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The lwlock scalability work introduced two race conditions into the lwlock variable support provided for xlog.c. First, and harmlessly on most platforms, it set/read the variable without the spinlock in some places. Secondly, due to the removal of the spinlock, it was possible that a backend missed changes to the variable's state if it changed in the wrong moment because checking the lock's state, the variable's state and the queuing are not protected by a single spinlock acquisition anymore. To fix first move resetting the variable's from LWLockAcquireWithVar to WALInsertLockRelease, via a new function LWLockReleaseClearVar. That prevents issues around waiting for a variable's value to change when a new locker has acquired the lock, but not yet set the value. Secondly re-check that the variable hasn't changed after enqueing, that prevents the issue that the lock has been released and already re-acquired by the time the woken up backend checks for the lock's state. Reported-By: Jeff Janes Analyzed-By: Heikki Linnakangas Reviewed-By: Heikki Linnakangas Discussion: 5592DB35.2060401@iki.fi Backpatch: 9.5, where the lwlock scalability went in
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