Re: lockdep-related warning in kernel/sched/deadline.c::find_lock_later_rq()

From: Luca Abeni
Date: Thu Oct 22 2015 - 03:06:43 EST


Hi,

On 10/22/2015 07:35 AM, Wanpeng Li wrote:
[...]
Now, if I understand correctly the issue is that dl_task_timer() does:
rq = task_rq_lock(p, &flags);
[...]
if (has_pushable_dl_tasks(rq))
push_dl_task(rq);
with task_rq_lock() that pins rq->lock and push_tl_task() that invokes
find_lock_later_rq() that unlocks rq->lock() while it is pinned.

I am not sure about how to fix this issue: as a first try, I did
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 142df26..5b1ba95 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -668,8 +668,11 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
* Queueing this task back might have overloaded rq, check if we need
* to kick someone away.
*/
- if (has_pushable_dl_tasks(rq))
+ if (has_pushable_dl_tasks(rq)) {
+ lockdep_unpin_lock(&rq->lock);
push_dl_task(rq);
+ lockdep_pin_lock(&rq->lock);
+ }
#endif

unlock:

This removes the warning, but I am not sure if it is the correct fix (is it
valid to unpin rq->lock, here?).

If someone can confirm that this is the correct approach, I'll test the patch a
little bit more and then I'll send a properly signed-off patch; otherwise, if
someone can suggest the correct approach I'll try it.

wake_up_new_task()
-> __task_rq_lock()
-> task_woken()
-> push_dl/rt_tasks()
-> push_dl/rt_task()

I think you also should consider the lockdep pin_lock in this path.
Well, I never triggered this warning for the task_woken() path, but now I
see it... Thanks!

If someone can confirm that unpinning before calling push/pull and pinning again
after the call is the correct thing to do, I'll send a proper patch taking into
account all the paths... But for the moment I am still not sure if unpinning
the lock here is ok.



Thanks,
Luca
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