Move memory barrier in UnlockBufHdr to before releasing the lock.

This bug appears to have been introduced late in the development of
48354581a4 ("Allow Pin/UnpinBuffer to operate in a lockfree
manner.").

Found while debugging a bug which turned out to be independent of the
commit mentioned above.

Backpatch: -

diff --git a/src/include/storage/buf_internals.h b/src/include/storage/buf_internals.h
index c9cae956a541e40deaba299b854553a4cc8562e1..511d7409e6ef850794aaa09d47faf732daf26000 100644 (file)
--- a/src/include/storage/buf_internals.h
+++ b/src/include/storage/buf_internals.h
@@ -242,8 +242,8 @@ extern PGDLLIMPORT LWLockMinimallyPadded *BufferIOLWLockArray;
 extern uint32 LockBufHdr(BufferDesc *desc);
 #define UnlockBufHdr(desc, s)  \
        do {    \
-               pg_atomic_write_u32(&(desc)->state, (s) & (~BM_LOCKED)); \
                pg_write_barrier(); \
+               pg_atomic_write_u32(&(desc)->state, (s) & (~BM_LOCKED)); \
        } while (0)