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>

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index 47b0a88c12a23a6d983eff4c3ff0e4bdf1347d2e..6d8db2f5c38d0d485199120f5c170492099eb54d 100644 (file)
--- 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);