On 04/14, Eric W. Biederman wrote:
>
> Oleg Nesterov <[email protected]> writes:
>
> > On 04/10, Roland McGrath wrote:
> >>
> >> I would be inclined to restructure the inner loop something like this:
> >>
> >> p = g;
> >> while (unlikely(p->mm == NULL)) {
> >> p = next_thread(p);
> >> if (p == g)
> >> break;
> >> }
> >> if (p->mm == mm) {
> >> /*
> >> * p->sighand can't disappear, but
> >> * may be changed by de_thread()
> >> */
> >> lock_task_sighand(p, &flags);
> >> zap_process(p);
> >> unlock_task_sighand(p, &flags);
> >> }
> >
> > Yes, I agree, this is much more understandable.
>
> There is one piece of zap_threads that still makes me uncomfortable.
>
> task_lock is used to protect p->mm.
> Therefore killing a process based upon p->mm == mm is racy
> with respect to sys_unshare I believe if we don't take
> task_lock.
Well, unshare(CLONE_VM) is not yet supported. Currently (as I see
it) mm->mmap_sem is enough to protect against changing ->mm. Yes,
exit_mm/exec_mmap take task_lock too, so it can be used as well.
Please correct my understanding.
I think it is better to take ->mmap_sem in sys_unshare, this path
is rare.
Oleg.
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