Move memory barrier in UnlockBufHdr to before releasing the lock.

anarazel · Alexander Korotkov · commit 5a24efd5b3c0 · 2016-07-11T11:31:21.000+03:00
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
@@ -230,8 +230,8 @@ typedef union BufferDescPadded
 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)
 
 
