From: Andres Freund Date: Tue, 31 Mar 2026 23:24:58 +0000 (-0400) Subject: bufmgr: Fix ordering of checks in PinBuffer() X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=c0af4eb4e71ecd4826ae6c9985f1d10ace5e1be6;p=thirdparty%2Fpostgresql.git bufmgr: Fix ordering of checks in PinBuffer() The check for skip_if_not_valid added in 819dc118c0f6 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 Discussion: https://postgr.es/m/5bf667f3-5270-4b19-a08f-0facbecdff68@postgrespro.ru --- diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index cd21ae3fc36..17499451ad2 100644 --- a/src/backend/storage/buffer/bufmgr.c +++ b/src/backend/storage/buffer/bufmgr.c @@ -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 */