Re: [ckrm-tech] [PATCH 1/7] containers (V7): Generic container system abstracted from cpusets code

Srivatsa Vaddagiri
2007-03-25T00:38:21Z

On Sat, Mar 24, 2007 at 12:25:59PM -0700, Paul Jackson wrote:
> > P.S : cpuset.c checks for PF_EXITING twice in attach_task(), while this
> > patch seems to be checking only once. Is that fine?
>
> I think the cpuset code is ok, because, as you note, it locks the task,
> picks off the cpuset pointer, and then checks a second time that the
> task still does not have PF_EXITING set:

Well afaics, PF_EXITING is set for the exiting task w/o taking any lock, which
makes this racy always.

> In the kernel/cpuset.c code for attach_task():
>
> task_lock(tsk);
> oldcs = tsk->cpuset;
> /*
> * After getting 'oldcs' cpuset ptr, be sure still not exiting.
> * If 'oldcs' might be the top_cpuset due to the_top_cpuset_hack
> * then fail this attach_task(), to avoid breaking top_cpuset.count.
> */
> if (tsk->flags & PF_EXITING) {

What if PF_EXITING is set after this check? If that happens then,

> task_unlock(tsk);
> mutex_unlock(&callback_mutex);
> put_task_struct(tsk);
> return -ESRCH;
> }

the following code becomes racy with cpuset_exit() ...

atomic_inc(&cs->count);
rcu_assign_pointer(tsk->cpuset, cs);
task_unlock(tsk);

--
Regards,
vatsa