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Clarify that Condition::wait() can spuriously wake up 

Also, remove the clarification that Condition::signal() wakes exactly
one thread as in the presence of spurious wake ups, this clarification
does not provide a safe guarantee to developers.

Bug: 34592766
Test: Build
Change-Id: I34df02e44a70a18fe04ceda858d002ef129c1fd9
This commit is contained in:
Tom Cherry 2017-02-27 17:02:02 -08:00
parent 7a5d535cd2
commit f1147f7dde
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,
* or unlock the mutex and continue. All threads calling wait() must
* 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 {
public:
@ -58,10 +63,11 @@ public:
explicit Condition(int type);
~Condition();
// Wait on the condition variable. Lock the mutex before calling.
// Note that spurious wake-ups may happen.
status_t wait(Mutex& mutex);
// same with relative timeout
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();
// Signal the condition variable, allowing one or all threads to continue.
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);
}
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);
}
inline void Condition::broadcast() {