Re: [PATCH 2/6] amd/amdkfd: Ignore return code of dma_fence_signal()

[Christian König]

On 11/27/25 10:48, Philipp Stanner wrote:
> On Wed, 2025-11-26 at 16:24 -0500, Kuehling, Felix wrote:
>>
>> On 2025-11-26 08:19, Philipp Stanner wrote:
>>> The return code of dma_fence_signal() is not really useful as there is
>>> nothing reasonable to do if a fence was already signaled. That return
>>> code shall be removed from the kernel.
>>>
>>> Ignore dma_fence_signal()'s return code.
>>
>> I think this is not correct. Looking at the comment in 
>> evict_process_worker, we use the return value to decide a race 
>> conditions where multiple threads are trying to signal the eviction 
>> fence. Only one of them should schedule the restore work. And the other 
>> ones need to increment the reference count to keep evictions balanced.
> 
> Thank you for pointing that out. Seems then amdkfd is the only user who
> actually relies on the feature. See below
> 
>>
>> Regards,
>>    Felix
>>
>>
>>>
>>> Suggested-by: Christian König <christian.koenig@xxxxxxx>
>>> Signed-off-by: Philipp Stanner <phasta@xxxxxxxxxx>
>>> ---
>>>   drivers/gpu/drm/amd/amdkfd/kfd_process.c | 5 ++---
>>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_process.c b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
>>> index ddfe30c13e9d..950fafa4b3c3 100644
>>> --- a/drivers/gpu/drm/amd/amdkfd/kfd_process.c
>>> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
>>> @@ -1986,7 +1986,6 @@ kfd_process_gpuid_from_node(struct kfd_process *p, struct kfd_node *node,
>>>   static int signal_eviction_fence(struct kfd_process *p)
>>>   {
>>>   	struct dma_fence *ef;
>>> -	int ret;
>>>   
>>>   	rcu_read_lock();
>>>   	ef = dma_fence_get_rcu_safe(&p->ef);
>>> @@ -1994,10 +1993,10 @@ static int signal_eviction_fence(struct kfd_process *p)
>>>   	if (!ef)
>>>   		return -EINVAL;
>>>   
>>> -	ret = dma_fence_signal(ef);
>>> +	dma_fence_signal(ef);
> 
> The issue now is that dma_fence_signal()'s return code is actually non-
> racy, because check + bit-set are protected by lock.
> 
> Christian's new spinlock series would add a lock function for fences:
> https://lore.kernel.org/dri-devel/20251113145332.16805-5-christian.koenig@xxxxxxx/
> 
> 
> So I suppose this should work:
> 
> dma_fence_lock_irqsave(ef, flags);
> if (dma_fence_test_signaled_flag(ef)) {
> 	dma_fence_unlock_irqrestore(ef, flags);
> 	return true;
> }
> dma_fence_signal_locked(ef);
> dma_fence_unlock_irqrestore(ef, flags);
> 
> return false;
> 
> 
> + some cosmetic adjustments for the boolean of course.
> 
> 
> Would that fly and be reasonable? @Felix, Christian.

I was just about to reply with the same idea when your mail arrived.

So yes looks totally reasonable to me.

Regards,
Christian.

> 
> 
> P.