linux-kernel - Re: [PATCH 2/2] drm/nouveau: Don't signal when killing the fence context

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Message-ID: <06210b9dc5e5ea8365295b77942c3ca030f02729.camel@mailbox.org>
Date: Thu, 22 May 2025 14:20:03 +0200
From: Philipp Stanner <phasta@...lbox.org>
To: Christian König <christian.koenig@....com>, Philipp
 Stanner <phasta@...nel.org>, Lyude Paul <lyude@...hat.com>, Danilo
 Krummrich <dakr@...nel.org>,  David Airlie <airlied@...il.com>, Simona
 Vetter <simona@...ll.ch>, Sumit Semwal <sumit.semwal@...aro.org>
Cc: dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org
Subject: Re: [PATCH 2/2] drm/nouveau: Don't signal when killing the fence
 context

On Thu, 2025-05-22 at 14:06 +0200, Christian König 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.

That has the advantage of centralizing the responsibility and
documenting it.

P.

> 
> Regards,
> Christian.
> 
> > 
> > Signed-off-by: Philipp Stanner <phasta@...nel.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))
>