20.05.2014, 12:16, "Juri Lelli" <juri.lelli@xxxxxxxxx>:
> Hi,
>
> On Tue, 20 May 2014 09:53:15 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
>> On Tue, May 20, 2014 at 09:08:53AM +0400, Kirill Tkhai wrote:
>>> 20.05.2014, 04:00, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>:
>>>> On Mon, May 19, 2014 at 11:31:19PM +0400, Kirill Tkhai wrote:
>>>>> @@ -513,9 +513,17 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>>>>> struct sched_dl_entity,
>>>>> dl_timer);
>>>>> struct task_struct *p = dl_task_of(dl_se);
>>>>> - struct rq *rq = task_rq(p);
>>>>> + struct rq *rq;
>>>>> +again:
>>>>> + rq = task_rq(p);
>>>>> raw_spin_lock(&rq->lock);
>>>>>
>>>>> + if (unlikely(rq != task_rq(p))) {
>>>>> + /* Task was moved, retrying. */
>>>>> + raw_spin_unlock(&rq->lock);
>>>>> + goto again;
>>>>> + }
>>>>> +
>>>> That thing is called: rq = __task_rq_lock(p);
>>> But p->pi_lock is not held. The problem is __task_rq_lock() has lockdep assert.
>>> Should we change it?
>> Ok, so now that I'm awake ;-)
>>
>> So the trivial problem as described by your initial changelog isn't
>> right, because we cannot call sched_setaffinity() on deadline tasks, or
>> rather we can, but we can't actually change the affinity mask.
>
> Well, if we disable AC we can. And I was able to recreate that race in
> that case.
>
>> Now I suppose the problem can still actually happen when you change the
>> root domain and trigger a effective affinity change that way.
>
> Yeah, I think here too.
>
>> That said, no leave it as you proposed, adding a *task_rq_lock() variant
>> without lockdep assert in will only confuse things, as normally we
>> really should be also taking ->pi_lock.
>>
>> The only reason we don't strictly need ->pi_lock now is because we're
>> guaranteed to have p->state == TASK_RUNNING here and are thus free of
>> ttwu races.
>
> Maybe we could add this as part of the comment.
Peter, Juri, thanks for comment. Hope, I understood you right :)
[PATCH] sched/dl: Fix race in dl_task_timer()
Throttled task is still on rq, and it may be moved to other cpu
if user is playing with sched_setaffinity(). Therefore, unlocked
task_rq() access makes the race.
Juri Lelli reports he got this race when dl_bandwidth_enabled()
was not set.
Other thing, pointed by Peter Zijlstra:
"Now I suppose the problem can still actually happen when
you change the root domain and trigger a effective affinity
change that way".
To fix that we do the same as made in __task_rq_lock(). We do not
use __task_rq_lock() itself, because it has a useful lockdep check,
which is not correct in case of dl_task_timer(). We do not need
pi_lock locked here. This case is an exception (PeterZ):
"The only reason we don't strictly need ->pi_lock now is because
we're guaranteed to have p->state == TASK_RUNNING here and are
thus free of ttwu races".
Signed-off-by: Kirill Tkhai <tkhai@xxxxxxxxx>
CC: Juri Lelli <juri.lelli@xxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v3.14
---
kernel/sched/deadline.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 800e99b..14bc348 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -513,9 +513,17 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
struct sched_dl_entity,
dl_timer);
struct task_struct *p = dl_task_of(dl_se);
- struct rq *rq = task_rq(p);
+ struct rq *rq;
+again:
+ rq = task_rq(p);
raw_spin_lock(&rq->lock);
+ if (rq != task_rq(p)) {
+ /* Task was moved, retrying. */
+ raw_spin_unlock(&rq->lock);
+ goto again;
+ }
+
/*
* We need to take care of a possible races here. In fact, the
* task might have changed its scheduling policy to something
--
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