linux-kernel - Re: [PATCH v2] drm/sched: Document race condition in drm_sched

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Message-Id: <DC1BGCY1JPKJ.7BHDGBYZDYMZ@kernel.org>
Date: Wed, 13 Aug 2025 14:58:45 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Philipp Stanner" <phasta@...nel.org>
Cc: "Matthew Brost" <matthew.brost@...el.com>,
 Christian König <ckoenig.leichtzumerken@...il.com>,
 "Maarten Lankhorst" <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard"
 <mripard@...nel.org>, "Thomas Zimmermann" <tzimmermann@...e.de>, "David
 Airlie" <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>,
 <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>, "James
 Flowers" <bold.zone2373@...tmail.com>
Subject: Re: [PATCH v2] drm/sched: Document race condition in
 drm_sched_fini()

On Wed Aug 13, 2025 at 10:56 AM CEST, Philipp Stanner wrote:
> In drm_sched_fini() all entities are marked as stopped - without taking
> the appropriate lock, because that would deadlock. That means that
> drm_sched_fini() and drm_sched_entity_push_job() can race against each
> other.
>
> This should most likely be fixed by establishing the rule that all
> entities associated with a scheduler must be torn down first. Then,
> however, the locking should be removed from drm_sched_fini() alltogether
> with an appropriate comment.
>
> Reported-by: James Flowers <bold.zone2373@...tmail.com>
> Link: https://lore.kernel.org/dri-devel/20250720235748.2798-1-bold.zone2373@fastmail.com/
> Signed-off-by: Philipp Stanner <phasta@...nel.org>
> ---
> Changes in v2:
>   - Fix typo.
> ---
>  drivers/gpu/drm/scheduler/sched_main.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 5a550fd76bf0..46119aacb809 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -1424,6 +1424,22 @@ void drm_sched_fini(struct drm_gpu_scheduler *sched)
>  			 * Prevents reinsertion and marks job_queue as idle,
>  			 * it will be removed from the rq in drm_sched_entity_fini()
>  			 * eventually
> +			 *
> +			 * FIXME:
> +			 * This lacks the proper spin_lock(&s_entity->lock) and
> +			 * is, therefore, a race condition. Most notably, it
> +			 * can race with drm_sched_entity_push_job(). The lock
> +			 * cannot be taken here, however, because this would
> +			 * lead to lock inversion -> deadlock.
> +			 *
> +			 * The best solution probably is to enforce the life
> +			 * time rule of all entities having to be torn down
> +			 * before their scheduler. Then, however, locking could
> +			 * be dropped alltogether from this function.

"Enforce the rule" is correct, since factually it's there, as a dependency in
the code.

Do we know which drivers violate this lifetime rule?

@Christian: What about amdgpu (for which the below was added to begin with)?

> +			 * For now, this remains a potential race in all
> +			 * drivers that keep entities alive for longer than
> +			 * the scheduler.
>  			 */
>  			s_entity->stopped = true;
>  		spin_unlock(&rq->lock);
> -- 
> 2.49.0