[PATCH] sched/debug: Fix SCHED_WARN_ON() to return a value on !CONFIG_SCHED_DEBUG as well

[Ingo Molnar]

* 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.
> 
> v2: use SCHED_WARN_ON to restrict when the test is run. This is a
>     macro for WARN_ON_ONCE, which is convenient.
> ---
>  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..44b94cfe02cb 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 (SCHED_WARN_ON(!se->on_rq))
> +			return;
>  		cfs_rq_of(se)->last = se;
> +	}

This won't build in the !CONFIG_SCHED_DEBUG case, because of the naive definition 
in sched.h:

#define SCHED_WARN_ON(x)        ((void)(x))

That should be changed to something like:

#define SCHED_WARN_ON(x)        ((void)(x))

I've applied the fix below. (untested at the moment)

Thanks,

	Ingo

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