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SubjectRe: [PATCH for-3.20-fixes] workqueue: fix hang involving racing cancel[_delayed]_work_sync()'s for PREEMPT_NONE
Hello, Tomeu.

On Tue, Mar 03, 2015 at 11:00:43AM +0100, Tomeu Vizoso wrote:
...
> [ 7.421803] PC is at wake_bit_function+0x18/0x6c
> [ 7.428168] LR is at __wake_up_common+0x5c/0x90
...
> [ 7.673183] [<c028f54c>] (wake_bit_function) from [<c028ed3c>] (__wake_up_common+0x5c/0x90)
> [ 7.683300] [<c028ed3c>] (__wake_up_common) from [<c028ef88>] (__wake_up+0x48/0x5c)
> [ 7.692744] [<c028ef88>] (__wake_up) from [<c026fdfc>] (__cancel_work_timer+0xe8/0x1ac)
> [ 7.702533] [<c026fdfc>] (__cancel_work_timer) from [<c026fedc>] (cancel_work_sync+0x1c/0x20)
> [ 7.712854] [<c026fedc>] (cancel_work_sync) from [<c02ce3bc>] (css_free_work_fn+0x174/0x2ec)
> [ 7.723099] [<c02ce3bc>] (css_free_work_fn) from [<c026f4d0>] (process_one_work+0x15c/0x3dc)
> [ 7.733339] [<c026f4d0>] (process_one_work) from [<c026ff6c>] (worker_thread+0x54/0x4e8)
> [ 7.743224] [<c026ff6c>] (worker_thread) from [<c02749d8>] (kthread+0xec/0x104)
> [ 7.752339] [<c02749d8>] (kthread) from [<c0210aa0>] (ret_from_fork+0x14/0x34)
> [ 7.761366] Code: e24cb004 e52de004 e8bd4000 e510400c (e5935000)

Hah, weird. How is your machine ending up in wake_bit_function() from
there? Can you please apply the following patch and report the dmesg
after crash?

Thanks.

---
kernel/workqueue.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2755,8 +2755,12 @@ static bool __cancel_work_timer(struct w
if (unlikely(ret == -ENOENT)) {
DEFINE_WAIT(wait);
prepare_to_wait(waitq, &wait, TASK_UNINTERRUPTIBLE);
- if (work_is_canceling(work))
+ WARN_ON_ONCE(wait.func != autoremove_wake_function);
+ if (work_is_canceling(work)) {
+ printk("XXX __cancel_work_timer: %d sleeping w/ %pf, wait=%p\n",
+ task_pid_nr(current), wait.func, &wait);
schedule();
+ }
finish_wait(waitq, &wait);
}
} while (unlikely(ret < 0));
@@ -2774,8 +2778,16 @@ static bool __cancel_work_timer(struct w
* visible there.
*/
smp_mb();
- if (waitqueue_active(waitq))
+ if (waitqueue_active(waitq)) {
+ wait_queue_t *cur;
+
+ spin_lock_irq(&waitq->lock);
+ list_for_each_entry(cur, &waitq->task_list, task_list)
+ printk("XXX __cancel_work_timer: waking up %pf, wait=%p\n",
+ cur->func, cur);
+ spin_unlock_irq(&waitq->lock);
wake_up(waitq);
+ }

return ret;
}


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