[PATCH 2/2] drm/scheduler: stop setting rq to NULL

[Andrey Grodzovsky]

On 08/06/2018 08:44 AM, Christian König wrote:
> Am 03.08.2018 um 16:54 schrieb Andrey Grodzovsky:
>>
>> [SNIP]
>>
>>>>
>>>>>
>>>>>     >
>>>>>     > Second of all, even after we removed the entity from rq in
>>>>>     > drm_sched_entity_flush to terminate any subsequent submissions
>>>>>     >
>>>>>     > to the queue the other thread doing push job can just
>>>>>     acquire again a
>>>>>     > run queue
>>>>>     >
>>>>>     > from drm_sched_entity_get_free_sched and continue submission
>>>>>
>>>>> Hi Christian
>>>>>
>>>>>     That is actually desired.
>>>>>
>>>>>     When another process is now using the entity to submit jobs
>>>>>     adding it
>>>>>     back to the rq is actually the right thing to do cause the
>>>>>     entity is
>>>>>     still in use.
>>>>>
>>>>
>>>> Yes, no problem if it's another process. But what about another 
>>>> thread from same process ? Is it a possible use case that 2 threads 
>>>> from same process submit to same entity concurrently ? If so and we 
>>>> specifically kill one, the other will not stop event if we want him 
>>>> to because current code makes him just require a rq for him self.
>>>
>>> Well you can't kill a single thread of a process (you can only 
>>> interrupt it), a SIGKILL will always kill the whole process.
>>
>> Is the following scenario possible and acceptable ?
>> 2 threads from same process working on same queue where thread A 
>> currently in drm_sched_entity_flush->wait_event_timeout (the process 
>> getting shut down because of SIGKILL sent by user)
>> while thread B still inside drm_sched_entity_push_job before 'if 
>> (reschedule)'. 'A' stopped waiting because queue became empty and 
>> then removes the entity queue from scheduler's run queue while
>> B goes inside 'reschedule' because it evaluates to true ('first' is 
>> true and all the rest of the conditions), acquires new rq, and later 
>> adds it back to scheduler (different one maybe) and keeps submitting 
>> jobs as much as he likes and then can be stack for up to 'timeout' 
>> time  in his drm_sched_entity_flush waiting for them.
>
> I'm not 100% sure but I don't think that can happen.
>
> See flushing the fd is done while dropping the fd, which happens only 
> after all threads of the process in question are killed.

Yea, this FDs handling is indeed a lot of gray area for me but as far as 
I remember flushing is done per each thread when exits (possibly due to 
a signal).
Now signals interception and processing (as a result of which .flush 
will get called if SIGKILL received) is done in some points amongst 
which is when returning from IOCTL.
So if first thread was at the very end of the CS ioctl when SIGKILL was 
received while the other one at the beginning  then I think we might see 
something like the scenario above.

Andrey

>
> Otherwise the flushing wouldn't make to much sense. In other words 
> imagine an application where a thread does a write() on a fd which is 
> killed.
>
> The idea of the flush is to preserve the data and that won't work if 
> that isn't correctly ordered.
>
>> My understanding was that introduction of entity->last is to force 
>> immediate termination job submissions by any thread from the 
>> terminating process.
>
> We could consider reordering that once more. Going to play out all 
> scenarios in my head over the weekend :)
>
> Christian.
>
>>
>> Andrey
>

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