fs: buffer: do not use unnecessary atomic operations when discarding buffers

Discarding buffers uses a bunch of atomic operations when discarding
buffers because ......  I can't think of a reason.  Use a cmpxchg loop to
clear all the necessary flags.  In most (all?) cases this will be a single
atomic operations.

[[email protected]: move BUFFER_FLAGS_DISCARD into the .c file]
Signed-off-by: Mel Gorman <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/buffer.c

@@ -1483,16 +1483,27 @@ EXPORT_SYMBOL(set_bh_page);
 /*
  * Called when truncating a buffer on a page completely.
  */
+
+/* Bits that are cleared during an invalidate */
+#define BUFFER_FLAGS_DISCARD \
+	(1 << BH_Mapped | 1 << BH_New | 1 << BH_Req | \
+	 1 << BH_Delay | 1 << BH_Unwritten)
+
 static void discard_buffer(struct buffer_head * bh)
 {
+	unsigned long b_state, b_state_old;
+
 	lock_buffer(bh);
 	clear_buffer_dirty(bh);
 	bh->b_bdev = NULL;
-	clear_buffer_mapped(bh);
-	clear_buffer_req(bh);
-	clear_buffer_new(bh);
-	clear_buffer_delay(bh);
-	clear_buffer_unwritten(bh);
+	b_state = bh->b_state;
+	for (;;) {
+		b_state_old = cmpxchg(&bh->b_state, b_state,
+				      (b_state & ~BUFFER_FLAGS_DISCARD));
+		if (b_state_old == b_state)
+			break;
+		b_state = b_state_old;
+	}
 	unlock_buffer(bh);
 }