xfs: reduce the number of atomic when locking a buffer after lookup

Avoid an extra atomic operation in the non-trylock case by only
doing a trylock if the XBF_TRYLOCK flag is set. This follows the
pattern in the IO path with NOWAIT semantics where the
"trylock-fail-lock" path showed 5-10% reduced throughput compared to
just using single lock call when not under NOWAIT conditions. So
make that same change here, too.

See commit 942491c9 ("xfs: fix AIM7 regression") for details.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
[hch: split from a larger patch]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/xfs_buf.c

@@ -534,11 +534,12 @@ xfs_buf_find_lock(
 	struct xfs_buf          *bp,
 	xfs_buf_flags_t		flags)
 {
-	if (!xfs_buf_trylock(bp)) {
-		if (flags & XBF_TRYLOCK) {
+	if (flags & XBF_TRYLOCK) {
+		if (!xfs_buf_trylock(bp)) {
 			XFS_STATS_INC(bp->b_mount, xb_busy_locked);
 			return -EAGAIN;
 		}
+	} else {
 		xfs_buf_lock(bp);
 		XFS_STATS_INC(bp->b_mount, xb_get_locked_waited);
 	}