Hi, On 2023-01-11 01:25:06 +0900, 斯波隼斗 wrote: > This question is about ClockSweepTick function and the code is below. > https://github.com/postgres/postgres/blob/24d2b2680a8d0e01b30ce8a41c4eb3b47aca5031/src/backend/storage/buffer/freelist.c#L146-L165 > > The value of expected, NBuffers, wrapped variable is fixed in the while > loop, so that when the value of expected variable is not equal to > StrategyControl->nextVictimBuffer, CAS operation fails and the while loop > will be run kind-of infinitely. > It is possible for this problem to occur when ClockSweepTick function is > concurrently called and nextVictimBuffer is incremented by other process > before CAS operation in the loop (ex: in this case, the value of expected > variable is NBuffers+1 while the value of nextVictimBuffer variable is > NBuffers+2. so CAS operation fails) > I think. `expected = originalVictim + 1;` line should be in while loop > (before acquiring spin lock) so that, even in the case above, expected > variable is incremented for each loop and CAS operation will be successful > at some point. > Is my understanding correct? If so, I will send PR for fixing this issue.
Yes, I think your understanding might be correct. Interesting that this apparently has never occurred. Yes, please send a patch. Thanks, Andres
