Re: [PATCH] sched/fair: WARN and refuse to set buddy when !se->on_rq

[Konstantin Khlebnikov]

On Mon, May 8, 2017 at 10:22 PM, Daniel Axtens <dja@xxxxxxxxxx> wrote:
> If we set a next or last buddy for a se that is not on_rq, we will
> end up taking a NULL pointer dereference in wakeup_preempt_entity
> via pick_next_task_fair.
>
> Detect when we would be about to do that, throw a warning and
> then refuse to actually set it.
>
> This has been suggested at least twice[0][1]: just do it.
>
> [0] https://marc.info/?l=linux-kernel&m=146651668921468&w=2
> [1] https://lkml.org/lkml/2016/6/16/663
>
> Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
>
> ---
>
> I recently had to debug a problem with these (we hadn't backported
> Konstantin's patches in this area) and this would have saved a lot
> of time/pain.

Yep, this is worth debug. But I think it should be under compile-time
config option.
Something like CONFIG_DEBUG_VM which enables VM_BUG_ON and VM_WARN_ON.

> ---
>  kernel/sched/fair.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d71109321841..9b988815b4ad 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6168,8 +6168,11 @@ static void set_last_buddy(struct sched_entity *se)
>         if (entity_is_task(se) && unlikely(task_of(se)->policy == SCHED_IDLE))
>                 return;
>
> -       for_each_sched_entity(se)
> +       for_each_sched_entity(se) {
> +               if (WARN_ON_ONCE(!se->on_rq))
> +                       return;
>                 cfs_rq_of(se)->last = se;
> +       }
>  }
>
>  static void set_next_buddy(struct sched_entity *se)
> @@ -6177,8 +6180,11 @@ static void set_next_buddy(struct sched_entity *se)
>         if (entity_is_task(se) && unlikely(task_of(se)->policy == SCHED_IDLE))
>                 return;
>
> -       for_each_sched_entity(se)
> +       for_each_sched_entity(se) {
> +               if (WARN_ON_ONCE(!se->on_rq))
> +                       return;
>                 cfs_rq_of(se)->next = se;
> +       }
>  }
>
>  static void set_skip_buddy(struct sched_entity *se)
> --
> 2.11.0
>