Nouveau - May 2025 - [PATCH 2/2] drm/nouveau: Don't signal when killing the fence context

On 5/22/25 14:20, Philipp Stanner wrote:
> On Thu, 2025-05-22 at 14:06 +0200, Christian K?nig wrote:
>> On 5/22/25 13:25, Philipp Stanner wrote:
>>> dma_fence_is_signaled_locked(), which is used in
>>> nouveau_fence_context_kill(), can signal fences below the surface
>>> through a callback.
>>>
>>> There is neither need for nor use in doing that when killing a
>>> fence
>>> context.
>>>
>>> Replace dma_fence_is_signaled_locked() with
>>> __dma_fence_is_signaled(), a
>>> function which only checks, never signals.
>>
>> That is not a good approach.
>>
>> Having the __dma_fence_is_signaled() means that other would be
>> allowed to call it as well.
>>
>> But nouveau can do that here only because it knows that the fence was
>> issued by nouveau.
>>
>> What nouveau can to is to test the signaled flag directly, but
that's
>> what you try to avoid as well.
> 
> There's many parties who check the bit already.
> 
> And if Nouveau is allowed to do that, one can just as well provide a
> wrapper for it.
No, exactly that's what is usually avoided in cases like this here.

See all the functions inside include/linux/dma-fence.h can be used by everybody.
It's basically the public interface of the dma_fence object.

So testing if a fence is signaled without calling the callback is only allowed
by whoever implemented the fence.

In other words nouveau can test nouveau fences, i915 can test i915 fences,
amdgpu can test amdgpu fences etc... But if you have the wrapper that makes it
officially allowed that nouveau starts testing i915 fences and that would be
problematic.

Regards,
Christian.
> 
> That has the advantage of centralizing the responsibility and
> documenting it.
> 
> P.
> 
>>
>> Regards,
>> Christian.
>>
>>>
>>> Signed-off-by: Philipp Stanner <phasta at kernel.org>
>>> ---
>>> ?drivers/gpu/drm/nouveau/nouveau_fence.c | 2 +-
>>> ?1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c
>>> b/drivers/gpu/drm/nouveau/nouveau_fence.c
>>> index d5654e26d5bc..993b3dcb5db0 100644
>>> --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
>>> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
>>> @@ -88,7 +88,7 @@ nouveau_fence_context_kill(struct
>>> nouveau_fence_chan *fctx, int error)
>>> ?
>>> ?	spin_lock_irqsave(&fctx->lock, flags);
>>> ?	list_for_each_entry_safe(fence, tmp, &fctx->pending, head)
>>> {
>>> -		if (error && !dma_fence_is_signaled_locked(&fence-
>>>> base))
>>> +		if (error && !__dma_fence_is_signaled(&fence-
>>>> base))
>>> ?			dma_fence_set_error(&fence->base, error);
>>> ?
>>> ?		if (nouveau_fence_signal(fence))
>>
>

On Thu, May 22, 2025 at 02:34:33PM +0200, Christian K?nig
wrote:
> See all the functions inside include/linux/dma-fence.h can be used by
everybody. It's basically the public interface of the dma_fence object.
As you write below, in certain cases it is valid to call this from drivers, so
it's not unreasonable to have it as part of the public API.
> So testing if a fence is signaled without calling the callback is only
allowed by whoever implemented the fence.
> 
> In other words nouveau can test nouveau fences, i915 can test i915 fences,
amdgpu can test amdgpu fences etc... But if you have the wrapper that makes it
officially allowed that nouveau starts testing i915 fences and that would be
problematic.
In general, I like the  __dma_fence_is_signaled() helper, because this way we
can document in which cases it is allowed to be used, i.e. the ones you descibe
above.

test_bit() can be called by anyone and there is no documentation comment
explaining that it is only allowed under certain conditions.

Having the __dma_fence_is_signaled() helper properly documented could get you
rid of having to explain in which case the test_bit() dance is allowed to do
over and over again. :-)

I also think the name is good, since the '__' prefix already implies
that there
are some restrictions on the use of this helper.