linux-kernel - Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup

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Date:   Tue, 5 Jun 2018 18:35:16 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Kohli, Gaurav" <gkohli@...eaurora.org>, tglx@...utronix.de,
        mpe@...erman.id.au, mingo@...nel.org, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against
 wakeup

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.