linux-kernel - Re: [PATCH] drm: Don't free jobs in wait_event

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Message-ID: <078332cf-ef58-5f76-5c49-8034435f7bea@arm.com>
Date:   Wed, 25 Sep 2019 17:00:45 +0100
From:   Steven Price <steven.price@....com>
To:     "Grodzovsky, Andrey" <Andrey.Grodzovsky@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        "Koenig, Christian" <Christian.Koenig@....com>
Cc:     "Deucher, Alexander" <Alexander.Deucher@....com>,
        Nayan Deshmukh <nayan26deshmukh@...il.com>,
        Sharat Masetty <smasetty@...eaurora.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] drm: Don't free jobs in wait_event_interruptible()

On 25/09/2019 16:56, Grodzovsky, Andrey wrote:
> On 9/25/19 11:14 AM, Steven Price wrote:
> 
>> drm_sched_cleanup_jobs() attempts to free finished jobs, however because
>> it is called as the condition of wait_event_interruptible() it must not
>> sleep. Unfortunately some free callbacks (notably for Panfrost) do sleep.
>>
>> Instead let's rename drm_sched_cleanup_jobs() to
>> drm_sched_get_cleanup_job() and simply return a job for processing if
>> there is one. The caller can then call the free_job() callback outside
>> the wait_event_interruptible() where sleeping is possible before
>> re-checking and returning to sleep if necessary.
>>
>> Signed-off-by: Steven Price <steven.price@....com>
>> ---
>>   drivers/gpu/drm/scheduler/sched_main.c | 44 ++++++++++++++------------
>>   1 file changed, 24 insertions(+), 20 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>> index 9a0ee74d82dc..0ed4aaa4e6d1 100644
>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> @@ -622,43 +622,41 @@ static void drm_sched_process_job(struct dma_fence *f, struct dma_fence_cb *cb)
>>   }
>>   
>>   /**
>> - * drm_sched_cleanup_jobs - destroy finished jobs
>> + * drm_sched_get_cleanup_job - fetch the next finished job to be destroyed
>>    *
>>    * @sched: scheduler instance
>>    *
>> - * Remove all finished jobs from the mirror list and destroy them.
>> + * Returns the next finished job from the mirror list (if there is one)
>> + * ready for it to be destroyed.
>>    */
>> -static void drm_sched_cleanup_jobs(struct drm_gpu_scheduler *sched)
>> +static struct drm_sched_job *
>> +drm_sched_get_cleanup_job(struct drm_gpu_scheduler *sched)
>>   {
>> +	struct drm_sched_job *job = NULL;
>>   	unsigned long flags;
>>   
>>   	/* Don't destroy jobs while the timeout worker is running */
>>   	if (sched->timeout != MAX_SCHEDULE_TIMEOUT &&
>>   	    !cancel_delayed_work(&sched->work_tdr))
>> -		return;
>> -
>> -
>> -	while (!list_empty(&sched->ring_mirror_list)) {
>> -		struct drm_sched_job *job;
>> +		return NULL;
>>   
>> -		job = list_first_entry(&sched->ring_mirror_list,
>> +	job = list_first_entry_or_null(&sched->ring_mirror_list,
>>   				       struct drm_sched_job, node);
>> -		if (!dma_fence_is_signaled(&job->s_fence->finished))
>> -			break;
>>   
>> -		spin_lock_irqsave(&sched->job_list_lock, flags);
>> +	spin_lock_irqsave(&sched->job_list_lock, flags);
>> +
>> +	if (job && dma_fence_is_signaled(&job->s_fence->finished)) {
>>   		/* remove job from ring_mirror_list */
>>   		list_del_init(&job->node);
>> -		spin_unlock_irqrestore(&sched->job_list_lock, flags);
>> -
>> -		sched->ops->free_job(job);
>> +	} else {
>> +		job = NULL;
>> +		/* queue timeout for next job */
>> +		drm_sched_start_timeout(sched);
>>   	}
>>   
>> -	/* queue timeout for next job */
>> -	spin_lock_irqsave(&sched->job_list_lock, flags);
>> -	drm_sched_start_timeout(sched);
>>   	spin_unlock_irqrestore(&sched->job_list_lock, flags);
>>   
>> +	return job;
>>   }
>>   
>>   /**
>> @@ -698,12 +696,18 @@ static int drm_sched_main(void *param)
>>   		struct drm_sched_fence *s_fence;
>>   		struct drm_sched_job *sched_job;
>>   		struct dma_fence *fence;
>> +		struct drm_sched_job *cleanup_job = NULL;
>>   
>>   		wait_event_interruptible(sched->wake_up_worker,
>> -					 (drm_sched_cleanup_jobs(sched),
>> +					 (cleanup_job = drm_sched_get_cleanup_job(sched)) ||
>>   					 (!drm_sched_blocked(sched) &&
>>   					  (entity = drm_sched_select_entity(sched))) ||
>> -					 kthread_should_stop()));
>> +					 kthread_should_stop());
> 
> 
> Can't we just call drm_sched_cleanup_jobs right here, remove all the 
> conditions in wait_event_interruptible (make it always true) and after 
> drm_sched_cleanup_jobs is called test for all those conditions and 
> return to sleep if they evaluate to false ? drm_sched_cleanup_jobs is 
> called unconditionally inside wait_event_interruptible anyway... This is 
> more of a question to Christian.

Christian may know better than me, but I think those conditions need to
be in wait_event_interruptible() to avoid race conditions. If we simply
replace all the conditions with a literal "true" then
wait_event_interruptible() will never actually sleep.

Steve