[PATCH v2] drm/sched: Fix dynamic job-flow control race

[Janne Grunau]

On Fri, Sep 13, 2024 at 01:23:01PM -0700, Rob Clark wrote:
> From: Rob Clark <robdclark at chromium.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 at asahilina.net>
> Fixes: a78422e9dff3 ("drm/sched: implement dynamic job-flow control")
> Closes: https://github.com/AsahiLinux/linux/issues/309
> Cc: stable at vger.kernel.org
> Signed-off-by: Rob Clark <robdclark at chromium.org>
> ---
>  drivers/gpu/drm/scheduler/sched_entity.c | 4 ++--
>  drivers/gpu/drm/scheduler/sched_main.c   | 7 ++-----
>  include/drm/gpu_scheduler.h              | 2 +-
>  3 files changed, 5 insertions(+), 8 deletions(-)

Tested for several hours with CONFIG_PREMPT=y and kasan with a similar
workload as in the github issue without reports or oopses.

Feel free to add
Tested-by: Janne Grunau <j at jannau.net>

thanks,
Janne