On Wed, 10 Oct 2007, Paul Menage wrote:

> On 10/6/07, David Rientjes <rientjes@google.com> wrote:
> >
> > It can race with sched_setaffinity().  It has to give up tasklist_lock as
> > well to call set_cpus_allowed() and can race
> >
> >         cpus_allowed = cpuset_cpus_allowed(p);
> >         cpus_and(new_mask, new_mask, cpus_allowed);
> >         retval = set_cpus_allowed(p, new_mask);
> >
> > and allow a task to have a cpu outside of the cpuset's new cpus_allowed if
> > you've taken it away between cpuset_cpus_allowed() and set_cpus_allowed().
> 
> cpuset_cpus_allowed() takes callback_mutex, which is held by
> update_cpumask() when it updates cs->cpus_allowed. So if we continue
> to hold callback_mutex across the task update loop this wouldn't be a
> race. Having said that, holding callback mutex for that long might not
> be a good idea.
> 

Moving the actual use of set_cpus_allowed() from the cgroup code to 
sched.c would probably be the best in terms of a clean interface.  Then 
you could protect the whole thing by sched_hotcpu_mutex, which is 
expressly designed for migrations.

Something like this:

	struct cgroup_iter it;
	struct task_struct *p, **tasks;
	int i = 0;

	cgroup_iter_start(cs->css.cgroup, &it);
	while ((p = cgroup_iter_next(cs->css.cgroup, &it))) {
		get_task_struct(p);
		tasks[i++] = p;
	}
	cgroup_iter_end(cs->css.cgroup, &it);

	while (--i >= 0) {
		sched_migrate_task(tasks[i], cs->cpus_allowed);
		put_task_struct(tasks[i]);
	}

kernel/sched.c:

	void sched_migrate_task(struct task_struct *task,
				cpumask_t cpus_allowed)
	{
		mutex_lock(&sched_hotcpu_mutex);
		set_cpus_allowed(task, cpus_allowed);
		mutex_unlock(&sched_hotcpu_mutex);
	}

It'd be faster to just pass **tasks to a sched.c function with the number 
of tasks to migrate to reduce the contention on sched_hotcpu_mutex, but 
then the put_task_struct() is left dangling over in cgroup code.  
sched_hotcpu_mutex should be rarely contended, anyway.

		David
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