releases/3.10.42/workqueue-make-rescuer_thread-empty-wq-maydays-list-before-exiting.patch

   1 From 4d595b866d2c653dc90a492b9973a834eabfa354 Mon Sep 17 00:00:00 2001
   2 From: Lai Jiangshan <laijs@cn.fujitsu.com>
   3 Date: Fri, 18 Apr 2014 11:04:16 -0400
   4 Subject: workqueue: make rescuer_thread() empty wq->maydays list before exiting
   5
   6 From: Lai Jiangshan <laijs@cn.fujitsu.com>
   7
   8 commit 4d595b866d2c653dc90a492b9973a834eabfa354 upstream.
   9
  10 After a @pwq is scheduled for emergency execution, other workers may
  11 consume the affectd work items before the rescuer gets to them.  This
  12 means that a workqueue many have pwqs queued on @wq->maydays list
  13 while not having any work item pending or in-flight.  If
  14 destroy_workqueue() executes in such condition, the rescuer may exit
  15 without emptying @wq->maydays.
  16
  17 This currently doesn't cause any actual harm.  destroy_workqueue() can
  18 safely destroy all the involved data structures whether @wq->maydays
  19 is populated or not as nobody access the list once the rescuer exits.
  20
  21 However, this is nasty and makes future development difficult.  Let's
  22 update rescuer_thread() so that it empties @wq->maydays after seeing
  23 should_stop to guarantee that the list is empty on rescuer exit.
  24
  25 tj: Updated comment and patch description.
  26
  27 Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
  28 Signed-off-by: Tejun Heo <tj@kernel.org>
  29 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  30
  31 ---
  32  kernel/workqueue.c |   21 ++++++++++++++++-----
  33  1 file changed, 16 insertions(+), 5 deletions(-)
  34
  35 --- a/kernel/workqueue.c
  36 +++ b/kernel/workqueue.c
  37 @@ -2362,6 +2362,7 @@ static int rescuer_thread(void *__rescue
  38         struct worker *rescuer = __rescuer;
  39         struct workqueue_struct *wq = rescuer->rescue_wq;
  40         struct list_head *scheduled = &rescuer->scheduled;
  41 +       bool should_stop;
  42
  43         set_user_nice(current, RESCUER_NICE_LEVEL);
  44
  45 @@ -2373,11 +2374,15 @@ static int rescuer_thread(void *__rescue
  46  repeat:
  47         set_current_state(TASK_INTERRUPTIBLE);
  48
  49 -       if (kthread_should_stop()) {
  50 -               __set_current_state(TASK_RUNNING);
  51 -               rescuer->task->flags &= ~PF_WQ_WORKER;
  52 -               return 0;
  53 -       }
  54 +       /*
  55 +        * By the time the rescuer is requested to stop, the workqueue
  56 +        * shouldn't have any work pending, but @wq->maydays may still have
  57 +        * pwq(s) queued.  This can happen by non-rescuer workers consuming
  58 +        * all the work items before the rescuer got to them.  Go through
  59 +        * @wq->maydays processing before acting on should_stop so that the
  60 +        * list is always empty on exit.
  61 +        */
  62 +       should_stop = kthread_should_stop();
  63
  64         /* see whether any pwq is asking for help */
  65         spin_lock_irq(&wq_mayday_lock);
  66 @@ -2429,6 +2434,12 @@ repeat:
  67
  68         spin_unlock_irq(&wq_mayday_lock);
  69
  70 +       if (should_stop) {
  71 +               __set_current_state(TASK_RUNNING);
  72 +               rescuer->task->flags &= ~PF_WQ_WORKER;
  73 +               return 0;
  74 +       }
  75 +
  76         /* rescuers should never participate in concurrency management */
  77         WARN_ON_ONCE(!(rescuer->flags & WORKER_NOT_RUNNING));
  78         schedule();