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

[Daniel Axtens]

Hi Ingo,

Good catch - thanks for picking that up and taking the patch!

Regards,
Daniel
> * 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
>
> ========================>
> From 6d3aed3d8a0573d0a6eb1160ccd0a0713f4dbc2f Mon Sep 17 00:00:00 2001
> From: Ingo Molnar <mingo@xxxxxxxxxx>
> Date: Tue, 20 Jun 2017 12:24:42 +0200
> Subject: [PATCH] sched/debug: Fix SCHED_WARN_ON() to return a value on !CONFIG_SCHED_DEBUG as well
>
> This definition of SCHED_WARN_ON():
>
>  #define SCHED_WARN_ON(x)        ((void)(x))
>
> is not fully compatible with the 'real' WARN_ON_ONCE() primitive, as it
> has no return value, so it cannot be used in conditionals.
>
> Fix it.
>
> Cc: Daniel Axtens <dja@xxxxxxxxxx>
> Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Mike Galbraith <efault@xxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  kernel/sched/sched.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index f2ef759a4cb6..e0329d10bdb8 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -39,9 +39,9 @@
>  #include "cpuacct.h"
>  
>  #ifdef CONFIG_SCHED_DEBUG
> -#define SCHED_WARN_ON(x)	WARN_ONCE(x, #x)
> +# define SCHED_WARN_ON(x)	WARN_ONCE(x, #x)
>  #else
> -#define SCHED_WARN_ON(x)	((void)(x))
> +# define SCHED_WARN_ON(x)	({ (void)(x), 0; })
>  #endif
>  
>  struct rq;