Messages in this thread | | | Date | Tue, 16 Aug 2022 18:11:03 -0400 | Subject | Re: [PATCH v5 3/3] cgroup/cpuset: Keep user set cpus affinity | From | Waiman Long <> |
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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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