Konstantin Khlebnikov <[email protected]> writes:
> On Mon, May 8, 2017 at 10:22 PM, Daniel Axtens <[email protected]> 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 <[email protected]>
>> Cc: Ben Segall <[email protected]>
>> Cc: Peter Zijlstra <[email protected]>
>> Cc: Thomas Gleixner <[email protected]>
>> Signed-off-by: Daniel Axtens <[email protected]>
>>
>> ---
>>
>> 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.
OK, will respin.
Regards,
Daniel
>
>> ---
>> 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
>>
