On 06/17/2014 03:17 PM, Andres Freund wrote:
LWLockWaitForVar() doesn't set releaseOk to true when waiting again. Isn't that a bug?
LWLockWaitForVar() waits in LW_WAIT_UNTIL_FREE mode, because it's not interested in acquiring the lock, it just wants to be woken up when it's released (or the "var" is updated). LWLockRelease doesn't clear releaseOK when it wakes up a LW_WAIT_UNTIL_FREE-mode waiter.
What if there's another locker coming in after LWLockWaitForVar() returns from the PGSemaphoreLock() but before it has acquire the spinlock? Now, it might be that it's unproblematic because of hte specific way these locks are used right now, but it doesn't seem like a good idea to leave it that way.
In that scenario, LWLockWaitForVar() will grab the spinlock, after the other process. What happens next depends on the whether the value of the variable it guards was changed. If it was, LWLockWaitForVar() will see that it changed, and return false without waiting again. If the value didn't change, it will sleep until the new locker releases the lock. In either case, I don't see a problem with releaseOK. It seems correct as it is.
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