linux-kernel - Re: [PATCH] sched: access local runqueue directly in single_task

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Date:	Fri, 18 Sep 2015 13:35:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Dominik Dingel <dingel@...ux.vnet.ibm.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>, stable@...r.kernel.org
Subject: Re: [PATCH] sched: access local runqueue directly in
 single_task_running

On Fri, Sep 18, 2015 at 01:26:53PM +0200, Paolo Bonzini wrote:
> > +++ b/kernel/sched/core.c
> > @@ -2614,13 +2614,13 @@ unsigned long nr_running(void)
> >  
> >  /*
> >   * Check if only the current task is running on the cpu.
> > + *
> > + * Caution result is subject to time-of-check-to-time-of-use race,
> > + * every caller is responsible to set up additional fences if necessary.
> 
> Let's expand it a bit more:
> 
>  * Caution: this function does not check that the caller has disabled
>  * preemption, thus the result might have a time-of-check-to-time-of-use
>  * race.  The caller is responsible to use this correctly, for example:
>  *
>  * - use it from a non-preemptable section
>  *
>  * - use it from a thread that is bound to a single CPU
>  *
>  * - use it in a loop where each iteration takes very little time
>  *   (e.g. a polling loop)
>  */
> 
> I'll include it in my pull request.

In which case:

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
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