Re: [PATCH v5 3/3] cgroup/cpuset: Keep user set cpus affinity

[Waiman Long]

On 8/16/22 16:15, Tejun Heo wrote:
> On Tue, Aug 16, 2022 at 03:27:34PM -0400, Waiman Long wrote:
> > +static int cpuset_set_cpus_allowed_ptr(struct task_struct *p,
> > +				       const struct cpumask *mask)
> > +{
> > +	cpumask_var_t new_mask;
> > +	int ret;
> > +
> > +	if (!READ_ONCE(p->user_cpus_ptr)) {
> > +		ret = set_cpus_allowed_ptr(p, mask);
> > +		/*
> > +		 * If user_cpus_ptr becomes set now, we are racing with
> > +		 * a concurrent sched_setaffinity(). So use the newly
> > +		 * set user_cpus_ptr and retry again.
> > +		 *
> > +		 * TODO: We cannot detect change in the cpumask pointed to
> > +		 * by user_cpus_ptr. We will have to add a sequence number
> > +		 * if such a race needs to be addressed.
> > +		 */
> This is too ugly and obviously broken. Let's please do it properly.

Actually, there is similar construct in __sched_setaffinity():

again:
        retval = __set_cpus_allowed_ptr(p, new_mask, SCA_CHECK);
        if (retval)
                goto out_free_new_mask;

        cpuset_cpus_allowed(p, cpus_allowed);
        if (!cpumask_subset(new_mask, cpus_allowed)) {
                /*
                 * We must have raced with a concurrent cpuset update.
                 * Just reset the cpumask to the cpuset's cpus_allowed.
                 */
                cpumask_copy(new_mask, cpus_allowed);
                goto again;
        }

It is hard to synchronize different subsystems atomically without 
running into locking issue. Let me think about what can be done in this 
case.

Is using a sequence number to check for race with retry good enough?

Cheers,
Longman