btrfs: Document a mutex lock/unlock sequence

1 files changed, 12 insertions, 0 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index a160f11465f8..fba1348cb2a0 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -756,6 +756,10 @@ again:
 
                         btrfs_release_path(path);
 
+                        /*
+                         * Mutex was contended, block until it's released and try
+                         * again
+                         */
                         mutex_lock(&head->mutex);
                         mutex_unlock(&head->mutex);
                         btrfs_put_delayed_ref(&head->node);
@@ -2297,6 +2301,10 @@ again:
                                 atomic_inc(&ref->refs);
 
                                 spin_unlock(&delayed_refs->lock);
+                                /*
+                                 * Mutex was contended, block until it's
+                                 * released and try again
+                                 */
                                 mutex_lock(&head->mutex);
                                 mutex_unlock(&head->mutex);
 
@@ -2363,6 +2371,10 @@ static noinline int check_delayed_ref(struct btrfs_trans_handle *trans,
 
                 btrfs_release_path(path);
 
+                /*
+                 * Mutex was contended, block until it's released and let
+                 * caller try again
+                 */
                 mutex_lock(&head->mutex);
                 mutex_unlock(&head->mutex);
                 btrfs_put_delayed_ref(&head->node);