On 09/09/2024 13:18, Christian König wrote:
Am 09.09.24 um 14:13 schrieb Philipp Stanner:
On Mon, 2024-09-09 at 13:29 +0200, Christian König wrote:
Am 09.09.24 um 11:44 schrieb Philipp Stanner:
On Fri, 2024-09-06 at 19:06 +0100, Tvrtko Ursulin wrote:
From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx>
Without the locking amdgpu currently can race
amdgpu_ctx_set_entity_priority() and drm_sched_job_arm(),
I would explicitly say "amdgpu's amdgpu_ctx_set_entity_priority()
races
through drm_sched_entity_modify_sched() with drm_sched_job_arm()".
The actual issue then seems to be drm_sched_job_arm() calling
drm_sched_entity_select_rq(). I would mention that, too.
leading to the
latter accesing potentially inconsitent entity->sched_list and
entity->num_sched_list pair.
The comment on drm_sched_entity_modify_sched() however says:
"""
* Note that this must be called under the same common lock for
@entity as
* drm_sched_job_arm() and drm_sched_entity_push_job(), or the
driver
needs to
* guarantee through some other means that this is never called
while
new jobs
* can be pushed to @entity.
"""
It is unclear if that is referring to this race or something
else.
That comment is indeed a bit awkward. Both
drm_sched_entity_push_job()
and drm_sched_job_arm() take rq_lock. But
drm_sched_entity_modify_sched() doesn't.
The comment was written in 981b04d968561. Interestingly, in
drm_sched_entity_push_job(), this "common lock" is mentioned with
the
soft requirement word "should" and apparently is more about keeping
sequence numbers in order when inserting.
I tend to think that the issue discovered by you is unrelated to
that
comment. But if no one can make sense of the comment, should it
maybe
be removed? Confusing comment is arguably worse than no comment
Agree, we probably mixed up in 981b04d968561 that submission needs a
common lock and that rq/priority needs to be protected by the
rq_lock.
There is also the big FIXME in the drm_sched_entity documentation
pointing out that this is most likely not implemented correctly.
I suggest to move the rq, priority and rq_lock fields together in the
drm_sched_entity structure and document that rq_lock is protecting
the two.
That could also be a great opportunity for improving the lock naming:
Well that comment made me laugh because I point out the same when the
scheduler came out ~8years ago and nobody cared about it since then.
But yeah completely agree :)
Maybe, but we need to keep in sight the fact some of these fixes may be
good to backport. In which case re-naming exercises are best left to follow.
Also..
void drm_sched_rq_update_fifo(struct drm_sched_entity *entity, ktime_t
ts)
{
/*
* Both locks need to be grabbed, one to protect from entity->rq
change
* for entity from within concurrent drm_sched_entity_select_rq
and the
* other to update the rb tree structure.
*/
spin_lock(&entity->rq_lock);
spin_lock(&entity->rq->lock);
.. I agree this is quite unredable and my initial reaction was a similar
ugh. However.. What names would you guys suggest and for what to make
this better and not lessen the logic of naming each individually?
Regards,
Tvrtko
[...]
P.
Then audit the code if all users of rq and priority actually hold the
correct locks while reading and writing them.
Regards,
Christian.
P.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx>
Fixes: b37aced31eb0 ("drm/scheduler: implement a function to
modify
sched list")
Cc: Christian König <christian.koenig@xxxxxxx>
Cc: Alex Deucher <alexander.deucher@xxxxxxx>
Cc: Luben Tuikov <ltuikov89@xxxxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx> # v5.7+
---
drivers/gpu/drm/scheduler/sched_entity.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/drivers/gpu/drm/scheduler/sched_entity.c
b/drivers/gpu/drm/scheduler/sched_entity.c
index 58c8161289fe..ae8be30472cd 100644
--- a/drivers/gpu/drm/scheduler/sched_entity.c
+++ b/drivers/gpu/drm/scheduler/sched_entity.c
@@ -133,8 +133,10 @@ void drm_sched_entity_modify_sched(struct
drm_sched_entity *entity,
{
WARN_ON(!num_sched_list || !sched_list);
+ spin_lock(&entity->rq_lock);
entity->sched_list = sched_list;
entity->num_sched_list = num_sched_list;
+ spin_unlock(&entity->rq_lock);
}
EXPORT_SYMBOL(drm_sched_entity_modify_sched);
