Re: [PATCH v3] psi: fix race between psi_trigger_create and psimon

[Suren Baghdasaryan]

On Tue, May 18, 2021 at 2:09 AM 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.
>
> 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
>

Please add this line in your description (see my comment in
https://lore.kernel.org/patchwork/patch/1430982/#1627215):

Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger
scheduling mechanism")

Generally, before posting a new version of your patch please follow these rules:
1. Read all the comments you already received and address all of them.
2. If you are unclear or disagree with a comment you need to reply to
it instead of posting a new version.

> Signed-off-by: ziwei.dai <ziwei.dai@xxxxxxxxxx>
> Signed-off-by: ke.wang <ke.wang@xxxxxxxxxx>
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@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
> ---
> ---
>  kernel/sched/psi.c | 29 +++++++++++++++++------------
>  1 file changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index cc25a3c..7b53217 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);
>         }
>
> @@ -1214,16 +1216,8 @@ static void psi_trigger_destroy(struct kref *ref)
>                 }
>         }
>
> -       mutex_unlock(&group->trigger_lock);
> -
> -       /*
> -        * Wait for both *trigger_ptr from psi_trigger_replace and
> -        * poll_task RCUs to complete their read-side critical sections
> -        * before destroying the trigger and optionally the poll_task
> -        */
> -       synchronize_rcu();
>         /*
> -        * Destroy the kworker after releasing trigger_lock to prevent a
> +        * Destroy psimon after releasing trigger_lock to prevent a
>          * deadlock while waiting for psi_poll_work to acquire trigger_lock
>          */
>         if (task_to_destroy) {
> @@ -1233,9 +1227,20 @@ static void psi_trigger_destroy(struct kref *ref)
>                  * But it might have been already scheduled before
>                  * that - deschedule it cleanly before destroying it.
>                  */
> -               del_timer_sync(&group->poll_timer);
> +               del_timer(&group->poll_timer);

Why are you refactoring the code instead of just moving del_timer()
into the trigger_lock protected area like I suggested in
https://lore.kernel.org/patchwork/patch/1429498/#1627196 ? Please
avoid unnecessary complexity and churn.

> +               mutex_unlock(&group->trigger_lock);
>                 kthread_stop(task_to_destroy);
> +       } else {
> +               mutex_unlock(&group->trigger_lock);
>         }
> +
> +       /*
> +        * Wait for both *trigger_ptr from psi_trigger_replace and
> +        * poll_task RCUs to complete their read-side critical sections
> +        * before destroying the trigger and optionally the poll_task
> +        */
> +       synchronize_rcu();
> +
>         kfree(t);
>  }
>
> --
> 1.9.1
>