[PATCH v4] drm/scheduler: Avoid accessing freed bad job.
Alex Deucher
alexdeucher at gmail.com
Thu Feb 6 14:49:51 UTC 2020
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?
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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