Re: [PATCH v6 4/5] sched: Handle set_cpus_allowed_ptr() & sched_setaffinity() race

[Waiman Long]

On 8/31/22 05:47, Peter Zijlstra wrote:
> On Thu, Aug 25, 2022 at 09:01:18PM -0400, Waiman Long wrote:
> > @@ -2722,6 +2734,7 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
> >   			complete = true;
> >   		}
> >   
> > +		swap_user_cpus_ptr(p, puser_mask);
> >   		task_rq_unlock(rq, p, rf);
> >   
> >   		if (push_task) {
> > @@ -2793,6 +2806,7 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
> >   		if (flags & SCA_MIGRATE_ENABLE)
> >   			p->migration_flags &= ~MDF_PUSH;
> >   
> > +		swap_user_cpus_ptr(p, puser_mask);
> >   		task_rq_unlock(rq, p, rf);
> >   
> >   		if (!stop_pending) {
> > @@ -2813,6 +2827,8 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
> >   				complete = true;
> >   			}
> >   		}
> > +
> > +		swap_user_cpus_ptr(p, puser_mask);
> >   		task_rq_unlock(rq, p, rf);
> >   
> >   		if (complete)
> I'm not at all sure about those.
>
> Would it not be much simpler to keep the update of cpus_mask and
> cpus_user_mask together, always ensuring that cpus_user_mask is a strict
> superset of cpus_mask ? That is, set_cpus_allowed_common() seems like
> the right place to me.
>
> I'm thinking this also means blowing away user_mask when we do a full
> reset of the cpus_mask when we do an affnity break.

Thanks for the suggestion. I will need to think about what will be best 
way to pass in those additional parameters there.

Cheers,
Longman