1 /* 2 * 3 * Copyright 2015 gRPC authors. 4 * 5 * Licensed under the Apache License, Version 2.0 (the "License"); 6 * you may not use this file except in compliance with the License. 7 * You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 * 17 */ 18 19 #ifndef GRPC_SUPPORT_SYNC_H 20 #define GRPC_SUPPORT_SYNC_H 21 22 /* Platform-specific type declarations of gpr_mu and gpr_cv. */ 23 #include <grpc/support/port_platform.h> 24 #include <grpc/support/time.h> /* for gpr_timespec */ 25 26 #ifdef __cplusplus 27 extern "C" { 28 #endif 29 30 /** Synchronization primitives for GPR. 31 32 The type gpr_mu provides a non-reentrant mutex (lock). 33 34 The type gpr_cv provides a condition variable. 35 36 The type gpr_once provides for one-time initialization. 37 38 The type gpr_event provides one-time-setting, reading, and 39 waiting of a void*, with memory barriers. 40 41 The type gpr_refcount provides an object reference counter, 42 with memory barriers suitable to control 43 object lifetimes. 44 45 The type gpr_stats_counter provides an atomic statistics counter. It 46 provides no memory barriers. 47 */ 48 49 #include <grpc/support/sync_generic.h> // IWYU pragma: export 50 51 #if defined(GPR_CUSTOM_SYNC) 52 #include <grpc/support/sync_custom.h> // IWYU pragma: export 53 #elif defined(GPR_ABSEIL_SYNC) 54 #include <grpc/support/sync_abseil.h> // IWYU pragma: export 55 #elif defined(GPR_POSIX_SYNC) 56 #include <grpc/support/sync_posix.h> // IWYU pragma: export 57 #elif defined(GPR_WINDOWS) 58 #include <grpc/support/sync_windows.h> // IWYU pragma: export 59 #else 60 #error Unable to determine platform for sync 61 #endif 62 63 /** --- Mutex interface --- 64 65 At most one thread may hold an exclusive lock on a mutex at any given time. 66 Actions taken by a thread that holds a mutex exclusively happen after 67 actions taken by all previous holders of the mutex. Variables of type 68 gpr_mu are uninitialized when first declared. */ 69 70 /** Initialize *mu. Requires: *mu uninitialized. */ 71 GPRAPI void gpr_mu_init(gpr_mu* mu); 72 73 /** Cause *mu no longer to be initialized, freeing any memory in use. Requires: 74 *mu initialized; no other concurrent operation on *mu. */ 75 GPRAPI void gpr_mu_destroy(gpr_mu* mu); 76 77 /** Wait until no thread has a lock on *mu, cause the calling thread to own an 78 exclusive lock on *mu, then return. May block indefinitely or crash if the 79 calling thread has a lock on *mu. Requires: *mu initialized. */ 80 GPRAPI void gpr_mu_lock(gpr_mu* mu); 81 82 /** Release an exclusive lock on *mu held by the calling thread. Requires: *mu 83 initialized; the calling thread holds an exclusive lock on *mu. */ 84 GPRAPI void gpr_mu_unlock(gpr_mu* mu); 85 86 /** Without blocking, attempt to acquire an exclusive lock on *mu for the 87 calling thread, then return non-zero iff success. Fail, if any thread holds 88 the lock; succeeds with high probability if no thread holds the lock. 89 Requires: *mu initialized. */ 90 GPRAPI int gpr_mu_trylock(gpr_mu* mu); 91 92 /** --- Condition variable interface --- 93 94 A while-loop should be used with gpr_cv_wait() when waiting for conditions 95 to become true. See the example below. Variables of type gpr_cv are 96 uninitialized when first declared. */ 97 98 /** Initialize *cv. Requires: *cv uninitialized. */ 99 GPRAPI void gpr_cv_init(gpr_cv* cv); 100 101 /** Cause *cv no longer to be initialized, freeing any memory in use. Requires: 102 *cv initialized; no other concurrent operation on *cv.*/ 103 GPRAPI void gpr_cv_destroy(gpr_cv* cv); 104 105 /** Atomically release *mu and wait on *cv. When the calling thread is woken 106 from *cv or the deadline abs_deadline is exceeded, execute gpr_mu_lock(mu) 107 and return whether the deadline was exceeded. Use 108 abs_deadline==gpr_inf_future for no deadline. abs_deadline can be either 109 an absolute deadline, or a GPR_TIMESPAN. May return even when not 110 woken explicitly. Requires: *mu and *cv initialized; the calling thread 111 holds an exclusive lock on *mu. */ 112 GPRAPI int gpr_cv_wait(gpr_cv* cv, gpr_mu* mu, gpr_timespec abs_deadline); 113 114 /** If any threads are waiting on *cv, wake at least one. 115 Clients may treat this as an optimization of gpr_cv_broadcast() 116 for use in the case where waking more than one waiter is not useful. 117 Requires: *cv initialized. */ 118 GPRAPI void gpr_cv_signal(gpr_cv* cv); 119 120 /** Wake all threads waiting on *cv. Requires: *cv initialized. */ 121 GPRAPI void gpr_cv_broadcast(gpr_cv* cv); 122 123 /** --- One-time initialization --- 124 125 gpr_once must be declared with static storage class, and initialized with 126 GPR_ONCE_INIT. e.g., 127 static gpr_once once_var = GPR_ONCE_INIT; */ 128 129 /** Ensure that (*init_function)() has been called exactly once (for the 130 specified gpr_once instance) and then return. 131 If multiple threads call gpr_once() on the same gpr_once instance, one of 132 them will call (*init_function)(), and the others will block until that call 133 finishes.*/ 134 GPRAPI void gpr_once_init(gpr_once* once, void (*init_function)(void)); 135 136 /** --- One-time event notification --- 137 138 These operations act on a gpr_event, which should be initialized with 139 gpr_ev_init(), or with GPR_EVENT_INIT if static, e.g., 140 static gpr_event event_var = GPR_EVENT_INIT; 141 It requires no destruction. */ 142 143 /** Initialize *ev. */ 144 GPRAPI void gpr_event_init(gpr_event* ev); 145 146 /** Set *ev so that gpr_event_get() and gpr_event_wait() will return value. 147 Requires: *ev initialized; value != NULL; no prior or concurrent calls to 148 gpr_event_set(ev, ...) since initialization. */ 149 GPRAPI void gpr_event_set(gpr_event* ev, void* value); 150 151 /** Return the value set by gpr_event_set(ev, ...), or NULL if no such call has 152 completed. If the result is non-NULL, all operations that occurred prior to 153 the gpr_event_set(ev, ...) set will be visible after this call returns. 154 Requires: *ev initialized. This operation is faster than acquiring a mutex 155 on most platforms. */ 156 GPRAPI void* gpr_event_get(gpr_event* ev); 157 158 /** Wait until *ev is set by gpr_event_set(ev, ...), or abs_deadline is 159 exceeded, then return gpr_event_get(ev). Requires: *ev initialized. Use 160 abs_deadline==gpr_inf_future for no deadline. When the event has been 161 signalled before the call, this operation is faster than acquiring a mutex 162 on most platforms. */ 163 GPRAPI void* gpr_event_wait(gpr_event* ev, gpr_timespec abs_deadline); 164 165 /** --- Reference counting --- 166 167 These calls act on the type gpr_refcount. It requires no destruction. */ 168 169 /** Initialize *r to value n. */ 170 GPRAPI void gpr_ref_init(gpr_refcount* r, int n); 171 172 /** Increment the reference count *r. Requires *r initialized. */ 173 GPRAPI void gpr_ref(gpr_refcount* r); 174 175 /** Increment the reference count *r. Requires *r initialized. 176 Crashes if refcount is zero */ 177 GPRAPI void gpr_ref_non_zero(gpr_refcount* r); 178 179 /** Increment the reference count *r by n. Requires *r initialized, n > 0. */ 180 GPRAPI void gpr_refn(gpr_refcount* r, int n); 181 182 /** Decrement the reference count *r and return non-zero iff it has reached 183 zero. . Requires *r initialized. */ 184 GPRAPI int gpr_unref(gpr_refcount* r); 185 186 /** Return non-zero iff the reference count of *r is one, and thus is owned 187 by exactly one object. */ 188 GPRAPI int gpr_ref_is_unique(gpr_refcount* r); 189 190 /** --- Stats counters --- 191 192 These calls act on the integral type gpr_stats_counter. It requires no 193 destruction. Static instances may be initialized with 194 gpr_stats_counter c = GPR_STATS_INIT; 195 Beware: These operations do not imply memory barriers. Do not use them to 196 synchronize other events. */ 197 198 /** Initialize *c to the value n. */ 199 GPRAPI void gpr_stats_init(gpr_stats_counter* c, intptr_t n); 200 201 /** *c += inc. Requires: *c initialized. */ 202 GPRAPI void gpr_stats_inc(gpr_stats_counter* c, intptr_t inc); 203 204 /** Return *c. Requires: *c initialized. */ 205 GPRAPI intptr_t gpr_stats_read(const gpr_stats_counter* c); 206 207 /** ==================Example use of interface=================== 208 A producer-consumer queue of up to N integers, 209 illustrating the use of the calls in this interface. */ 210 #if 0 211 212 #define N 4 213 214 typedef struct queue { 215 gpr_cv non_empty; /* Signalled when length becomes non-zero. */ 216 gpr_cv non_full; /* Signalled when length becomes non-N. */ 217 gpr_mu mu; /* Protects all fields below. 218 (That is, except during initialization or 219 destruction, the fields below should be accessed 220 only by a thread that holds mu.) */ 221 int head; /* Index of head of queue 0..N-1. */ 222 int length; /* Number of valid elements in queue 0..N. */ 223 int elem[N]; /* elem[head .. head+length-1] are queue elements. */ 224 } queue; 225 226 /* Initialize *q. */ 227 void queue_init(queue *q) { 228 gpr_mu_init(&q->mu); 229 gpr_cv_init(&q->non_empty); 230 gpr_cv_init(&q->non_full); 231 q->head = 0; 232 q->length = 0; 233 } 234 235 /* Free storage associated with *q. */ 236 void queue_destroy(queue *q) { 237 gpr_mu_destroy(&q->mu); 238 gpr_cv_destroy(&q->non_empty); 239 gpr_cv_destroy(&q->non_full); 240 } 241 242 /* Wait until there is room in *q, then append x to *q. */ 243 void queue_append(queue *q, int x) { 244 gpr_mu_lock(&q->mu); 245 /* To wait for a predicate without a deadline, loop on the negation of the 246 predicate, and use gpr_cv_wait(..., gpr_inf_future) inside the loop 247 to release the lock, wait, and reacquire on each iteration. Code that 248 makes the condition true should use gpr_cv_broadcast() on the 249 corresponding condition variable. The predicate must be on state 250 protected by the lock. */ 251 while (q->length == N) { 252 gpr_cv_wait(&q->non_full, &q->mu, gpr_inf_future); 253 } 254 if (q->length == 0) { /* Wake threads blocked in queue_remove(). */ 255 /* It's normal to use gpr_cv_broadcast() or gpr_signal() while 256 holding the lock. */ 257 gpr_cv_broadcast(&q->non_empty); 258 } 259 q->elem[(q->head + q->length) % N] = x; 260 q->length++; 261 gpr_mu_unlock(&q->mu); 262 } 263 264 /* If it can be done without blocking, append x to *q and return non-zero. 265 Otherwise return 0. */ 266 int queue_try_append(queue *q, int x) { 267 int result = 0; 268 if (gpr_mu_trylock(&q->mu)) { 269 if (q->length != N) { 270 if (q->length == 0) { /* Wake threads blocked in queue_remove(). */ 271 gpr_cv_broadcast(&q->non_empty); 272 } 273 q->elem[(q->head + q->length) % N] = x; 274 q->length++; 275 result = 1; 276 } 277 gpr_mu_unlock(&q->mu); 278 } 279 return result; 280 } 281 282 /* Wait until the *q is non-empty or deadline abs_deadline passes. If the 283 queue is non-empty, remove its head entry, place it in *head, and return 284 non-zero. Otherwise return 0. */ 285 int queue_remove(queue *q, int *head, gpr_timespec abs_deadline) { 286 int result = 0; 287 gpr_mu_lock(&q->mu); 288 /* To wait for a predicate with a deadline, loop on the negation of the 289 predicate or until gpr_cv_wait() returns true. Code that makes 290 the condition true should use gpr_cv_broadcast() on the corresponding 291 condition variable. The predicate must be on state protected by the 292 lock. */ 293 while (q->length == 0 && 294 !gpr_cv_wait(&q->non_empty, &q->mu, abs_deadline)) { 295 } 296 if (q->length != 0) { /* Queue is non-empty. */ 297 result = 1; 298 if (q->length == N) { /* Wake threads blocked in queue_append(). */ 299 gpr_cv_broadcast(&q->non_full); 300 } 301 *head = q->elem[q->head]; 302 q->head = (q->head + 1) % N; 303 q->length--; 304 } /* else deadline exceeded */ 305 gpr_mu_unlock(&q->mu); 306 return result; 307 } 308 #endif /* 0 */ 309 310 #ifdef __cplusplus 311 } // extern "C" 312 #endif 313 314 #endif /* GRPC_SUPPORT_SYNC_H */ 315