Re: [PATCH v6] psi: fix race between psi_trigger_create/destroy

[Peter Zijlstra]

On Thu, May 20, 2021 at 07:11:08PM -0700, Suren Baghdasaryan wrote:
> On Thu, May 20, 2021 at 7:07 PM Huangzhaoyang <huangzhaoyang@xxxxxxxxx> wrote:
> >
> > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >
> > Race detected between psi_trigger_destroy/create as shown below, which
> > cause panic by accessing invalid psi_system->poll_wait->wait_queue_entry
> > and psi_system->poll_timer->entry->next. Under this modification, the
> > race window is removed by initialising poll_wait and poll_timer in
> > group_init which are executed only once at beginning.
> >
> > psi_trigger_destroy                      psi_trigger_create
> > mutex_lock(trigger_lock);
> > rcu_assign_pointer(poll_task, NULL);
> > mutex_unlock(trigger_lock);
> >                                         mutex_lock(trigger_lock);
> >                                         if (!rcu_access_pointer(group->poll_task)) {
> >
> >                                                 timer_setup(poll_timer, poll_timer_fn, 0);
> >
> >                                                 rcu_assign_pointer(poll_task, task);
> >                                         }
> >                                         mutex_unlock(trigger_lock);
> >
> > synchronize_rcu();
> > del_timer_sync(poll_timer); <-- poll_timer has been reinitialized by
> > psi_trigger_create
> >
> > So, trigger_lock/RCU correctly protects destruction of group->poll_task but
> > misses this race affecting poll_timer and poll_wait.
> >
> > Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger
> > scheduling mechanism")
> >
> > Signed-off-by: ziwei.dai <ziwei.dai@xxxxxxxxxx>
> > Signed-off-by: ke.wang <ke.wang@xxxxxxxxxx>
> > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>

This is not a valid SoB chain though; please (re)read Documentation/process/submitting-patches.rst.

> Looks good. Thanks!
> Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>