From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Kohli, Gaurav" <gkohli@codeaurora.org>,
tglx@linutronix.de, mpe@ellerman.id.au, mingo@kernel.org,
bigeasy@linutronix.de, linux-kernel@vger.kernel.org,
linux-arm-msm@vger.kernel.org,
Neeraj Upadhyay <neeraju@codeaurora.org>,
Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup
Date: Tue, 5 Jun 2018 18:35:16 +0200 [thread overview]
Message-ID: <20180605163515.GB24053@redhat.com> (raw)
In-Reply-To: <20180605154053.GB12235@hirez.programming.kicks-ass.net>
On 06/05, Peter Zijlstra wrote:
>
> On Tue, Jun 05, 2018 at 05:22:12PM +0200, Peter Zijlstra wrote:
>
> > > OK, but __kthread_parkme() can be preempted before it calls schedule(), so the
> > > caller still can be migrated? Plus kthread_park_complete() can be called twice.
> >
> > Argh... I forgot TASK_DEAD does the whole thing with preempt_disable().
> > Let me stare at that a bit.
>
> This should ensure we only ever complete when we read PARKED, right?
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8d59b259af4a..e513b4600796 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2641,7 +2641,7 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev,
> * past. prev == current is still correct but we need to recalculate this_rq
> * because prev may have moved to another CPU.
> */
> -static struct rq *finish_task_switch(struct task_struct *prev)
> +static struct rq *finish_task_switch(struct task_struct *prev, bool preempt)
> __releases(rq->lock)
> {
> struct rq *rq = this_rq();
> @@ -2674,7 +2674,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> *
> * We must observe prev->state before clearing prev->on_cpu (in
> * finish_task), otherwise a concurrent wakeup can get prev
> - * running on another CPU and we could rave with its RUNNING -> DEAD
> + * running on another CPU and we could race with its RUNNING -> DEAD
> * transition, resulting in a double drop.
> */
> prev_state = prev->state;
> @@ -2720,7 +2720,8 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> break;
>
> case TASK_PARKED:
> - kthread_park_complete(prev);
> + if (!preempt)
> + kthread_park_complete(prev);
Yes, but this won't fix the race decribed by Kohli...
Plus this complicates the schedule() paths for the very special case, and to me
it seems that all this kthread_park/unpark logic needs some serious cleanups...
Not that I can suggest something better right now.
Oleg.
next prev parent reply other threads:[~2018-06-05 16:35 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-25 8:33 [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup Gaurav Kohli
2018-04-25 20:09 ` Peter Zijlstra
2018-04-26 4:04 ` Kohli, Gaurav
2018-04-26 9:14 ` Peter Zijlstra
2018-04-26 8:41 ` Peter Zijlstra
2018-04-26 8:57 ` Peter Zijlstra
2018-04-26 15:53 ` Kohli, Gaurav
2018-04-30 11:17 ` Peter Zijlstra
2018-05-01 7:50 ` Kohli, Gaurav
2018-05-01 10:18 ` Peter Zijlstra
2018-05-01 10:40 ` Peter Zijlstra
2018-05-01 10:40 ` Kohli, Gaurav
2018-05-01 11:31 ` Peter Zijlstra
2018-05-01 11:46 ` Kohli, Gaurav
2018-05-01 13:19 ` Peter Zijlstra
2018-05-02 5:15 ` Kohli, Gaurav
2018-05-02 8:20 ` Peter Zijlstra
2018-05-02 10:13 ` Kohli, Gaurav
2018-05-07 11:09 ` Kohli, Gaurav
2018-05-07 11:23 ` Kohli, Gaurav
2018-06-05 11:13 ` Kohli, Gaurav
2018-06-05 15:08 ` Oleg Nesterov
2018-06-05 15:22 ` Peter Zijlstra
2018-06-05 15:40 ` Peter Zijlstra
2018-06-05 16:35 ` Oleg Nesterov [this message]
2018-06-05 18:21 ` Kohli, Gaurav
2018-06-05 20:13 ` Peter Zijlstra
2018-06-06 13:51 ` Oleg Nesterov
2018-06-06 15:03 ` Peter Zijlstra
2018-06-06 15:04 ` Peter Zijlstra
2018-06-06 15:22 ` Peter Zijlstra
2018-06-06 18:59 ` Peter Zijlstra
2018-06-07 8:30 ` Kohli, Gaurav
2018-05-01 10:44 ` Peter Zijlstra
2018-04-26 16:02 ` Andrea Parri
2018-04-26 16:18 ` Oleg Nesterov
2018-04-30 11:20 ` Peter Zijlstra
2018-04-30 11:56 ` Peter Zijlstra
2018-04-28 6:43 ` [lkp-robot] [kthread/smpboot] cad8e99675: inconsistent{IN-HARDIRQ-W}->{HARDIRQ-ON-W}usage kernel test robot
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