Re: [PATCH v2] drm/sched: Protect entity->last_scheduled with spinlock
From: Tvrtko Ursulin
Date: Tue Jun 30 2026 - 06:48:24 EST
On 30/06/2026 11:06, Philipp Stanner wrote:
On Tue, 2026-06-30 at 10:23 +0100, Tvrtko Ursulin wrote:
On 26/06/2026 09:19, Philipp Stanner wrote:
The entity->last_scheduled field has always been set and read with
special RCU functions in addition to memory barriers. There is no
obvious reason for that, since the entity lock is available and taken at
all places that evaluate the last_scheduled field. The only exception is
drm_sched_entity_error(), which is not performance critical in any way.
I agree this looks odd since all call sites apart from
drm_sched_entity_error() use
"rcu_dereference_check(entity->last_scheduled, true);" ie. "ignore" the RCU.
Btw this was added in:
commit 70102d77ff22dd88a0111b1c3bac5099ac5d0425
Author: Christian König <christian.koenig@xxxxxxx>
Date: Mon Apr 17 17:32:11 2023 +0200
drm/scheduler: add drm_sched_entity_error and use rcu for
last_scheduled
You may want to add this as a reference in the commit message.
I did git-blame for that commit. It looks like this:
drm/scheduler: add drm_sched_entity_error and use rcu for last_scheduled
Switch to using RCU handling for the last scheduled job and add a
function to return the error code of it.
It's a good example of why I think it's so vital to write verbose
commit messages. The only way to find out why this was added is to ask
the author, if he's still around [which is the case in this case].
I can't see the value of adding a link? That commit says "add foo" and
my commit says "remove foo because it achieves nothing".
It achieves something, there is a little bit more detail to it. You could write:
Commit xxx added RCU without documenting that it needed it for lockless access to blah blah, while all other call sites needed to explicitly and forcefully silence the RCU checker, etc...
Anyway, I don't intend to write the commit text but my point is value can be added while being accurate. If you add a patch which says "there is no obvious reason", well, lets leave historical breadcrumbs to make it less non-obvious, now that we know what it was.
I guess it relied on dma-fence RCU destruction to enable lockless
lookups from the AMD submit path. Given how many other locks we have in
those paths it is probably noise to have one more so maybe it is a win
to remove some barriers and those rcu_dereference_check-true lines. I
think Christian will need to comment.
My argument is more that locks are the right tool to use unless there
is proof to the contrary.
Improve robustness, readability and maintainability by replacing RCU and
barriers with the lock.
As a preparational step, while at it, also guard spsc_queue_pop() with
the lock, since spsc_queue is deprecated and supposed to be replaced
with a locked list.
You would have said to split the logical changes into separate patches.
Me? :D
In this case, a lock that did not exist is added from nowhere. But I
tend to think that you are right. We could leave spsc_queue lockless
for now. That's cleaner.
[…]
struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
{
+ /* Helper to avoid dropping the reference while the entity lock is held,
+ * just to have some more robustness.
+ */
+ struct dma_fence *prev_last_scheduled;
struct drm_sched_job *sched_job;
sched_job = drm_sched_entity_queue_peek(entity);
@@ -523,19 +532,20 @@ struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
if (entity->guilty && atomic_read(entity->guilty))
dma_fence_set_error(&sched_job->s_fence->finished, -ECANCELED);
- dma_fence_put(rcu_dereference_check(entity->last_scheduled, true));
- rcu_assign_pointer(entity->last_scheduled,
- dma_fence_get(&sched_job->s_fence->finished));
+ spin_lock(&entity->lock);
+ prev_last_scheduled = entity->last_scheduled;
+ entity->last_scheduled = dma_fence_get(&sched_job->s_fence->finished);
- /*
- * If the queue is empty we allow drm_sched_entity_select_rq() to
- * locklessly access ->last_scheduled. This only works if we set the
- * pointer before we dequeue and if we a write barrier here.
+ /* A recent rework required taking the spinlock above. Since spsc_queue
+ * is scheduled for removal as per the DRM-TODO-list, we access it here
+ * locked already to prepare for that cleanup.
+ *
+ * TODO: Fully replace spsc_queue with a locked (h)list.
*/
- smp_wmb();
-
spsc_queue_pop(&entity->job_queue);
+ spin_unlock(&entity->lock);
+ dma_fence_put(prev_last_scheduled);
drm_sched_rq_pop_entity(entity);
Notice the entity->lock ends up cycled twice for no good reason (second
Getting rid of hard to understand barriers + RCU *is* a _very_ good
reason.
I will re-phrase - there is no reason to cycle the lock twice while proposing the removal of barriers and RCU. They are completely orthogonal.
is in drm_sched_rq_pop_entity()). So I would suggest you somehow reduce
that to once. Probably just pull out entity->lock out of the
drm_sched_rq_pop_entity() to drm_sched_entity_pop_job()?
Can you see danger in sense of a significant performance regression
because of that?
It's just unsightly. Last year or so I removed the very same lazy pattern from the scheduler so lets not re-add it.
d42a254633c7 ("drm/sched: Optimise drm_sched_entity_push_job")
36caa026b28a ("drm/sched: Avoid double re-lock on the job free path")
I guess if you do that then the "while at it" part of the commit message
can be "upgraded" to "spsc_queue_pop() being under the lock as a
consequence of the rework" and then no need to split it.
I agree with you that it should be *downgraded* instead.
Dropping that part of change? Works for me.
/* Jobs and entities might have different lifecycles. Since we're
@@ -561,21 +571,15 @@ void drm_sched_entity_select_rq(struct drm_sched_entity *entity)
if (spsc_queue_count(&entity->job_queue))
return;
- /*
- * Only when the queue is empty are we guaranteed that
- * drm_sched_run_job_work() cannot change entity->last_scheduled. To
- * enforce ordering we need a read barrier here. See
- * drm_sched_entity_pop_job() for the other side.
- */
- smp_rmb();
-
- fence = rcu_dereference_check(entity->last_scheduled, true);
+ spin_lock(&entity->lock);
+ fence = entity->last_scheduled;
/* stay on the same engine if the previous job hasn't finished */
- if (fence && !dma_fence_is_signaled(fence))
+ if (fence && !dma_fence_is_signaled(fence)) {
+ spin_unlock(&entity->lock);
Have you tried with lockdep to see if there are any hidden lock
inversions with this?
As far as I could grep really no one touches the entity lock (which is
not surprising, since the entire drm_sched design resolves around the
central philosophy: "NEVER use a spinlock unless you absolutely have
to". When you look at the old code and documentation, you see that
locks were really only ever used to protect lists.
Anyways. This is the scheduler's fence. It can never implement any
callback to someone who might interfere with the entity lock, can it?
Right, just my trauma from past encounters with opportunistic signalling.
Regards,
Tvrtko
I also wonder if we could demote this to a flag check only and remove
any doubt. I don't think opportunistic signalling matter in this code path.
With the new fence API, where we can bypass the ops, that would
probably be the more canonical code. But that's then indeed something
for a separate patch.
P.