Re: [PATCH] sched/fair: Fix dereference NULL sched domain during select_idle_sibling

[Wanpeng Li <kernellwp@gmail.com>]

2016-10-09 1:06 GMT+08:00 Peter Zijlstra <peterz@infradead.org>:
> On Sat, Oct 08, 2016 at 06:24:38PM +0800, Wanpeng Li wrote:
>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 543b2f2..03a6620 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5472,19 +5472,29 @@ static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd
>>   */
>>  static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
>>  {
>> -     struct sched_domain *this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>
> So select_idle_cpu() <- select_idle_sibling() is called from two places,
> both which already hold rcu_read_lock() afaict.

Agreed.

>
> This would've insta-triggered a rcu-lockdep splat otherwise I think.

CONFIG_LOCKDEP_SUPPORT=y
CONFIG_LOCKDEP=y
CONFIG_DEBUG_LOCKDEP=y
CONFIG_DEBUG_LOCK_ALLOC=y

So it is interesting why not a rcu-lockdep splat. :)

>
> That is, selsect_task_rq_fair() has rcu_read_lock() taken when calling
> this, and task_numa_compare() does too.
>
>> +     struct sched_domain *this_sd;
>>       u64 avg_idle = this_rq()->avg_idle;
>> -     u64 avg_cost = this_sd->avg_scan_cost;
>> +     u64 avg_cost;
>>       u64 time, cost;
>>       s64 delta;
>>       int cpu, wrap;
>>
>> +     rcu_read_lock();
>> +     this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>> +     if (!this_sd) {
>> +             cpu = -1;
>> +             goto unlock;
>> +     }
>
> Yes, this is the part that was missing. We need to test this_sd after
> the lookup.
>
> Thanks for looking at this!

NP, I will send out v2 soon. :)

Regards,
Wanpeng Li