workqueue: make rescuer_thread() empty wq->maydays list before exiting

After a @pwq is scheduled for emergency execution, other workers may
consume the affectd work items before the rescuer gets to them.  This
means that a workqueue many have pwqs queued on @wq->maydays list
while not having any work item pending or in-flight.  If
destroy_workqueue() executes in such condition, the rescuer may exit
without emptying @wq->maydays.

This currently doesn't cause any actual harm.  destroy_workqueue() can
safely destroy all the involved data structures whether @wq->maydays
is populated or not as nobody access the list once the rescuer exits.

However, this is nasty and makes future development difficult.  Let's
update rescuer_thread() so that it empties @wq->maydays after seeing
should_stop to guarantee that the list is empty on rescuer exit.

tj: Updated comment and patch description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org # v3.10+

1 files changed, 16 insertions, 5 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 3150b217c936..6ba0c6054224 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2398,6 +2398,7 @@ static int rescuer_thread(void *__rescuer)
         struct worker *rescuer = __rescuer;
         struct workqueue_struct *wq = rescuer->rescue_wq;
         struct list_head *scheduled = &rescuer->scheduled;
+        bool should_stop;
 
         set_user_nice(current, RESCUER_NICE_LEVEL);
 
@@ -2409,11 +2410,15 @@ static int rescuer_thread(void *__rescuer)
 repeat:
         set_current_state(TASK_INTERRUPTIBLE);
 
-        if (kthread_should_stop()) {
-                __set_current_state(TASK_RUNNING);
-                rescuer->task->flags &= ~PF_WQ_WORKER;
-                return 0;
-        }
+        /*
+         * By the time the rescuer is requested to stop, the workqueue
+         * shouldn't have any work pending, but @wq->maydays may still have
+         * pwq(s) queued.  This can happen by non-rescuer workers consuming
+         * all the work items before the rescuer got to them.  Go through
+         * @wq->maydays processing before acting on should_stop so that the
+         * list is always empty on exit.
+         */
+        should_stop = kthread_should_stop();
 
         /* see whether any pwq is asking for help */
         spin_lock_irq(&wq_mayday_lock);
@@ -2459,6 +2464,12 @@ repeat:
 
         spin_unlock_irq(&wq_mayday_lock);
 
+        if (should_stop) {
+                __set_current_state(TASK_RUNNING);
+                rescuer->task->flags &= ~PF_WQ_WORKER;
+                return 0;
+        }
+
         /* rescuers should never participate in concurrency management */
         WARN_ON_ONCE(!(rescuer->flags & WORKER_NOT_RUNNING));
         schedule();