Le 04/11/2025 à 13:43, Philipp Stanner a écrit :
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: …
---
OK.
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.
OK.
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?
Since drm_sched_entity_kill_jobs_cb uses drm_sched_entity_kill_jobs and vice
versa, I'll have to forward declare one of the 2 functions anyway.
+
[…]
+/* 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.
As discussed offline, "f" is simply the fence being signaled, I'm not sure it
warrants a comment.
Regards,
Pierre-Eric
Optional.
Rest of the code looks good. No further objections.
P.
