Re: [PATCH v3 2/3] sched: Provide copy_user_cpus_mask() to copy out user mask

[Waiman Long]

On 8/15/22 04:58, Peter Zijlstra wrote:
> On Fri, Aug 12, 2022 at 04:39:28PM -0400, Waiman Long wrote:
> > Since accessing the content of the user_cpus_ptr requires lock protection
> > to ensure its validity, provide a helper function copy_user_cpus_mask()
> > to facilitate its reading.
> Sure, but this is atrocious.
>
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2619,6 +2619,24 @@ void release_user_cpus_ptr(struct task_struct *p)
> >   	kfree(clear_user_cpus_ptr(p));
> >   }
> >   
> > +/*
> > + * Return the copied mask pointer or NULL if user mask not available.
> > + */
> > +struct cpumask *copy_user_cpus_mask(struct task_struct *p,
> > +				    struct cpumask *user_mask)
> > +{
> > +	struct rq_flags rf;
> > +	struct rq *rq = task_rq_lock(p, &rf);
> > +	struct cpumask *mask = NULL;
> > +
> > +	if (p->user_cpus_ptr) {
> > +		cpumask_copy(user_mask, p->user_cpus_ptr);
> > +		mask = user_mask;
> > +	}
> > +	task_rq_unlock(rq, p, &rf);
> > +	return mask;
> > +}
> For reading the mask you only need one of those locks, and I would
> suggest p->pi_lock is much less contended than rq->lock.
Right. pi_lock should be enough for read access. Will make the change.

Thanks,
Longman