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c0af4eb4e7
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