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 * Provides a mutex that can be used to synchronise coroutines 117 */ 118 struct CoWaitRecord; 119 typedef struct CoMutex { 120 /* Count of pending lockers; 0 for a free mutex, 1 for an 121 * uncontended mutex. 122 */ 123 unsigned locked; 124 125 /* Context that is holding the lock. Useful to avoid spinning 126 * when two coroutines on the same AioContext try to get the lock. :) 127 */ 128 AioContext *ctx; 129 130 /* A queue of waiters. Elements are added atomically in front of 131 * from_push. to_pop is only populated, and popped from, by whoever 132 * is in charge of the next wakeup. This can be an unlocker or, 133 * through the handoff protocol, a locker that is about to go to sleep. 134 */ 135 QSLIST_HEAD(, CoWaitRecord) from_push, to_pop; 136 137 unsigned handoff, sequence; 138 139 Coroutine *holder; 140 } CoMutex; 141 142 /** 143 * Initialises a CoMutex. This must be called before any other operation is used 144 * on the CoMutex. 145 */ 146 void qemu_co_mutex_init(CoMutex *mutex); 147 148 /** 149 * Locks the mutex. If the lock cannot be taken immediately, control is 150 * transferred to the caller of the current coroutine. 151 */ 152 void coroutine_fn qemu_co_mutex_lock(CoMutex *mutex); 153 154 /** 155 * Unlocks the mutex and schedules the next coroutine that was waiting for this 156 * lock to be run. 157 */ 158 void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex); 159 160 161 /** 162 * CoQueues are a mechanism to queue coroutines in order to continue executing 163 * them later. They are similar to condition variables, but they need help 164 * from an external mutex in order to maintain thread-safety. 165 */ 166 typedef struct CoQueue { 167 QSIMPLEQ_HEAD(, Coroutine) entries; 168 } CoQueue; 169 170 /** 171 * Initialise a CoQueue. This must be called before any other operation is used 172 * on the CoQueue. 173 */ 174 void qemu_co_queue_init(CoQueue *queue); 175 176 /** 177 * Adds the current coroutine to the CoQueue and transfers control to the 178 * caller of the coroutine. The mutex is unlocked during the wait and 179 * locked again afterwards. 180 */ 181 void coroutine_fn qemu_co_queue_wait(CoQueue *queue, CoMutex *mutex); 182 183 /** 184 * Restarts the next coroutine in the CoQueue and removes it from the queue. 185 * 186 * Returns true if a coroutine was restarted, false if the queue is empty. 187 */ 188 bool coroutine_fn qemu_co_queue_next(CoQueue *queue); 189 190 /** 191 * Restarts all coroutines in the CoQueue and leaves the queue empty. 192 */ 193 void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue); 194 195 /** 196 * Enter the next coroutine in the queue 197 */ 198 bool qemu_co_enter_next(CoQueue *queue); 199 200 /** 201 * Checks if the CoQueue is empty. 202 */ 203 bool qemu_co_queue_empty(CoQueue *queue); 204 205 206 typedef struct CoRwlock { 207 int pending_writer; 208 int reader; 209 CoMutex mutex; 210 CoQueue queue; 211 } CoRwlock; 212 213 /** 214 * Initialises a CoRwlock. This must be called before any other operation 215 * is used on the CoRwlock 216 */ 217 void qemu_co_rwlock_init(CoRwlock *lock); 218 219 /** 220 * Read locks the CoRwlock. If the lock cannot be taken immediately because 221 * of a parallel writer, control is transferred to the caller of the current 222 * coroutine. 223 */ 224 void qemu_co_rwlock_rdlock(CoRwlock *lock); 225 226 /** 227 * Write Locks the mutex. If the lock cannot be taken immediately because 228 * of a parallel reader, control is transferred to the caller of the current 229 * coroutine. 230 */ 231 void qemu_co_rwlock_wrlock(CoRwlock *lock); 232 233 /** 234 * Unlocks the read/write lock and schedules the next coroutine that was 235 * waiting for this lock to be run. 236 */ 237 void qemu_co_rwlock_unlock(CoRwlock *lock); 238 239 /** 240 * Yield the coroutine for a given duration 241 * 242 * Behaves similarly to co_sleep_ns(), but the sleeping coroutine will be 243 * resumed when using aio_poll(). 244 */ 245 void coroutine_fn co_aio_sleep_ns(AioContext *ctx, QEMUClockType type, 246 int64_t ns); 247 248 /** 249 * Yield until a file descriptor becomes readable 250 * 251 * Note that this function clobbers the handlers for the file descriptor. 252 */ 253 void coroutine_fn yield_until_fd_readable(int fd); 254 255 #endif /* QEMU_COROUTINE_H */ 256