drm/sched: Fix deadlock in drm_sched_entity_kill_jobs_cb

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 don'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().

Cc: stable@vger.kernel.org # v6.2+
Fixes: 2fdb8a8f07c2 ("drm/scheduler: rework entity flush, kill and fini")
Link: https://gitlab.freedesktop.org/mesa/mesa/-/issues/13908
Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Suggested-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
[phasta: commit message nits]
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patch.msgid.link/20251104095358.15092-1-pierre-eric.pelloux-prayer@amd.com

diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
index c8e949f4a5687c135a323087c69fee9cc2edcf1e..fe174a4857be7f10ed96b583f6754a21ffd4367c 100644 (file)
--- 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);
+
 static void drm_sched_entity_kill_jobs_work(struct work_struct *wrk)
 {
        struct drm_sched_job *job = container_of(wrk, typeof(*job), work);
-
-       drm_sched_fence_scheduled(job->s_fence, NULL);
-       drm_sched_fence_finished(job->s_fence, -ESRCH);
-       WARN_ON(job->s_fence->parent);
-       job->sched->ops->free_job(job);
-}
-
-/* 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);
+       struct dma_fence *f;
        unsigned long index;
 
-       dma_fence_put(f);
-
        /* Wait for all dependencies to avoid data corruptions */
        xa_for_each(&job->dependencies, index, f) {
                struct drm_sched_fence *s_fence = to_drm_sched_fence(f);
@@ -220,6 +209,21 @@ static void drm_sched_entity_kill_jobs_cb(struct dma_fence *f,
                dma_fence_put(f);
        }
 
+       drm_sched_fence_scheduled(job->s_fence, NULL);
+       drm_sched_fence_finished(job->s_fence, -ESRCH);
+       WARN_ON(job->s_fence->parent);
+       job->sched->ops->free_job(job);
+}
+
+/* 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);
+
        INIT_WORK(&job->work, drm_sched_entity_kill_jobs_work);
        schedule_work(&job->work);
 }