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)
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
Pushed to drm-misc-fixes.
Thx, Pierre-Eric!
P.