Re: [PATCH V3 04/10] sched/core: Prevent race condition between cpuset and __sched_setscheduler()

Juri Lelli
2018-02-14T10:49:41Z

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@linaro.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.