linux-kernel - Re: [PATCH V3 04/10] sched/core: Prevent race condition between cpuset and __sched

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Message-ID: <20180214104935.GS12979@localhost.localdomain>
Date:   Wed, 14 Feb 2018 11:49:35 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     peterz@...radead.org, lizefan@...wei.com, mingo@...hat.com,
        rostedt@...dmis.org, claudio@...dence.eu.com, bristot@...hat.com,
        tommaso.cucinotta@...tannapisa.it, luca.abeni@...tannapisa.it,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 04/10] sched/core: Prevent race condition between
 cpuset and __sched_setscheduler()

On 14/02/18 11:36, Juri Lelli wrote:
> Hi Mathieu,
> 
> On 13/02/18 13:32, Mathieu Poirier wrote:
> > No synchronisation mechanism exist between the cpuset subsystem and calls
> > to function __sched_setscheduler().  As such it is possible that new root
> > domains are created on the cpuset side while a deadline acceptance test
> > is carried out in __sched_setscheduler(), leading to a potential oversell
> > of CPU bandwidth.
> > 
> > By making available the cpuset_mutex to the core scheduler it is possible
> > to prevent situations such as the one described above from happening.
> > 
> > Signed-off-by: Mathieu Poirier <mathieu.poirier@...aro.org>
> > ---
> 
> [...]
> 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index f727c3d0064c..0d8badcf1f0f 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4176,6 +4176,13 @@ static int __sched_setscheduler(struct task_struct *p,
> >  	}
> >  
> >  	/*
> > +	 * Make sure we don't race with the cpuset subsystem where root
> > +	 * domains can be rebuilt or modified while operations like DL
> > +	 * admission checks are carried out.
> > +	 */
> > +	cpuset_lock();
> > +
> > +	/*
> 
> Mmm, I'm afraid we can't do this. __sched_setscheduler might be called
> from interrupt contex by normalize_rt_tasks().

Maybe conditionally grabbing it if pi is true could do? I guess we don't
care much about domains when sysrq.