On Tue, 2025-11-04 at 10:53 +0100, Pierre-Eric Pelloux-Prayer wrote:
> The Mesa issue referenced below pointed out a possible deadlock:
>
> [ 1231.611031] Possible interrupt unsafe locking scenario:
>
> [ 1231.611033] CPU0 CPU1
> [ 1231.611034] ---- ----
> [ 1231.611035] lock(&xa->xa_lock#17);
> [ 1231.611038] local_irq_disable();
> [ 1231.611039] lock(&fence->lock);
> [ 1231.611041] lock(&xa->xa_lock#17);
> [ 1231.611044] <Interrupt>
> [ 1231.611045] lock(&fence->lock);
> [ 1231.611047]
> *** DEADLOCK ***
>
> In this example, CPU0 would be any function accessing job->dependencies
> through the xa_* functions that doesn't disable interrupts (eg:
> drm_sched_job_add_dependency, drm_sched_entity_kill_jobs_cb).
>
> CPU1 is executing drm_sched_entity_kill_jobs_cb as a fence signalling
> callback so in an interrupt context. It will deadlock when trying to
> grab the xa_lock which is already held by CPU0.
>
> Replacing all xa_* usage by their xa_*_irq counterparts would fix
> this issue, but Christian pointed out another issue: dma_fence_signal
> takes fence.lock and so does dma_fence_add_callback.
>
> dma_fence_signal() // locks f1.lock
> -> drm_sched_entity_kill_jobs_cb()
> -> foreach dependencies
> -> dma_fence_add_callback() // locks f2.lock
>
> This will deadlock if f1 and f2 share the same spinlock.
>
> To fix both issues, the code iterating on dependencies and re-arming them
> is moved out to drm_sched_entity_kill_jobs_work.
>
> v2: reworded commit message (Philipp)
> v3: added Fixes tag (Philipp)
Thx for the update.
In the future please put the changelog below between a pair of '---'
---
v2: …
v3: …
---
Some things I have unfortunately overlooked below.
>
> Fixes: 2fdb8a8f07c2 ("drm/scheduler: rework entity flush, kill and fini")
We should +Cc stable. It's a deadlock after all.
> Link: https://gitlab.freedesktop.org/mesa/mesa/-/issues/13908
> Reported-by: Mikhail Gavrilov <[email protected]>
> Suggested-by: Christian König <[email protected]>
> Reviewed-by: Christian König <[email protected]>
> Signed-off-by: Pierre-Eric Pelloux-Prayer <[email protected]>
> ---
> drivers/gpu/drm/scheduler/sched_entity.c | 34 +++++++++++++-----------
> 1 file changed, 19 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c
> b/drivers/gpu/drm/scheduler/sched_entity.c
> index c8e949f4a568..fe174a4857be 100644
> --- a/drivers/gpu/drm/scheduler/sched_entity.c
> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
> @@ -173,26 +173,15 @@ int drm_sched_entity_error(struct drm_sched_entity
> *entity)
> }
> EXPORT_SYMBOL(drm_sched_entity_error);
>
> +static void drm_sched_entity_kill_jobs_cb(struct dma_fence *f,
> + struct dma_fence_cb *cb);
It's far better to move the function up instead. Can you do that?
> +
>
[…]
> +/* Signal the scheduler finished fence when the entity in question is
> killed. */
> +static void drm_sched_entity_kill_jobs_cb(struct dma_fence *f,
> + struct dma_fence_cb *cb)
> +{
> + struct drm_sched_job *job = container_of(cb, struct drm_sched_job,
> + finish_cb);
> +
> + dma_fence_put(f);
It would be great if we knew what fence is being dropped here and why.
I know you're just moving the pre-existing code, but if you should
know, informing about that via comment would be great.
Optional.
Rest of the code looks good. No further objections.
P.
