On Wed, 24 May 2017, Petr Mladek wrote:
> On Thu 2017-05-18 14:00:43, Miroslav Benes wrote:
> > If a task sleeps in a set of patched functions uninterruptibly, it could
> > block the whole transition process indefinitely. Thus it may be useful
> > to clear its TIF_PATCH_PENDING to allow the process to finish.
> >
> > Admin can do that now by writing 2 to force sysfs attribute in livepatch
> > sysfs directory. TIF_PATCH_PENDING is then cleared for all tasks and the
> > transition can finish successfully.
> >
> > Important note! Use wisely. Admin must be sure that it is safe to
> > execute such action. This means that it must be checked that by doing so
> > the consistency model guarantees are not violated.
> >
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > index bb61aaa196d3..d057a34510e6 100644
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -591,3 +591,19 @@ void klp_send_fake_signal(void)
> > }
> > read_unlock(&tasklist_lock);
> > }
> > +
> > +/*
> > + * Drop TIF_PATCH_PENDING of all tasks on admin's request. This forces an
> > + * existing transition to finish.
> > + */
> > +void klp_unmark_tasks(void)
> > +{
> > + struct task_struct *g, *task;
> > +
> > + pr_warn("all tasks marked as migrated on admin's request\n");
> > +
> > + read_lock(&tasklist_lock);
> > + for_each_process_thread(g, task)
> > + klp_update_patch_state(task);
> > + read_unlock(&tasklist_lock);
>
> This should get called under klp_mutex. The following race comes to my mind:
>
> CPU0: CPU1:
>
> klp_transition_work_fn()
> klp_try_complete_transition()
> for_each_process()
> if (!klp_try_switch_task(task))
>
> # success
>
> klp_complete_transition()
>
> for_each_process()
> task->patch_state = KLP_UNDEFINED;
>
>
> klp_unmark_tasks()
> for_each_process()
> klp_update_patch_state()
> task->patch_state =
> klp_target_state;
>
> klp_target_state = KLP_UNDEFINED;
>
> => CPU1 might happily set an obsolete state and create a mess.
This should not happen. klp_update_patch_state() use
test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING) and only if true,
task->patch_state is set.
And all TIF_PATCH_PENDING are cleared when you get
klp_complete_transition().
> It would be possible to solve this by reodering, barriers.
> But much better solution seems to serialize both actions
> using klp_mutex.
>
> In fact, I would suggest to take klp_mutex in force_store()
> and do all actions synchronously, including the check
> of klp_transition_patch.
I still think it is better not do it. klp_unmark_tasks() does nothing else
than tasks already do. They call klp_update_patch_state() by themselves
and they do not grab klp_mutex lock for doing that. klp_unmark_tasks()
only forces this action.
On the other hand, I do not see a problem in doing that. We already have a
relationship between klp_mutex and tasklist_lock defined elsewhere, so it
is safe. It would only serialize things needlessly.
Thanks,
Miroslav
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