[PATCH v4] drm/scheduler: Avoid accessing freed bad job.
Christian König
christian.koenig at amd.com
Thu Feb 6 14:51:31 UTC 2020
Am 06.02.20 um 15:49 schrieb Alex Deucher:
> On Thu, Feb 6, 2020 at 6:50 AM Christian König
> <ckoenig.leichtzumerken at gmail.com> wrote:
>> Am 06.02.20 um 12:10 schrieb Lucas Stach:
>>> Hi all,
>>>
>>> On Mi, 2020-02-05 at 19:24 +0100, Lucas Stach wrote:
>>>> Hi Andrey,
>>>>
>>>> This commit breaks all drivers, which may bail out of the timeout
>>>> processing as they wish to extend the timeout (etnaviv, v3d).
>>>>
>>>> Those drivers currently just return from the timeout handler before
>>>> calling drm_sched_stop(), which means with this commit applied we are
>>>> removing the first job from the ring_mirror_list, but never put it
>>>> back. This leads to jobs getting lost from the ring mirror, which then
>>>> causes quite a bit of fallout like unsignaled fences.
>>>>
>>>> Not sure yet what to do about it, we can either add a function to add
>>>> the job back to the ring_mirror if the driver wants to extend the
>>>> timeout, or we could look for another way to stop
>>>> drm_sched_cleanup_jobs from freeing jobs that are currently in timeout
>>>> processing.
>>> So after thinking about this a bit more my opinion is that we need to
>>> revert this change for now and go back to the drawing board for the
>>> scheduler timeout handling.
>>>
>>> Right now this starts to feel like a big midlayer mistake with all the
>>> very intricate intertwining between the drivers and the scheduler. The
>>> rules on when it's safe to manipulate the ring mirror and when
>>> completed jobs are signaled and freed are not really well specified.
>>> The fact that we need to mutate state in order to get rid of races
>>> instead of having a single big "timeout processing is owner of the
>>> scheduler state for now" is a big fat warning sign IMHO.
>> Yes, that strongly feels like a hack to me as well. But I didn't had
>> time and still haven't to take a closer look and suggest something better.
>>
> In that case, can someone send me a revert?
Well a revert would break our driver.
The real solution is that somebody needs to sit down, gather ALL the
requirements and then come up with a solution which is clean and works
for everyone.
Christian.
>
> Alex
>
>
>> Christian.
>>
>>> It took me far longer than I'd like to admit to understand the failure
>>> mode with fences not getting signaled after a GPU hang. The back and
>>> forth between scheduler and driver code makes things really hard to
>>> follow.
>>>
>>> Regards,
>>> Lucas
>>>
>>>> Regards,
>>>> Lucas
>>>>
>>>> On Mo, 2019-11-25 at 15:51 -0500, Andrey Grodzovsky wrote:
>>>>> Problem:
>>>>> Due to a race between drm_sched_cleanup_jobs in sched thread and
>>>>> drm_sched_job_timedout in timeout work there is a possiblity that
>>>>> bad job was already freed while still being accessed from the
>>>>> timeout thread.
>>>>>
>>>>> Fix:
>>>>> Instead of just peeking at the bad job in the mirror list
>>>>> remove it from the list under lock and then put it back later when
>>>>> we are garanteed no race with main sched thread is possible which
>>>>> is after the thread is parked.
>>>>>
>>>>> v2: Lock around processing ring_mirror_list in drm_sched_cleanup_jobs.
>>>>>
>>>>> v3: Rebase on top of drm-misc-next. v2 is not needed anymore as
>>>>> drm_sched_get_cleanup_job already has a lock there.
>>>>>
>>>>> v4: Fix comments to relfect latest code in drm-misc.
>>>>>
>>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
>>>>> Reviewed-by: Christian König <christian.koenig at amd.com>
>>>>> Tested-by: Emily Deng <Emily.Deng at amd.com>
>>>>> ---
>>>>> drivers/gpu/drm/scheduler/sched_main.c | 27 +++++++++++++++++++++++++++
>>>>> 1 file changed, 27 insertions(+)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>>>> index 6774955..1bf9c40 100644
>>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>>> @@ -284,10 +284,21 @@ static void drm_sched_job_timedout(struct work_struct *work)
>>>>> unsigned long flags;
>>>>>
>>>>> sched = container_of(work, struct drm_gpu_scheduler, work_tdr.work);
>>>>> +
>>>>> + /* Protects against concurrent deletion in drm_sched_get_cleanup_job */
>>>>> + spin_lock_irqsave(&sched->job_list_lock, flags);
>>>>> job = list_first_entry_or_null(&sched->ring_mirror_list,
>>>>> struct drm_sched_job, node);
>>>>>
>>>>> if (job) {
>>>>> + /*
>>>>> + * Remove the bad job so it cannot be freed by concurrent
>>>>> + * drm_sched_cleanup_jobs. It will be reinserted back after sched->thread
>>>>> + * is parked at which point it's safe.
>>>>> + */
>>>>> + list_del_init(&job->node);
>>>>> + spin_unlock_irqrestore(&sched->job_list_lock, flags);
>>>>> +
>>>>> job->sched->ops->timedout_job(job);
>>>>>
>>>>> /*
>>>>> @@ -298,6 +309,8 @@ static void drm_sched_job_timedout(struct work_struct *work)
>>>>> job->sched->ops->free_job(job);
>>>>> sched->free_guilty = false;
>>>>> }
>>>>> + } else {
>>>>> + spin_unlock_irqrestore(&sched->job_list_lock, flags);
>>>>> }
>>>>>
>>>>> spin_lock_irqsave(&sched->job_list_lock, flags);
>>>>> @@ -370,6 +383,20 @@ void drm_sched_stop(struct drm_gpu_scheduler *sched, struct drm_sched_job *bad)
>>>>> kthread_park(sched->thread);
>>>>>
>>>>> /*
>>>>> + * Reinsert back the bad job here - now it's safe as
>>>>> + * drm_sched_get_cleanup_job cannot race against us and release the
>>>>> + * bad job at this point - we parked (waited for) any in progress
>>>>> + * (earlier) cleanups and drm_sched_get_cleanup_job will not be called
>>>>> + * now until the scheduler thread is unparked.
>>>>> + */
>>>>> + if (bad && bad->sched == sched)
>>>>> + /*
>>>>> + * Add at the head of the queue to reflect it was the earliest
>>>>> + * job extracted.
>>>>> + */
>>>>> + list_add(&bad->node, &sched->ring_mirror_list);
>>>>> +
>>>>> + /*
>>>>> * Iterate the job list from later to earlier one and either deactive
>>>>> * their HW callbacks or remove them from mirror list if they already
>>>>> * signaled.
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