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: Chris Mason <chris.mason@oracle.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index 47b0a88c12a2..6d8db2f5c38d 100644
--- a/fs/btrfs/locking.c
+++ b/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);