linux-kernel - Re: [PATCH v2 1/2] sched: schedule_tail() should disable preemption

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Date:	Thu, 09 Oct 2014 01:37:28 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] sched: schedule_tail() should disable preemption

В Ср, 08/10/2014 в 21:36 +0200, Oleg Nesterov пишет:
> On 10/08, Oleg Nesterov wrote:
> >
> > Another problem is that finish_task_switch() itself runs with preempt
> > enabled after finish_lock_switch(). If nothing else this means that
> > ->sched_in() notifier can't trust its "cpu" arg.
> 
> OOPS, this obviously can't happen, ->preempt_notifiers must be empty.
> Remove this part from the changelog, please see v2.
> 
> I am not sure about finish_arch_post_lock_switch() ... but probably
> it should be fine without preempt_disable.
> 
> -------------------------------------------------------------------------------
> Subject: [PATCH v2 1/2] sched: schedule_tail() should disable preemption
> 
> finish_task_switch() enables preemption, so post_schedule(rq) can be
> called on the wrong (and even dead) CPU. Afaics, nothing really bad
> can happen, but in this case we can wrongly clear rq->post_schedule
> on that CPU. And this simply looks wrong in any case.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  kernel/sched/core.c |   11 +++++------
>  1 files changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 703c7e6..3f267e8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2277,15 +2277,14 @@ static inline void post_schedule(struct rq *rq)
>  asmlinkage __visible void schedule_tail(struct task_struct *prev)
>  	__releases(rq->lock)
>  {
> -	struct rq *rq = this_rq();
> +	struct rq *rq;
>  
> +	/* finish_task_switch() drops rq->lock and enables preemtion */
> +	preempt_disable();

Maybe, the code would look simpler if we change
init_task_preempt_count() and create new tasks
with preempt_count() == 2, so this preempt_disable()
won't be necessary. But it's more or less subjectively.

Reviewed-by: Kirill Tkhai <ktkhai@...allels.com>

> +	rq = this_rq();
>  	finish_task_switch(rq, prev);
> -
> -	/*
> -	 * FIXME: do we need to worry about rq being invalidated by the
> -	 * task_switch?
> -	 */
>  	post_schedule(rq);
> +	preempt_enable();
>  
>  	if (current->set_child_tid)
>  		put_user(task_pid_vnr(current), current->set_child_tid);




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