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 back to a coroutine's caller
  81  *
  82  * This function does not return until the coroutine is re-entered using
  83  * qemu_coroutine_enter().
  84  */
  85 void coroutine_fn qemu_coroutine_yield(void);
  86
  87 /**
  88  * Get the currently executing coroutine
  89  */
  90 Coroutine *coroutine_fn qemu_coroutine_self(void);
  91
  92 /**
  93  * Return whether or not currently inside a coroutine
  94  *
  95  * This can be used to write functions that work both when in coroutine context
  96  * and when not in coroutine context.  Note that such functions cannot use the
  97  * coroutine_fn annotation since they work outside coroutine context.
  98  */
  99 bool qemu_in_coroutine(void);
 100
 101 /**
 102  * Return true if the coroutine is currently entered
 103  *
 104  * A coroutine is "entered" if it has not yielded from the current
 105  * qemu_coroutine_enter() call used to run it.  This does not mean that the
 106  * coroutine is currently executing code since it may have transferred control
 107  * to another coroutine using qemu_coroutine_enter().
 108  *
 109  * When several coroutines enter each other there may be no way to know which
 110  * ones have already been entered.  In such situations this function can be
 111  * used to avoid recursively entering coroutines.
 112  */
 113 bool qemu_coroutine_entered(Coroutine *co);
 114
 115
 116 /**
 117  * CoQueues are a mechanism to queue coroutines in order to continue executing
 118  * them later. They provide the fundamental primitives on which coroutine locks
 119  * are built.
 120  */
 121 typedef struct CoQueue {
 122     QSIMPLEQ_HEAD(, Coroutine) entries;
 123 } CoQueue;
 124
 125 /**
 126  * Initialise a CoQueue. This must be called before any other operation is used
 127  * on the CoQueue.
 128  */
 129 void qemu_co_queue_init(CoQueue *queue);
 130
 131 /**
 132  * Adds the current coroutine to the CoQueue and transfers control to the
 133  * caller of the coroutine.
 134  */
 135 void coroutine_fn qemu_co_queue_wait(CoQueue *queue);
 136
 137 /**
 138  * Restarts the next coroutine in the CoQueue and removes it from the queue.
 139  *
 140  * Returns true if a coroutine was restarted, false if the queue is empty.
 141  */
 142 bool coroutine_fn qemu_co_queue_next(CoQueue *queue);
 143
 144 /**
 145  * Restarts all coroutines in the CoQueue and leaves the queue empty.
 146  */
 147 void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue);
 148
 149 /**
 150  * Enter the next coroutine in the queue
 151  */
 152 bool qemu_co_enter_next(CoQueue *queue);
 153
 154 /**
 155  * Checks if the CoQueue is empty.
 156  */
 157 bool qemu_co_queue_empty(CoQueue *queue);
 158
 159
 160 /**
 161  * Provides a mutex that can be used to synchronise coroutines
 162  */
 163 struct CoWaitRecord;
 164 typedef struct CoMutex {
 165     /* Count of pending lockers; 0 for a free mutex, 1 for an
 166      * uncontended mutex.
 167      */
 168     unsigned locked;
 169
 170     /* Context that is holding the lock.  Useful to avoid spinning
 171      * when two coroutines on the same AioContext try to get the lock. :)
 172      */
 173     AioContext *ctx;
 174
 175     /* A queue of waiters.  Elements are added atomically in front of
 176      * from_push.  to_pop is only populated, and popped from, by whoever
 177      * is in charge of the next wakeup.  This can be an unlocker or,
 178      * through the handoff protocol, a locker that is about to go to sleep.
 179      */
 180     QSLIST_HEAD(, CoWaitRecord) from_push, to_pop;
 181
 182     unsigned handoff, sequence;
 183
 184     Coroutine *holder;
 185 } CoMutex;
 186
 187 /**
 188  * Initialises a CoMutex. This must be called before any other operation is used
 189  * on the CoMutex.
 190  */
 191 void qemu_co_mutex_init(CoMutex *mutex);
 192
 193 /**
 194  * Locks the mutex. If the lock cannot be taken immediately, control is
 195  * transferred to the caller of the current coroutine.
 196  */
 197 void coroutine_fn qemu_co_mutex_lock(CoMutex *mutex);
 198
 199 /**
 200  * Unlocks the mutex and schedules the next coroutine that was waiting for this
 201  * lock to be run.
 202  */
 203 void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex);
 204
 205 typedef struct CoRwlock {
 206     bool writer;
 207     int reader;
 208     CoQueue queue;
 209 } CoRwlock;
 210
 211 /**
 212  * Initialises a CoRwlock. This must be called before any other operation
 213  * is used on the CoRwlock
 214  */
 215 void qemu_co_rwlock_init(CoRwlock *lock);
 216
 217 /**
 218  * Read locks the CoRwlock. If the lock cannot be taken immediately because
 219  * of a parallel writer, control is transferred to the caller of the current
 220  * coroutine.
 221  */
 222 void qemu_co_rwlock_rdlock(CoRwlock *lock);
 223
 224 /**
 225  * Write Locks the mutex. If the lock cannot be taken immediately because
 226  * of a parallel reader, control is transferred to the caller of the current
 227  * coroutine.
 228  */
 229 void qemu_co_rwlock_wrlock(CoRwlock *lock);
 230
 231 /**
 232  * Unlocks the read/write lock and schedules the next coroutine that was
 233  * waiting for this lock to be run.
 234  */
 235 void qemu_co_rwlock_unlock(CoRwlock *lock);
 236
 237 /**
 238  * Yield the coroutine for a given duration
 239  *
 240  * Behaves similarly to co_sleep_ns(), but the sleeping coroutine will be
 241  * resumed when using aio_poll().
 242  */
 243 void coroutine_fn co_aio_sleep_ns(AioContext *ctx, QEMUClockType type,
 244                                   int64_t ns);
 245
 246 /**
 247  * Yield until a file descriptor becomes readable
 248  *
 249  * Note that this function clobbers the handlers for the file descriptor.
 250  */
 251 void coroutine_fn yield_until_fd_readable(int fd);
 252
 253 #endif /* QEMU_COROUTINE_H */