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

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