bufmgr: Use atomic sub for unpinning buffers

The prior commit made it legal to modify BufferDesc.state while the buffer
header spinlock is held. This allows us to replace the CAS loop
inUnpinBufferNoOwner() with an atomic sub. This improves scalability
significantly. See the prior commits for more background.

Reviewed-by: Matthias van de Meent <[email protected]>
Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff

Author: anarazel

src/backend/storage/buffer/bufmgr.c

@@ -3267,7 +3267,6 @@ UnpinBufferNoOwner(BufferDesc *buf)
 	ref->refcount--;
 	if (ref->refcount == 0)
 	{
-		uint32		buf_state;
 		uint32		old_buf_state;
 
 		/*
@@ -3285,33 +3284,11 @@ UnpinBufferNoOwner(BufferDesc *buf)
 		 */
 		Assert(!LWLockHeldByMe(BufferDescriptorGetContentLock(buf)));
 
-		/*
-		 * Decrement the shared reference count.
-		 *
-		 * Since buffer spinlock holder can update status using just write,
-		 * it's not safe to use atomic decrement here; thus use a CAS loop.
-		 *
-		 * TODO: The above requirement does not hold anymore, in a future
-		 * commit this will be rewritten to release the pin in a single atomic
-		 * operation.
-		 */
-		old_buf_state = pg_atomic_read_u32(&buf->state);
-		for (;;)
-		{
-			if (old_buf_state & BM_LOCKED)
-				old_buf_state = WaitBufHdrUnlocked(buf);
-
-			buf_state = old_buf_state;
-
-			buf_state -= BUF_REFCOUNT_ONE;
-
-			if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
-											   buf_state))
-				break;
-		}
+		/* decrement the shared reference count */
+		old_buf_state = pg_atomic_fetch_sub_u32(&buf->state, BUF_REFCOUNT_ONE);
 
 		/* Support LockBufferForCleanup() */
-		if (buf_state & BM_PIN_COUNT_WAITER)
+		if (old_buf_state & BM_PIN_COUNT_WAITER)
 			WakePinCountWaiter(buf);
 
 		ForgetPrivateRefCountEntry(ref);