[PATCH 1/3] drm/sched: Document run_job() refcount hazard

[Christian König]

Am 09.01.25 um 14:37 schrieb Philipp Stanner:
> From: Philipp Stanner <pstanner at redhat.com>
>
> drm_sched_backend_ops.run_job() returns a dma_fence for the scheduler.
> That fence is signalled by the driver once the hardware completed the
> associated job. The scheduler does not increment the reference count on
> that fence, but implicitly expects to inherit this fence from run_job().
>
> This is relatively subtle and prone to misunderstandings.
>
> This implies that, to keep a reference for itself, a driver needs to
> call dma_fence_get() in addition to dma_fence_init() in that callback.
>
> It's further complicated by the fact that the scheduler even decrements
> the refcount in drm_sched_run_job_work() since it created a new
> reference in drm_sched_fence_scheduled(). It does, however, still use
> its pointer to the fence after calling dma_fence_put() - which is safe
> because of the aforementioned new reference, but actually still violates
> the refcounting rules.
>
> Improve the explanatory comment for that decrement.
>
> Move the call to dma_fence_put() to the position behind the last usage
> of the fence.
>
> Document the necessity to increment the reference count in
> drm_sched_backend_ops.run_job().
>
> Signed-off-by: Philipp Stanner <pstanner at redhat.com>
> ---
>   drivers/gpu/drm/scheduler/sched_main.c | 10 +++++++---
>   include/drm/gpu_scheduler.h            | 19 +++++++++++++++----
>   2 files changed, 22 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 57da84908752..5f46c01eb01e 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -1218,15 +1218,19 @@ static void drm_sched_run_job_work(struct work_struct *w)
>   	drm_sched_fence_scheduled(s_fence, fence);
>   
>   	if (!IS_ERR_OR_NULL(fence)) {
> -		/* Drop for original kref_init of the fence */
> -		dma_fence_put(fence);
> -
>   		r = dma_fence_add_callback(fence, &sched_job->cb,
>   					   drm_sched_job_done_cb);
>   		if (r == -ENOENT)
>   			drm_sched_job_done(sched_job, fence->error);
>   		else if (r)
>   			DRM_DEV_ERROR(sched->dev, "fence add callback failed (%d)\n", r);
> +
> +		/*
> +		 * s_fence took a new reference to fence in the call to
> +		 * drm_sched_fence_scheduled() above. The reference passed by
> +		 * run_job() above is now not needed any longer. Drop it.
> +		 */
> +		dma_fence_put(fence);
>   	} else {
>   		drm_sched_job_done(sched_job, IS_ERR(fence) ?
>   				   PTR_ERR(fence) : 0);
> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
> index 95e17504e46a..d5cd2a78f27c 100644
> --- a/include/drm/gpu_scheduler.h
> +++ b/include/drm/gpu_scheduler.h
> @@ -420,10 +420,21 @@ struct drm_sched_backend_ops {
>   					 struct drm_sched_entity *s_entity);
>   
>   	/**
> -         * @run_job: Called to execute the job once all of the dependencies
> -         * have been resolved.  This may be called multiple times, if
> -	 * timedout_job() has happened and drm_sched_job_recovery()
> -	 * decides to try it again.
> +	 * @run_job: Called to execute the job once all of the dependencies
> +	 * have been resolved. This may be called multiple times, if
> +	 * timedout_job() has happened and drm_sched_job_recovery() decides to
> +	 * try it again.

I just came to realize that this hack with calling run_job multiple 
times won't work any more with this patch here.

The background is that you can't allocate memory for a newly returned 
fence and as far as I know no driver pre allocates multiple HW fences 
for a job.

So at least amdgpu used to re-use the same HW fence as return over and 
over again, just re-initialized the reference count. I removed that hack 
from amdgpu, but just FYI it could be that other driver did the same.

Apart from that concern I think that this patch is really the right 
thing and that driver hacks relying on the order of dropping references 
are fundamentally broken approaches.

So feel free to add Reviewed-by: Christian König <christian.koenig at amd.com>.

Regards,
Christian.

> +	 *
> +	 * @sched_job: the job to run
> +	 *
> +	 * Returns: dma_fence the driver must signal once the hardware has
> +	 *	completed the job ("hardware fence").
> +	 *
> +	 * Note that the scheduler expects to 'inherit' its own reference to
> +	 * this fence from the callback. It does not invoke an extra
> +	 * dma_fence_get() on it. Consequently, this callback must take a
> +	 * reference for the scheduler, and additional ones for the driver's
> +	 * respective needs.
>   	 */
>   	struct dma_fence *(*run_job)(struct drm_sched_job *sched_job);
>