Btrfs: Check for a blocking lock before taking the spin

This reduces contention on the extent buffer spin locks by testing for a
blocking lock before trying to take the spinlock.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/locking.c

@@ -71,12 +71,13 @@ void btrfs_clear_lock_blocking(struct extent_buffer *eb)
 static int btrfs_spin_on_block(struct extent_buffer *eb)
 {
 	int i;
+
 	for (i = 0; i < 512; i++) {
-		cpu_relax();
 		if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 			return 1;
 		if (need_resched())
 			break;
+		cpu_relax();
 	}
 	return 0;
 }
@@ -102,6 +103,7 @@ int btrfs_try_spin_lock(struct extent_buffer *eb)
 
 	/* spin for a bit on the BLOCKING flag */
 	for (i = 0; i < 2; i++) {
+		cpu_relax();
 		if (!btrfs_spin_on_block(eb))
 			break;
 
@@ -148,6 +150,9 @@ int btrfs_tree_lock(struct extent_buffer *eb)
 	DEFINE_WAIT(wait);
 	wait.func = btrfs_wake_function;
 
+	if (!btrfs_spin_on_block(eb))
+		goto sleep;
+
 	while(1) {
 		spin_nested(eb);
 
@@ -165,9 +170,10 @@ int btrfs_tree_lock(struct extent_buffer *eb)
 		 * spin for a bit, and if the blocking flag goes away,
 		 * loop around
 		 */
+		cpu_relax();
 		if (btrfs_spin_on_block(eb))
 			continue;
-
+sleep:
 		prepare_to_wait_exclusive(&eb->lock_wq, &wait,
 					  TASK_UNINTERRUPTIBLE);