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Subject: Re: drm_sched with panfrost crash on T820
To:     Steven Price <steven.price@arm.com>, daniel@ffwll.ch,
        airlied@linux.ie,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
Cc:     Tomeu Vizoso <tomeu.vizoso@collabora.com>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        "open list:ARM/Amlogic Meson..." <linux-amlogic@lists.infradead.org>,
        Erico Nunes <nunes.erico@gmail.com>,
        Rob Herring <robh@kernel.org>
References: <e450fbe6-dec7-2704-59c2-db7e869d67f5@baylibre.com>
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 <26ae2a4d-8df1-e8db-3060-41638ed63e2a@arm.com>
From:   Neil Armstrong <narmstrong@baylibre.com>
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Organization: Baylibre
Message-ID: <1eec2f1b-0467-cd4d-aa22-23c70388ac0f@baylibre.com>
Date:   Fri, 27 Sep 2019 13:27:23 +0200
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In-Reply-To: <26ae2a4d-8df1-e8db-3060-41638ed63e2a@arm.com>
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Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi Steven,

Thanks for your prompt reaction !

On 27/09/2019 12:48, Steven Price wrote:
> On 27/09/2019 10:55, Steven Price wrote:
> [...]
>> One obvious issue with the DRM scheduler is that there is a call to
>> cancel_delayed_work() in drm_sched_stop() which to me looks like it
>> should be cancel_delayed_work_sync() to ensure that the timeout handling
>> has completed.
>>
>> However in the above scenario a _sync() variety would then cause a
>> deadlock (one thread has pfdev->reset_lock and is waiting for the other
>> thread which is trying to take the lock).
>>
>> So we need to update Panfrost so that it can coordinate the reset
>> between schedulers. Can you try something like the following (untested):
> 
> And actually testing it I of course discover it doesn't quite work. We
> do need the cancel_delayed_work_sync() in the DRM scheduler (when
> stopping a different scheduler) and we need to avoid holding the
> reset_lock during the drm_sched_stop() call to prevent deadlocking with
> another thread handling a timeout.

Yep the first patch wasn't fixing the issue all the time.

> 
> Can you give the following patch a spin? I don't have a great
> reproduction case, so it would be good to get some confidence it fixes
> the problem.

Running it right now.

Thanks,
Neil

> 
> ----8<----
> From 521a286789260197ae94f698932ebf369efc45ad Mon Sep 17 00:00:00 2001
> From: Steven Price <steven.price@arm.com>
> Date: Fri, 27 Sep 2019 11:42:40 +0100
> Subject: [PATCH] drm/panfrost: Handle resetting on timeout better
> 
> Panfrost uses multiple schedulers (one for each slot, so 2 in reality),
> and on a timeout has to stop all the schedulers to safely perform a
> reset. However more than one scheduler can trigger a timeout at the same
> time. This race condition results in jobs being freed while they are
> still in use.
> 
> Modify drm_sched_stop() to call cancel_delayed_work_sync() when stopping
> a different scheduler to the one belonging to the passed in job.
> panfrost_job_timedout() is also modified to only allow one thread at a
> time to handle the reset. Any subsequent threads simply return assuming
> that the first thread will handle the situation.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
>  drivers/gpu/drm/panfrost/panfrost_device.h |  2 ++
>  drivers/gpu/drm/panfrost/panfrost_job.c    | 11 ++++++++++-
>  drivers/gpu/drm/scheduler/sched_main.c     |  5 ++++-
>  3 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_device.h b/drivers/gpu/drm/panfrost/panfrost_device.h
> index f503c566e99f..6441c7fba6c4 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_device.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_device.h
> @@ -99,6 +99,8 @@ struct panfrost_device {
>  		unsigned long cur_volt;
>  		struct panfrost_devfreq_slot slot[NUM_JOB_SLOTS];
>  	} devfreq;
> +
> +	bool is_resetting;
>  };
>  
>  struct panfrost_mmu {
> diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c b/drivers/gpu/drm/panfrost/panfrost_job.c
> index 05c85f45a0de..1b2019e08b43 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_job.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_job.c
> @@ -388,13 +388,21 @@ static void panfrost_job_timedout(struct drm_sched_job *sched_job)
>  
>  	mutex_lock(&pfdev->reset_lock);
>  
> +	if (pfdev->is_resetting) {
> +		mutex_unlock(&pfdev->reset_lock);
> +		return;
> +	}
> +	pfdev->is_resetting = true;
> +
> +	mutex_unlock(&pfdev->reset_lock);
> +
>  	for (i = 0; i < NUM_JOB_SLOTS; i++)
>  		drm_sched_stop(&pfdev->js->queue[i].sched, sched_job);
>  
>  	if (sched_job)
>  		drm_sched_increase_karma(sched_job);
>  
> -	/* panfrost_core_dump(pfdev); */
> +	mutex_lock(&pfdev->reset_lock);
>  
>  	panfrost_devfreq_record_transition(pfdev, js);
>  	panfrost_device_reset(pfdev);
> @@ -406,6 +414,7 @@ static void panfrost_job_timedout(struct drm_sched_job *sched_job)
>  	for (i = 0; i < NUM_JOB_SLOTS; i++)
>  		drm_sched_start(&pfdev->js->queue[i].sched, true);
>  
> +	pfdev->is_resetting = false;
>  	mutex_unlock(&pfdev->reset_lock);
>  }
>  
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 148468447ba9..bc6d1862ec8a 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -415,7 +415,10 @@ void drm_sched_stop(struct drm_gpu_scheduler *sched, struct drm_sched_job *bad)
>  	 * this TDR finished and before the newly restarted jobs had a
>  	 * chance to complete.
>  	 */
> -	cancel_delayed_work(&sched->work_tdr);
> +	if (bad->sched != sched)
> +		cancel_delayed_work_sync(&sched->work_tdr);
> +	else
> +		cancel_delayed_work(&sched->work_tdr);
>  }
>  
>  EXPORT_SYMBOL(drm_sched_stop);
>