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

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Date:   Wed, 25 Sep 2019 16:07:34 +0000
From:   "Grodzovsky, Andrey" <Andrey.Grodzovsky@....com>
To:     Steven Price <steven.price@....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 9/25/19 12:00 PM, Steven Price wrote:

> 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

Yes you right, it won't work as I missed that condition is evaluated as 
first step in wait_event_interruptible before it sleeps.

Andrey