Subject: can while loop in ClockSweepTick function be kind of infinite loop in some cases?

From: 斯波隼斗 <[email-masked]>
Date: Wed, 11 Jan 2023 01:25:06 +0900

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.

Thank you in advance
Hayato Shiba