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    SubjectRe: [PATCH 3/4] threadgroup: extend threadgroup_lock() to cover exit and exec
    Hello,

    Sorry for the late reply.

    Of course I am in no position to ack the changes in this code, I do not
    fell I understand it enough. But afaics this series is fine.

    A couple of questions.

    On 09/05, Tejun Heo wrote:
    >
    > For exec, threadgroup_[un]lock() are updated to also grab and release
    > cred_guard_mutex.

    OK, this means that we do not need

    cgroups-more-safe-tasklist-locking-in-cgroup_attach_proc.patch
    http://marc.info/?l=linux-mm-commits&m=131491135428326&w=2

    Ben, what do you think?

    > With this change, threadgroup_lock() guarantees that the target
    > threadgroup will remain stable - no new task will be added, no new
    > PF_EXITING will be set and exec won't happen.

    To me, this is the only "contradictory" change,

    > --- a/kernel/exit.c
    > +++ b/kernel/exit.c
    > @@ -936,6 +936,12 @@ NORET_TYPE void do_exit(long code)
    > schedule();
    > }
    >
    > + /*
    > + * @tsk's threadgroup is going through changes - lock out users
    > + * which expect stable threadgroup.
    > + */
    > + threadgroup_change_begin(tsk);
    > +
    > exit_irq_thread();
    >
    > exit_signals(tsk); /* sets PF_EXITING */
    > @@ -1018,10 +1024,6 @@ NORET_TYPE void do_exit(long code)
    > kfree(current->pi_state_cache);
    > #endif
    > /*
    > - * Make sure we are holding no locks:
    > - */
    > - debug_check_no_locks_held(tsk);
    > - /*
    > * We can do this unlocked here. The futex code uses this flag
    > * just to verify whether the pi state cleanup has been done
    > * or not. In the worst case it loops once more.
    > @@ -1039,6 +1041,12 @@ NORET_TYPE void do_exit(long code)
    > preempt_disable();
    > exit_rcu();
    >
    > + /*
    > + * Release threadgroup and make sure we are holding no locks.
    > + */
    > + threadgroup_change_done(tsk);

    I am wondering, can't we narrow the scope of threadgroup_change_begin/done
    in do_exit() path?

    The code after 4/4 still has to check PF_EXITING, this is correct. And yes,
    with this patch PF_EXITING becomes stable under ->group_rwsem. But, it seems,
    we do not really need this?

    I mean, can't we change cgroup_exit() to do threadgroup_change_begin/done
    instead? We do not really care about PF_EXITING, we only need to ensure that
    we can't race with cgroup_exit(), right?

    Say, cgroup_attach_proc() does

    do {
    if (tsk->flags & PF_EXITING)
    continue;

    flex_array_put_ptr(group, tsk);
    } while_each_thread();

    Yes, this tsk can call do_exit() and set PF_EXITING right after the check
    but this is fine. The only guarantee we need is: if it has already called
    cgroup_exit() we can not miss PF_EXITING, and if cgroup_exit() takes the
    same sem this should be true. And, otoh, if we do not see PF_EXITING then
    we can not race with cgroup_exit(), it should block on ->group_rwsem hold
    by us.

    If I am right, afaics the only change 4/4 needs is that it should not add
    WARN_ON_ONCE(tsk->flags & PF_EXITING) into cgroup_task_migrate().

    What do you think?

    Oleg.



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