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Commit5ffb7c7

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De-pessimize ConditionVariableCancelSleep().
Commitb91dd9d was concerned with a theoretical problem with ournon-atomic condition variable operations. If you stop sleeping, andthen cancel the sleep in a separate step, you might be signaled inbetween, and that could be lost. That doesn't matter for callers ofConditionVariableBroadcast(), but callers of ConditionVariableSignal()might be upset if a signal went missing like this.Commitbc971f4 interacted badly with that logic, because it doesn'tuse ConditionVariableSleep(), which would normally put us back in thewait list. ConditionVariableCancelSleep() would be confused and thinkwe'd received an extra signal, and try to forward it to another backend,resulting in wakeup storms.New idea: ConditionVariableCancelSleep() can just return true if we'vebeen signaled. Hypothetical users of ConditionVariableSignal() wouldthen still have a way to deal with rare lost signals if they areconcerned about that problem.Back-patch to 16, wherebc971f4 arrived.Reported-by: Tomas Vondra <tomas.vondra@enterprisedb.com>Reviewed-by: Andres Freund <andres@anarazel.de>Discussion:https://postgr.es/m/2840876b-4cfe-240f-0a7e-29ffd66711e7%40enterprisedb.com
1 parent82a4eda commit5ffb7c7

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2 files changed

+7
-11
lines changed

2 files changed

+7
-11
lines changed

‎src/backend/storage/lmgr/condition_variable.c

Lines changed: 6 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -223,15 +223,17 @@ ConditionVariableTimedSleep(ConditionVariable *cv, long timeout,
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*
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* Do nothing if nothing is pending; this allows this function to be called
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* during transaction abort to clean up any unfinished CV sleep.
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*
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* Return true if we've been signaled.
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*/
227-
void
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bool
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ConditionVariableCancelSleep(void)
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{
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ConditionVariable*cv=cv_sleep_target;
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boolsignaled= false;
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if (cv==NULL)
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return;
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return false;
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SpinLockAcquire(&cv->mutex);
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if (proclist_contains(&cv->wakeup,MyProc->pgprocno,cvWaitLink))
@@ -240,15 +242,9 @@ ConditionVariableCancelSleep(void)
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signaled= true;
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SpinLockRelease(&cv->mutex);
242244

243-
/*
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* If we've received a signal, pass it on to another waiting process, if
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* there is one. Otherwise a call to ConditionVariableSignal() might get
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* lost, despite there being another process ready to handle it.
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*/
248-
if (signaled)
249-
ConditionVariableSignal(cv);
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cv_sleep_target=NULL;
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returnsignaled;
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}
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/*

‎src/include/storage/condition_variable.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -56,7 +56,7 @@ extern void ConditionVariableInit(ConditionVariable *cv);
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externvoidConditionVariableSleep(ConditionVariable*cv,uint32wait_event_info);
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externboolConditionVariableTimedSleep(ConditionVariable*cv,longtimeout,
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uint32wait_event_info);
59-
externvoidConditionVariableCancelSleep(void);
59+
externboolConditionVariableCancelSleep(void);
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/*
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* Optionally, ConditionVariablePrepareToSleep can be called before entering

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