[PATCH] drm/sched: Document run_job() refcount hazard

[Christian König]

Am 20.12.24 um 14:18 schrieb Danilo Krummrich:
> On Fri, Dec 20, 2024 at 01:53:34PM +0100, Christian König wrote:
>> Am 20.12.24 um 13:45 schrieb Philipp Stanner:
>>> From: Philipp Stanner <pstanner at redhat.com>
>>>
>>> drm_sched_backend_ops.run_job() returns a dma_fence for the scheduler.
>>> That fence is signalled by the driver once the hardware completed the
>>> associated job. The scheduler does not increment the reference count on
>>> that fence, but implicitly expects to inherit this fence from run_job().
>>>
>>> This is relatively subtle and prone to misunderstandings.
>>>
>>> This implies that, to keep a reference for itself, a driver needs to
>>> call dma_fence_get() in addition to dma_fence_init() in that callback.
>>>
>>> It's further complicated by the fact that the scheduler even decrements
>>> the refcount in drm_sched_run_job_work() since it created a new
>>> reference in drm_sched_fence_scheduled(). It does, however, still use
>>> its pointer to the fence after calling dma_fence_put() - which is safe
>>> because of the aforementioned new reference, but actually still violates
>>> the refcounting rules.
>>>
>>> Improve the explanatory comment for that decrement.
>>>
>>> Move the call to dma_fence_put() to the position behind the last usage
>>> of the fence.
>>>
>>> Document the necessity to increment the reference count in
>>> drm_sched_backend_ops.run_job().
>>>
>>> Cc: Christian König <christian.koenig at amd.com>
>>> Cc: Tvrtko Ursulin <tursulin at ursulin.net>
>>> Cc: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
>>> Signed-off-by: Philipp Stanner <pstanner at redhat.com>
>>> ---
>>>    drivers/gpu/drm/scheduler/sched_main.c | 10 +++++++---
>>>    include/drm/gpu_scheduler.h            | 20 ++++++++++++++++----
>>>    2 files changed, 23 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 7ce25281c74c..d6f8df39d848 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> +	 *
>>> +	 * @sched_job: the job to run
>>> +	 *
>>> +	 * Returns: dma_fence the driver must signal once the hardware has
>>> +	 *	completed the job ("hardware fence").
>>> +	 *
>>> +	 * Note that the scheduler expects to 'inherit' its own reference to
>>> +	 * this fence from the callback. It does not invoke an extra
>>> +	 * dma_fence_get() on it. Consequently, this callback must return a
>>> +	 * fence whose refcount is at least 2: One for the scheduler's
>>> +	 * reference returned here, another one for the reference kept by the
>>> +	 * driver.
>> Well the driver actually doesn't need any extra reference. The scheduler
>> just needs to guarantee that this reference isn't dropped before it is
>> signaled.
> I think he means the reference the driver's fence context has to have in order
> to signal that thing eventually.

Yeah, but this is usually a weak reference. IIRC most drivers don't 
increment the reference count for the reference they keep to signal a fence.

It's expected that the consumers of the dma_fence keep the fence alive 
at least until it is signaled. That's why we have this nice warning in 
dma_fence_release().

On the other hand I completely agree it would be more defensive if 
drivers increment the reference count for the reference they keep for 
signaling.

So if we want to document that the fence reference count should at least 
be 2 we somehow need to enforce this with a warning for example.

Regards,
Christian.



>
>> Regards,
>> Christian.
>>
>>>    	 */
>>>    	struct dma_fence *(*run_job)(struct drm_sched_job *sched_job);