[PATCH v6] drm/sched: Make sure we wait for all dependencies in kill_jobs_cb()

[Luben Tuikov]

On 2023-06-19 03:19, Boris Brezillon wrote:
> drm_sched_entity_kill_jobs_cb() logic is omitting the last fence popped
> from the dependency array that was waited upon before
> drm_sched_entity_kill() was called (drm_sched_entity::dependency field),
> so we're basically waiting for all dependencies except one.
> 
> In theory, this wait shouldn't be needed because resources should have
> their users registered to the dma_resv object, thus guaranteeing that
> future jobs wanting to access these resources wait on all the previous
> users (depending on the access type, of course). But we want to keep
> these explicit waits in the kill entity path just in case.
> 
> Let's make sure we keep all dependencies in the array in
> drm_sched_job_dependency(), so we can iterate over the array and wait
> in drm_sched_entity_kill_jobs_cb().
> 
> We also make sure we wait on drm_sched_fence::finished if we were
> originally asked to wait on drm_sched_fence::scheduled. In that case,
> we assume the intent was to delegate the wait to the firmware/GPU or
> rely on the pipelining done at the entity/scheduler level, but when
> killing jobs, we really want to wait for completion not just scheduling.
> 
> v6:
> - Back to v4 implementation
> - Add Christian's R-b
> 
> v5:
> - Flag deps on which we should only wait for the scheduled event
>   at insertion time
> 
> v4:
> - Fix commit message
> - Fix a use-after-free bug
> 
> v3:
> - Always wait for drm_sched_fence::finished fences in
>   drm_sched_entity_kill_jobs_cb() when we see a sched_fence
> 
> v2:
> - Don't evict deps in drm_sched_job_dependency()

Hmm, why is this in reverse chronological order?
It's very confusing.

> 
> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
> Suggested-by: "Christian König" <christian.koenig at amd.com>
> Reviewed-by: "Christian König" <christian.koenig at amd.com>

These three lines would usually come after the CCs.

Regards,
Luben

> Cc: Frank Binns <frank.binns at imgtec.com>
> Cc: Sarah Walker <sarah.walker at imgtec.com>
> Cc: Donald Robson <donald.robson at imgtec.com>
> Cc: Luben Tuikov <luben.tuikov at amd.com>
> Cc: David Airlie <airlied at gmail.com>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Sumit Semwal <sumit.semwal at linaro.org>
> Cc: "Christian König" <christian.koenig at amd.com>
> ---
>  drivers/gpu/drm/scheduler/sched_entity.c | 41 +++++++++++++++++++-----
>  1 file changed, 33 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
> index 68e807ae136a..ec41d82d0141 100644
> --- a/drivers/gpu/drm/scheduler/sched_entity.c
> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
> @@ -176,16 +176,32 @@ static void drm_sched_entity_kill_jobs_cb(struct dma_fence *f,
>  {
>  	struct drm_sched_job *job = container_of(cb, struct drm_sched_job,
>  						 finish_cb);
> -	int r;
> +	unsigned long index;
>  
>  	dma_fence_put(f);
>  
>  	/* Wait for all dependencies to avoid data corruptions */
> -	while (!xa_empty(&job->dependencies)) {
> -		f = xa_erase(&job->dependencies, job->last_dependency++);
> -		r = dma_fence_add_callback(f, &job->finish_cb,
> -					   drm_sched_entity_kill_jobs_cb);
> -		if (!r)
> +	xa_for_each(&job->dependencies, index, f) {
> +		struct drm_sched_fence *s_fence = to_drm_sched_fence(f);
> +
> +		if (s_fence && f == &s_fence->scheduled) {
> +			/* The dependencies array had a reference on the scheduled
> +			 * fence, and the finished fence refcount might have
> +			 * dropped to zero. Use dma_fence_get_rcu() so we get
> +			 * a NULL fence in that case.
> +			 */
> +			f = dma_fence_get_rcu(&s_fence->finished);
> +
> +			/* Now that we have a reference on the finished fence,
> +			 * we can release the reference the dependencies array
> +			 * had on the scheduled fence.
> +			 */
> +			dma_fence_put(&s_fence->scheduled);
> +		}
> +
> +		xa_erase(&job->dependencies, index);
> +		if (f && !dma_fence_add_callback(f, &job->finish_cb,
> +						 drm_sched_entity_kill_jobs_cb))
>  			return;
>  
>  		dma_fence_put(f);
> @@ -415,8 +431,17 @@ static struct dma_fence *
>  drm_sched_job_dependency(struct drm_sched_job *job,
>  			 struct drm_sched_entity *entity)
>  {
> -	if (!xa_empty(&job->dependencies))
> -		return xa_erase(&job->dependencies, job->last_dependency++);
> +	struct dma_fence *f;
> +
> +	/* We keep the fence around, so we can iterate over all dependencies
> +	 * in drm_sched_entity_kill_jobs_cb() to ensure all deps are signaled
> +	 * before killing the job.
> +	 */
> +	f = xa_load(&job->dependencies, job->last_dependency);
> +	if (f) {
> +		job->last_dependency++;
> +		return dma_fence_get(f);
> +	}
>  
>  	if (job->sched->ops->prepare_job)
>  		return job->sched->ops->prepare_job(job, entity);