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

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