Btrfs: reduce calls to wake_up on uncontended locks

The btrfs locks were unconditionally calling wake_up as the
locks were released.  This lead to extra thrashing on the waitqueue,
especially for locks that were dominated by readers.

Signed-off-by: Chris Mason <chris.mason@fusionio.com>

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index 272f911..a44eff0 100644 (file)
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -78,13 +78,15 @@ void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
                write_lock(&eb->lock);
                WARN_ON(atomic_read(&eb->spinning_writers));
                atomic_inc(&eb->spinning_writers);
-               if (atomic_dec_and_test(&eb->blocking_writers))
+               if (atomic_dec_and_test(&eb->blocking_writers) &&
+                   waitqueue_active(&eb->write_lock_wq))
                        wake_up(&eb->write_lock_wq);
        } else if (rw == BTRFS_READ_LOCK_BLOCKING) {
                BUG_ON(atomic_read(&eb->blocking_readers) == 0);
                read_lock(&eb->lock);
                atomic_inc(&eb->spinning_readers);
-               if (atomic_dec_and_test(&eb->blocking_readers))
+               if (atomic_dec_and_test(&eb->blocking_readers) &&
+                   waitqueue_active(&eb->read_lock_wq))
                        wake_up(&eb->read_lock_wq);
        }
        return;
@@ -199,7 +201,8 @@ void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
        }
        btrfs_assert_tree_read_locked(eb);
        WARN_ON(atomic_read(&eb->blocking_readers) == 0);
-       if (atomic_dec_and_test(&eb->blocking_readers))
+       if (atomic_dec_and_test(&eb->blocking_readers) &&
+           waitqueue_active(&eb->read_lock_wq))
                wake_up(&eb->read_lock_wq);
        atomic_dec(&eb->read_locks);
 }
@@ -247,8 +250,9 @@ void btrfs_tree_unlock(struct extent_buffer *eb)
        if (blockers) {
                WARN_ON(atomic_read(&eb->spinning_writers));
                atomic_dec(&eb->blocking_writers);
-               smp_wmb();
-               wake_up(&eb->write_lock_wq);
+               smp_mb();
+               if (waitqueue_active(&eb->write_lock_wq))
+                       wake_up(&eb->write_lock_wq);
        } else {
                WARN_ON(atomic_read(&eb->spinning_writers) != 1);
                atomic_dec(&eb->spinning_writers);