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Allow ConditionVariable[PrepareTo]Sleep to auto-switch between CVs.

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The original coding here insisted that callers manually cancel any prepared
sleep for one condition variable before starting a sleep on another one.
While that's not a huge burden today, it seems like a gotcha that will bite
us in future if the use of condition variables increases; anything we can
do to make the use of this API simpler and more robust is attractive.
Hence, allow these functions to automatically switch their attention to
a different CV when required.  This is safe for the same reason it was OK
for commit aced5a92b to let a broadcast operation cancel any prepared CV
sleep: whenever we return to the other test-and-sleep loop, we will
automatically re-prepare that CV, paying at most an extra test of that
loop's exit condition.

Back-patch to v10 where condition variables were introduced.  Ordinarily
we would probably not back-patch a change like this, but since it does not
invalidate any coding pattern that was legal before, it seems safe enough.
Furthermore, there's an open bug in replorigin_drop() for which the
simplest fix requires this.  Even if we chose to fix that in some more
complicated way, the hazard would remain that we might back-patch some
other bug fix that requires this behavior.

Patch by me, reviewed by Thomas Munro.

Discussion: https://postgr.es/m/2437.1515368316@sss.pgh.pa.us
This commit is contained in:
Tom Lane 2018-01-09 11:39:10 -05:00
parent 921059bd66
commit 13db3b9363
2 changed files with 15 additions and 15 deletions
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26
src/backend/storage/lmgr/condition_variable.c
View File

@ -55,10 +55,6 @@ ConditionVariableInit(ConditionVariable *cv)
* condition between calling ConditionVariablePrepareToSleep and calling
* ConditionVariableSleep. If that is inconvenient, omit calling
* ConditionVariablePrepareToSleep.
*
* Only one condition variable can be used at a time, ie,
* ConditionVariableCancelSleep must be called before any attempt is made
* to sleep on a different condition variable.
*/
void
ConditionVariablePrepareToSleep(ConditionVariable *cv)
@ -81,10 +77,15 @@ ConditionVariablePrepareToSleep(ConditionVariable *cv)
}
/*
* It's not legal to prepare a sleep until the previous sleep has been
* completed or canceled.
* If some other sleep is already prepared, cancel it; this is necessary
* because we have just one static variable tracking the prepared sleep,
* and also only one cvWaitLink in our PGPROC. It's okay to do this
* because whenever control does return to the other test-and-sleep loop,
* its ConditionVariableSleep call will just re-establish that sleep as
* the prepared one.
*/
Assert(cv_sleep_target == NULL);
if (cv_sleep_target != NULL)
ConditionVariableCancelSleep();
/* Record the condition variable on which we will sleep. */
cv_sleep_target = cv;
@ -133,16 +134,16 @@ ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
* is recommended because it avoids manipulations of the wait list, or not
* met initially, in which case preparing first is better because it
* avoids one extra test of the exit condition.
*
* If we are currently prepared to sleep on some other CV, we just cancel
* that and prepare this one; see ConditionVariablePrepareToSleep.
*/
if (cv_sleep_target == NULL)
if (cv_sleep_target != cv)
{
ConditionVariablePrepareToSleep(cv);
return;
}
/* Any earlier condition variable sleep must have been canceled. */
Assert(cv_sleep_target == cv);
do
{
CHECK_FOR_INTERRUPTS();
@ -265,7 +266,8 @@ ConditionVariableBroadcast(ConditionVariable *cv)
* prepared CV sleep. The next call to ConditionVariableSleep will take
* care of re-establishing the lost state.
*/
ConditionVariableCancelSleep();
if (cv_sleep_target != NULL)
ConditionVariableCancelSleep();
/*
* Inspect the state of the queue. If it's empty, we have nothing to do.

4
src/include/storage/condition_variable.h
View File

@ -40,9 +40,7 @@ extern void ConditionVariableInit(ConditionVariable *cv);
* ConditionVariableSleep. Spurious wakeups are possible, but should be
* infrequent. After exiting the loop, ConditionVariableCancelSleep must
* be called to ensure that the process is no longer in the wait list for
* the condition variable. Only one condition variable can be used at a
* time, ie, ConditionVariableCancelSleep must be called before any attempt
* is made to sleep on a different condition variable.
* the condition variable.
*/
extern void ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info);
extern void ConditionVariableCancelSleep(void);