[PATCH 2/2] drm/scheduler: Remove obsolete spinlock.

[Christian König]

drm_sched_fence_create() assigns a sequence number to the fence it creates.

Now drm_sched_fence_create() is called by drm_sched_job_init() to 
initialize the jobs we want to push on the scheduler queue.

When you now call drm_sched_entity_push_job() without a protecting lock 
it can happen that you push two (or even more) job with reversed 
sequence numbers.

Since the sequence numbers are used to determine job completion order 
reversing them can seriously mess things up.

So the spin lock should be superfluous, if it isn't we have a much 
larger bug we need to fix.

Christian.

Am 16.05.2018 um 13:15 schrieb Andrey Grodzovsky:
> Can you please elaborate more, maybe give an example - I don't 
> understand yet the problematic scenario.
>
> Andrey
>
>
> On 05/16/2018 02:50 AM, Christian König wrote:
>> No, that spinlock is indeed incorrect. I
>>
>> See even when we protect the spsc queue with a spinlock that doesn't 
>> make it correct. It can happen that the jobs pushed to the queue are 
>> reversed in their sequence order and that can cause severe problems 
>> in the memory management.
>>
>> Christian.
>>
>> Am 16.05.2018 um 05:33 schrieb Grodzovsky, Andrey:
>>> Yeah, that what I am not sure about... It's lockless in a sense of 
>>> single producer single consumer but not for multiple concurrent 
>>> producers... So now I think this spinlock should stay there... It 
>>> just looked useless to me at first sight...
>>>
>>> Andrey
>>>
>>> ________________________________________
>>> From: Zhou, David(ChunMing)
>>> Sent: 15 May 2018 23:04:44
>>> To: Grodzovsky, Andrey; amd-gfx at lists.freedesktop.org; 
>>> dri-devel at lists.freedesktop.org
>>> Cc: Koenig, Christian
>>> Subject: Re: [PATCH 2/2] drm/scheduler: Remove obsolete spinlock.
>>>
>>>
>>>
>>> On 2018年05月16日 03:31, Andrey Grodzovsky wrote:
>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
>>>> ---
>>>>    drivers/gpu/drm/scheduler/gpu_scheduler.c | 4 ----
>>>>    include/drm/gpu_scheduler.h               | 1 -
>>>>    2 files changed, 5 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/scheduler/gpu_scheduler.c 
>>>> b/drivers/gpu/drm/scheduler/gpu_scheduler.c
>>>> index 1f1dd70..2569a63 100644
>>>> --- a/drivers/gpu/drm/scheduler/gpu_scheduler.c
>>>> +++ b/drivers/gpu/drm/scheduler/gpu_scheduler.c
>>>> @@ -140,7 +140,6 @@ int drm_sched_entity_init(struct 
>>>> drm_gpu_scheduler *sched,
>>>>        entity->last_scheduled = NULL;
>>>>
>>>>        spin_lock_init(&entity->rq_lock);
>>>> -     spin_lock_init(&entity->queue_lock);
>>>>        spsc_queue_init(&entity->job_queue);
>>>>
>>>>        atomic_set(&entity->fence_seq, 0);
>>>> @@ -424,11 +423,8 @@ void drm_sched_entity_push_job(struct 
>>>> drm_sched_job *sched_job,
>>>>
>>>>        trace_drm_sched_job(sched_job, entity);
>>>>
>>>> -     spin_lock(&entity->queue_lock);
>>>>        first = spsc_queue_push(&entity->job_queue, 
>>>> &sched_job->queue_node);
>>>>
>>>> -     spin_unlock(&entity->queue_lock);
>>> Is your spsc safely to be added simultaneously?
>>>
>>> Regards,
>>> David Zhou
>>>> -
>>>>        /* first job wakes up scheduler */
>>>>        if (first) {
>>>>                /* Add the entity to the run queue */
>>>> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
>>>> index 350a62c..683eb65 100644
>>>> --- a/include/drm/gpu_scheduler.h
>>>> +++ b/include/drm/gpu_scheduler.h
>>>> @@ -56,7 +56,6 @@ struct drm_sched_entity {
>>>>        spinlock_t                      rq_lock;
>>>>        struct drm_gpu_scheduler        *sched;
>>>>
>>>> -     spinlock_t                      queue_lock;
>>>>        struct spsc_queue               job_queue;
>>>>
>>>>        atomic_t                        fence_seq;
>>
>
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