I wrote:
> Actually ... perhaps a better design would be to have
> ConditionVariable[PrepareTo]Sleep auto-cancel any prepared sleep for
> a different condition variable, analogously to what we just did in
> ConditionVariableBroadcast, on the same theory that whenever control
> returns to the other CV wait loop it can re-establish the relevant
> state easily enough. I have to think that if the use of CVs grows
> much, the existing restriction is going to become untenable anyway,
> so why not just get rid of it?
Concretely, as per attached.
regards, tom lane
diff --git a/src/backend/storage/lmgr/condition_variable.c b/src/backend/storage/lmgr/condition_variable.c
index 0b9d676..b01864c 100644
--- a/src/backend/storage/lmgr/condition_variable.c
+++ b/src/backend/storage/lmgr/condition_variable.c
@@ -50,10 +50,6 @@ ConditionVariableInit(ConditionVariable *cv)
* However, if the first test of the exit condition is likely to succeed,
* it's more efficient to omit the ConditionVariablePrepareToSleep call.
* See comments in ConditionVariableSleep for more detail.
- *
- * 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)
@@ -76,10 +72,14 @@ 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.
+ * 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;
@@ -128,16 +128,16 @@ ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
* 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();
diff --git a/src/include/storage/condition_variable.h b/src/include/storage/condition_variable.h
index 7dac477..32e645c 100644
--- a/src/include/storage/condition_variable.h
+++ b/src/include/storage/condition_variable.h
@@ -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);
