Re: [PATCH v5] psi: fix race between psi_trigger_create and psimon

[Suren Baghdasaryan]

Thanks! The code looks good to me, just a couple nits below.

On Wed, May 19, 2021 at 6:56 PM Huangzhaoyang <huangzhaoyang@xxxxxxxxx> wrote:
>
> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
>
> Race detected between psimon_new and psimon_old 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.

I still think describing this as a race between psi_trigger_create and
psi_trigger_destroy would be better. Description in abstract terms
like "psimon_new" and "psimon_old" is not very clear IMO. Feel free to
borrow from my reply at
https://lore.kernel.org/patchwork/patch/1429498/#1627054, but I won't
object to the current version as well.

>
> psi_trigger_create      psimon_new     psimon_old
>  init_waitqueue_head                    finish_wait
>                                           spin_lock(lock_old)
>   spin_lock_init(lock_new)
>  wake_up_process(psimon_new)
>
>                         finish_wait
>                           spin_lock(lock_new)
>                             list_del       list_del
>
> 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>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

> ---
> v2: change del_timer_sync to del_timer in psi_trigger_destroy
> v3: remove timer_setup within psi_tirgger_create
>     protect del_timer by extending the critical section of mutex_lock
> v4: amend fix information on comment
> v5: delete the poll_timer while assigning the task to NULL
> ---
> ---
>  kernel/sched/psi.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index cc25a3c..075501e 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -182,6 +182,8 @@ struct psi_group psi_system = {
>
>  static void psi_avgs_work(struct work_struct *work);
>
> +static void poll_timer_fn(struct timer_list *t);
> +
>  static void group_init(struct psi_group *group)
>  {
>         int cpu;
> @@ -201,6 +203,8 @@ static void group_init(struct psi_group *group)
>         memset(group->polling_total, 0, sizeof(group->polling_total));
>         group->polling_next_update = ULLONG_MAX;
>         group->polling_until = 0;
> +       init_waitqueue_head(&group->poll_wait);
> +       timer_setup(&group->poll_timer, poll_timer_fn, 0);
>         rcu_assign_pointer(group->poll_task, NULL);
>  }
>
> @@ -1157,9 +1161,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
>                         return ERR_CAST(task);
>                 }
>                 atomic_set(&group->poll_wakeup, 0);
> -               init_waitqueue_head(&group->poll_wait);
>                 wake_up_process(task);
> -               timer_setup(&group->poll_timer, poll_timer_fn, 0);
>                 rcu_assign_pointer(group->poll_task, task);
>         }
>
> @@ -1211,6 +1213,7 @@ static void psi_trigger_destroy(struct kref *ref)
>                                         group->poll_task,
>                                         lockdep_is_held(&group->trigger_lock));
>                         rcu_assign_pointer(group->poll_task, NULL);
> +                       del_timer(&group->poll_timer);
>                 }
>         }
>
> @@ -1223,17 +1226,14 @@ static void psi_trigger_destroy(struct kref *ref)
>          */
>         synchronize_rcu();
>         /*
> -        * Destroy the kworker after releasing trigger_lock to prevent a
> +        * Destroy psimon after releasing trigger_lock to prevent a

Technically this should read as "Stop kthread" instead of "Destroy psimon".

>          * deadlock while waiting for psi_poll_work to acquire trigger_lock
>          */
>         if (task_to_destroy) {
>                 /*
>                  * After the RCU grace period has expired, the worker
>                  * can no longer be found through group->poll_task.
> -                * But it might have been already scheduled before
> -                * that - deschedule it cleanly before destroying it.
>                  */
> -               del_timer_sync(&group->poll_timer);
>                 kthread_stop(task_to_destroy);
>         }
>         kfree(t);
> --
> 1.9.1
>