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

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Date:   Wed, 14 Feb 2018 17:31:45 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Li Zefan <lizefan@...wei.com>, Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        "luca.abeni" <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 08:33, Mathieu Poirier wrote:
> On 14 February 2018 at 04:27, Juri Lelli <juri.lelli@...hat.com> wrote:
> > On 14/02/18 11:49, Juri Lelli wrote:
> >> 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.
> >
> > Ops.. just got this. :/
> 
> 
> Arrghhh... Back to the drawing board.
> 

Eh.. even though the warning simply happens because unlocking of
cpuset lock is missing

--->8---
@@ -4312,6 +4312,7 @@ static int __sched_setscheduler(struct task_struct *p,                                                            
        /* Avoid rq from going away on us: */                       
        preempt_disable();        
        task_rq_unlock(rq, p, &rf);                                 
+       cpuset_unlock();          
                                  
        if (pi)                   
                rt_mutex_adjust_pi(p);
--->8---

Still grabbing it is a no-go, as do_sched_setscheduler calls
sched_setscheduler from inside an RCU read-side critical section.

So, back to the drawing board indeed. :/

Thanks,

- Juri