is used on the CoRwlock
 256  */
 257 void qemu_co_rwlock_init(CoRwlock *lock);
 258
 259 /**
 260  * Read locks the CoRwlock. If the lock cannot be taken immediately because
 261  * of a parallel writer, control is transferred to the caller of the current
 262  * coroutine.
 263  */
 264 void qemu_co_rwlock_rdlock(CoRwlock *lock);
 265
 266 /**
 267  * Write Locks the CoRwlock from a reader.  This is a bit more efficient than
 268  * @qemu_co_rwlock_unlock followed by a separate @qemu_co_rwlock_wrlock.
 269  * Note that if the lock cannot be upgraded immediately, control is transferred
 270  * to the caller of the current coroutine; another writer might run while
 271  * @qemu_co_rwlock_upgrade blocks.
 272  */
 273 void qemu_co_rwlock_upgrade(CoRwlock *lock);
 274
 275 /**
 276  * Downgrades a write-side critical section to a reader.  Downgrading with
 277  * @qemu_co_rwlock_downgrade never blocks, unlike @qemu_co_rwlock_unlock
 278  * followed by @qemu_co_rwlock_rdlock.  This makes it more efficient, but
 279  * may also sometimes be necessary for correctness.
 280  */
 281 void qemu_co_rwlock_downgrade(CoRwlock *lock);
 282
 283 /**
 284  * Write Locks the mutex. If the lock cannot be taken immediately because
 285  * of a parallel reader, control is transferred to the caller of the current
 286  * coroutine.
 287  */
 288 void qemu_co_rwlock_wrlock(CoRwlock *lock);
 289
 290 /**
 291  * Unlocks the read/write lock and schedules the next coroutine that was
 292  * waiting for this lock to be run.
 293  */
 294 void qemu_co_rwlock_unlock(CoRwlock *lock);
 295
 296 typedef struct QemuCoSleep {
 297     Coroutine *to_wake;
 298 } QemuCoSleep;
 299
 300 /**
 301  * Yield the coroutine for a given duration. Initializes @w so that,
 302  * during this yield, it can be passed to qemu_co_sleep_wake() to
 303  * terminate the sleep.
 304  */
 305 void coroutine_fn qemu_co_sleep_ns_wakeable(QemuCoSleep *w,
 306                                             QEMUClockType type, int64_t ns);
 307
 308 /**
 309  * Yield the coroutine until the next call to qemu_co_sleep_wake.
 310  */
 311 void coroutine_fn qemu_co_sleep(QemuCoSleep *w);
 312
 313 static inline void coroutine_fn qemu_co_sleep_ns(QEMUClockType type, int64_t ns)
 314 {
 315     QemuCoSleep w = { 0 };
 316     qemu_co_sleep_ns_wakeable(&w, type, ns);
 317 }
 318
 319 /**
 320  * Wake a coroutine if it is sleeping in qemu_co_sleep_ns. The timer will be
 321  * deleted. @sleep_state must be the variable whose address was given to
 322  * qemu_co_sleep_ns() and should be checked to be non-NULL before calling
 323  * qemu_co_sleep_wake().
 324  */
 325 void qemu_co_sleep_wake(QemuCoSleep *w);
 326
 327 /**
 328  * Yield until a file descriptor becomes readable
 329  *
 330  * Note that this function clobbers the handlers for the file descriptor.
 331  */
 332 void coroutine_fn yield_until_fd_readable(int fd);
 333
 334 #include "qemu/lockable.h"
 335
 336 #endif /* QEMU_COROUTINE_H */