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SubjectRe: drm_sched with panfrost crash on T820
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Hi,

On 27/09/2019 13:27, Neil Armstrong wrote:
> 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.

First run gave me (while applying on v5.3):
[ 307.969230] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000c0
[...]
[ 308.029358] Hardware name: Khadas VIM2 (DT)
[ 308.033510] Workqueue: events drm_sched_job_timedout
[ 308.038416] pstate: 80000005 (Nzcv daif -PAN -UAO)
[ 308.043160] pc : drm_sched_start+0x88/0x138
[ 308.047300] lr : drm_sched_start+0xb0/0x138
[...]
[ 308.133635] Call trace:
[ 308.136052] drm_sched_start+0x88/0x138
[ 308.139847] panfrost_job_timedout+0x1cc/0x208
[ 308.144245] drm_sched_job_timedout+0x44/0xa8
[ 308.148560] process_one_work+0x1e0/0x320
[ 308.152524] worker_thread+0x40/0x450
[ 308.156149] kthread+0x124/0x128
[ 308.159342] ret_from_fork+0x10/0x18
[ 308.162879] Code: 54000280 f9400862 52800020 aa1a03e1 (f940605b)
[ 308.168914] ---[ end trace 256b7f5faec101d2 ]---

Bad pointer seems to be struct dma_fence *fence = s_job->s_fence->parent that
could be a NULL return from panfrost_job_run().

Neil
>
> 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);
>>
>

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