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bufmgr: Fix ordering of checks in PinBuffer()

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The check for skip_if_not_valid added in 819dc118c0 was put at the start of
the loop. A CAS loop in theory does allow to make that check in a race free
manner. However, just after the check, there's a
    old_buf_state = WaitBufHdrUnlocked(buf);
which introduces a race, because it would allow BM_VALID to be cleared, after
the skip_if_not_valid check.

Fix by restarting the loop after WaitBufHdrUnlocked().

Reported-by: Yura Sokolov <y.sokolov@postgrespro.ru>
Discussion: https://postgr.es/m/5bf667f3-5270-4b19-a08f-0facbecdff68@postgrespro.ru
This commit is contained in:
Andres Freund 2026-03-31 19:24:58 -04:00
parent 273d26b75e
commit c0af4eb4e7
1 changed files with 6 additions and 1 deletions
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7
src/backend/storage/buffer/bufmgr.c
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@ -3281,9 +3281,14 @@ PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy,
* We're not allowed to increase the refcount while the buffer
* header spinlock is held. Wait for the lock to be released.
*/
if (old_buf_state & BM_LOCKED)
if (unlikely(old_buf_state & BM_LOCKED))
{
old_buf_state = WaitBufHdrUnlocked(buf);
/* perform checks at the top of the loop again */
continue;
}
buf_state = old_buf_state;
/* increase refcount */