Btrfs: fix async worker startup race

After a new worker thread starts, it is placed into the
list of idle threads.  But, this may race with a
check for idle done by the worker thread itself, resulting
in a double list_add operation.

This fix adds a check to make sure the idle thread addition
is done properly.

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

1 files changed, 11 insertions, 3 deletions

diff --git a/fs/btrfs/async-thread.c b/fs/btrfs/async-thread.c
index 6ea5cd0a595f..73df627ab8ab 100644
--- a/fs/btrfs/async-thread.c
+++ b/fs/btrfs/async-thread.c
@@ -74,7 +74,12 @@ static void check_idle_worker(struct btrfs_worker_thread *worker)
                 unsigned long flags;
                 spin_lock_irqsave(&worker->workers->lock, flags);
                 worker->idle = 1;
-                list_move(&worker->worker_list, &worker->workers->idle_list);
+
+                /* the list may be empty if the worker is just starting */
+                if (!list_empty(&worker->worker_list)) {
+                        list_move(&worker->worker_list,
+                                 &worker->workers->idle_list);
+                }
                 spin_unlock_irqrestore(&worker->workers->lock, flags);
         }
 }
@@ -90,8 +95,11 @@ static void check_busy_worker(struct btrfs_worker_thread *worker)
                 unsigned long flags;
                 spin_lock_irqsave(&worker->workers->lock, flags);
                 worker->idle = 0;
-                list_move_tail(&worker->worker_list,
-                               &worker->workers->worker_list);
+
+                if (!list_empty(&worker->worker_list)) {
+                        list_move_tail(&worker->worker_list,
+                                      &worker->workers->worker_list);
+                }
                 spin_unlock_irqrestore(&worker->workers->lock, flags);
         }
 }