Move memory barrier in UnlockBufHdr to before releasing the lock.

anarazel · anarazel · commit 87a3023c60f4 · 2016-05-30T15:35:53.000-07:00
This bug appears to have been introduced late in the development of 4835458 ("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
@@ -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)
 
 
