----- On Feb 16, 2018, at 11:53 AM, Mark Rutland mark.rutl...@arm.com wrote:
> Hi,
>
> On Thu, Feb 15, 2018 at 10:08:56PM +0000, Mathieu Desnoyers wrote:
>> My current theory: do_exit() gets preempted after having set current->mm
>> to NULL, and after having issued mmput(), which brings the mm_count down
>> to 0.
>>
>> Unfortunately, if the scheduler switches from a userspace thread
>> to a kernel thread, context_switch() loads prev->active_mm which still
>> points to the now-freed mm, mmgrab the mm, and eventually does mmdrop
>> in finish_task_switch().
>
> For this to happen, we need to get to the mmput() in exit_mm() with:
>
> mm->mm_count == 1
> mm->mm_users == 1
> mm == active_mm
>
> ... but AFAICT, this cannot happen.
>
> If there's no context_switch between clearing current->mm and the
> mmput(), then mm->mm_count >= 2, thanks to the prior mmgrab() and the
> active_mm reference (in mm_count) that context_switch+finish_task_switch
> manage.
>
> If there is a context_switch between the two, then AFAICT, either:
>
> a) The task re-inherits its old mm as active_mm, and mm_count >= 2. In
> context_switch we mmgrab() the active_mm to inherit it, and in
> finish_task_switch() we drop the oldmm, balancing the mmgrab() with
> an mmput().
>
> e.g we go task -> kernel_task -> task
>
> b) At some point, another user task is scheduled, and we switch to its
> mm. We don't mmgrab() the active_mm, but we mmdrop() the oldmm, which
> means mm_count >= 1. Since we witched to a new mm, if we switch back
> to the first task, it cannot have its own mm as active_mm.
>
> e.g. we go task -> other_task -> task
>
> I suspect we have a bogus mmdrop or mmput elsewhere, and do_exit() and
> finish_task_switch() aren't to blame.
Currently reviewing: fs/proc/base.c: __set_oom_adj()
/*
* Make sure we will check other processes sharing the mm if this is
* not vfrok which wants its own oom_score_adj.
* pin the mm so it doesn't go away and get reused after task_unlock
*/
if (!task->vfork_done) {
struct task_struct *p = find_lock_task_mm(task);
if (p) {
if (atomic_read(&p->mm->mm_users) > 1) {
mm = p->mm;
mmgrab(mm);
}
task_unlock(p);
}
}
Considering that mmput() done by exit_mm() is done outside of the
task_lock critical section, I wonder how the mm_users load is
synchronized ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
