[PATCH 3.13 072/212] workqueue: make rescuer_thread() empty wq->maydays list before exiting

Kamal Mostafa
2014-06-17T21:42:46Z

From: Lai Jiangshan <laijs@cn.fujitsu.com>

commit 4d595b866d2c653dc90a492b9973a834eabfa354 upstream.

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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
kernel/workqueue.c | 21 ++++++++++++++++-----
1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 6b1b746..df9d7e8 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2391,6 +2391,7 @@ 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);

@@ -2402,11 +2403,15 @@ 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);
@@ -2452,6 +2457,12 @@ 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();
--
1.9.1