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In LWLockRelease() (and in 9.4+ LWLockUpdateVar()) we release enqueued waiters using PGSemaphoreUnlock(). As there are other sources of such unlocks backends only wake up if MyProc->lwWaiting is set to false; which is only done in the aforementioned functions. Before this commit there were dangers because the store to lwWaitLink could become visible before the store to lwWaitLink. This could both happen due to compiler reordering (on most compilers) and on some platforms due to the CPU reordering stores. The possible consequence of this is that a backend stops waiting before lwWaitLink is set to NULL. If that backend then tries to acquire another lock and has to wait there the list could become corrupted once the lwWaitLink store is finally performed. Add a write memory barrier to prevent that issue. Unfortunately the barrier support has been only added in 9.2. Given that the issue has not knowingly been observed in praxis it seems sufficient to prohibit compiler reordering using volatile for 9.0 and 9.1. Actual problems due to compiler reordering are more likely anyway. Discussion: 20140210134625.GA15246@awork2.anarazel.de |
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