cpuset: attach_task() vs sched_setaffinity() race?

[Oleg Nesterov <oleg@tv-sign.ru>]

After the brief look at kernel/cpuset.c, it seems that attach_task() should
guarantee that the task can't use CPUs outside of cpuset->cpus_allowed.

But this looks racy wrt sched_setaffinity() which does

	cpus_allowed = cpuset_cpus_allowed(p);
	// callback_mutex is free
	set_cpus_allowed(p);

What if attach_task()->set_cpus_allowed() happens in between?


Another question: update_cpumask(cs) does nothing with the tasks attached to
that cpuset, why? It may take a while before the task actually migrates to the
new CPU.

Thanks,

Oleg.