linux-kernel - [PATCH] drm/sched: Fix dynamic job-flow control race

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Message-ID: <20240913165326.8856-1-robdclark@gmail.com>
Date: Fri, 13 Sep 2024 09:53:25 -0700
From: Rob Clark <robdclark@...il.com>
To: dri-devel@...ts.freedesktop.org
Cc: Rob Clark <robdclark@...omium.org>,
	Asahi Lina <lina@...hilina.net>,
	Luben Tuikov <ltuikov89@...il.com>,
	Matthew Brost <matthew.brost@...el.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>,
	Daniel Vetter <daniel@...ll.ch>,
	Danilo Krummrich <dakr@...hat.com>,
	linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] drm/sched: Fix dynamic job-flow control race

From: Rob Clark <robdclark@...omium.org>

Fixes a race condition reported here: https://github.com/AsahiLinux/linux/issues/309#issuecomment-2238968609

The whole premise of lockless access to a single-producer-single-
consumer queue is that there is just a single producer and single
consumer.  That means we can't call drm_sched_can_queue() (which is
about queueing more work to the hw, not to the spsc queue) from
anywhere other than the consumer (wq).

This call in the producer is just an optimization to avoid scheduling
the consuming worker if it cannot yet queue more work to the hw.  It
is safe to drop this optimization to avoid the race condition.

Suggested-by: Asahi Lina <lina@...hilina.net>
Fixes: a78422e9dff3 ("drm/sched: implement dynamic job-flow control")
Signed-off-by: Rob Clark <robdclark@...omium.org>
---
 drivers/gpu/drm/scheduler/sched_main.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
index ab53ab486fe6..1af1dbe757d5 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -1020,8 +1020,7 @@ EXPORT_SYMBOL(drm_sched_job_cleanup);
 void drm_sched_wakeup(struct drm_gpu_scheduler *sched,
 		      struct drm_sched_entity *entity)
 {
-	if (drm_sched_can_queue(sched, entity))
-		drm_sched_run_job_queue(sched);
+	drm_sched_run_job_queue(sched);
 }
 
 /**
-- 
2.46.0