linux-kernel - Re: [PATCH v2] sched/fair: WARN and refuse to set buddy when !se->on

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Date:   Tue, 20 Jun 2017 10:41:38 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Daniel Axtens <dja@...ens.net>, linux-kernel@...r.kernel.org,
        peterz@...radead.org, mingo@...hat.com
Cc:     Ben Segall <bsegall@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] sched/fair: WARN and refuse to set buddy when
 !se->on_rq



On 19.06.2017 17:05, Daniel Axtens wrote:
> Hi Konstantin and Peter,
> 
> Just checking if this version was OK with you - I hadn't heard anything
> and I noticed it's not in -next so I just wanted to check to see if
> there were any other changes you wanted.

Looks good for me. Exactly that debug helped me alot at that time.

> 
> Regards,
> Daniel
> 
> 
> Daniel Axtens <dja@...ens.net> writes:
> 
>> 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@...dex-team.ru>
>> Cc: Ben Segall <bsegall@...gle.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Signed-off-by: Daniel Axtens <dja@...ens.net>
>>
>> ---
>>
>> 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;
>> +	}
>>   }
>>   
>>   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 (SCHED_WARN_ON(!se->on_rq))
>> +			return;
>>   		cfs_rq_of(se)->next = se;
>> +	}
>>   }
>>   
>>   static void set_skip_buddy(struct sched_entity *se)
>> -- 
>> 2.11.0