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Merge "Clarify that Condition::wait() can spuriously wake up" am: 514e0aa2e2

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am: 675e05c48a

Change-Id: Ib6107e4f31dbf45984e4e318e5a115a8567a358b
This commit is contained in:
Tom Cherry 2017-03-06 19:56:36 +00:00 • committed by android-build-merger
commit df8fc09716
1 changed files with 7 additions and 12 deletions
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19
libutils/include/utils/Condition.h
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@ -41,6 +41,11 @@ namespace android {
* call wait(), then either re-wait() if things aren't quite what you want, * call wait(), then either re-wait() if things aren't quite what you want,
* or unlock the mutex and continue. All threads calling wait() must * or unlock the mutex and continue. All threads calling wait() must
* use the same mutex for a given Condition. * use the same mutex for a given Condition.
*
* On Android and Apple platforms, these are implemented as a simple wrapper
* around pthread condition variables. Care must be taken to abide by
* the pthreads semantics, in particular, a boolean predicate must
* be re-evaluated after a wake-up, as spurious wake-ups may happen.
*/ */
class Condition { class Condition {
public: public:
@ -58,10 +63,11 @@ public:
explicit Condition(int type); explicit Condition(int type);
~Condition(); ~Condition();
// Wait on the condition variable. Lock the mutex before calling. // Wait on the condition variable. Lock the mutex before calling.
// Note that spurious wake-ups may happen.
status_t wait(Mutex& mutex); status_t wait(Mutex& mutex);
// same with relative timeout // same with relative timeout
status_t waitRelative(Mutex& mutex, nsecs_t reltime); status_t waitRelative(Mutex& mutex, nsecs_t reltime);
// Signal the condition variable, allowing exactly one thread to continue. // Signal the condition variable, allowing one thread to continue.
void signal(); void signal();
// Signal the condition variable, allowing one or all threads to continue. // Signal the condition variable, allowing one or all threads to continue.
void signal(WakeUpType type) { void signal(WakeUpType type) {
@ -142,17 +148,6 @@ inline status_t Condition::waitRelative(Mutex& mutex, nsecs_t reltime) {
return -pthread_cond_timedwait(&mCond, &mutex.mMutex, &ts); return -pthread_cond_timedwait(&mCond, &mutex.mMutex, &ts);
} }
inline void Condition::signal() { inline void Condition::signal() {
/*
* POSIX says pthread_cond_signal wakes up "one or more" waiting threads.
* However bionic follows the glibc guarantee which wakes up "exactly one"
* waiting thread.
*
* man 3 pthread_cond_signal
* pthread_cond_signal restarts one of the threads that are waiting on
* the condition variable cond. If no threads are waiting on cond,
* nothing happens. If several threads are waiting on cond, exactly one
* is restarted, but it is not specified which.
*/
pthread_cond_signal(&mCond); pthread_cond_signal(&mCond);
} }
inline void Condition::broadcast() { inline void Condition::broadcast() {