[Cluster-devel] [PATCH v3 07/13] locks: avoid taking global lock if possible when waking up blocked waiters

[Jeff Layton <jlayton@redhat.com>]

Since we always hold the i_lock when inserting a new waiter onto the
fl_block list, we can avoid taking the global lock at all if we find
that it's empty when we go to wake up blocked waiters.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
 fs/locks.c |   17 ++++++++++++++---
 1 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/locks.c b/fs/locks.c
index 8f56651..a8f3b33 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -532,7 +532,10 @@ static void locks_delete_block(struct file_lock *waiter)
  * the order they blocked. The documentation doesn't require this but
  * it seems like the reasonable thing to do.
  *
- * Must be called with file_lock_lock held!
+ * Must be called with both the i_lock and file_lock_lock held. The fl_block
+ * list itself is protected by the file_lock_list, but by ensuring that the
+ * i_lock is also held on insertions we can avoid taking the file_lock_lock
+ * in some cases when we see that the fl_block list is empty.
  */
 static void __locks_insert_block(struct file_lock *blocker,
 					struct file_lock *waiter)
@@ -560,8 +563,16 @@ static void locks_insert_block(struct file_lock *blocker,
  */
 static void locks_wake_up_blocks(struct file_lock *blocker)
 {
+	/*
+	 * Avoid taking global lock if list is empty. This is safe since new
+	 * blocked requests are only added to the list under the i_lock, and
+	 * the i_lock is always held here.
+	 */
+	if (list_empty(&blocker->fl_block))
+		return;
+
 	spin_lock(&file_lock_lock);
-	while (!list_empty(&blocker->fl_block)) {
+	do {
 		struct file_lock *waiter;
 
 		waiter = list_first_entry(&blocker->fl_block,
@@ -571,7 +582,7 @@ static void locks_wake_up_blocks(struct file_lock *blocker)
 			waiter->fl_lmops->lm_notify(waiter);
 		else
 			wake_up(&waiter->fl_wait);
-	}
+	} while (!list_empty(&blocker->fl_block));
 	spin_unlock(&file_lock_lock);
 }
 
-- 
1.7.1