postgres.git / summary / log / commit / refs

commit    c0af4eb4e71ecd4826ae6c9985f1d10ace5e1be6
Author:   Andres Freund <andres@anarazel.de>
Date:     Tue Mar 31 23:24:58 2026 +0000

    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 <y.sokolov@postgrespro.ru>
    Discussion: https://postgr.es/m/5bf667f3-5270-4b19-a08f-0facbecdff68@postgrespro.ru


 src/backend/storage/buffer/bufmgr.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

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 */


[parent: 273d26b75e78]