Re: [PATCH 1/3] drm/nouveau: Prevent signaled fences in pending list

Philipp Stanner
2025-04-10T13:09:53Z

On Thu, 2025-04-10 at 14:58 +0200, Christian König wrote:
> Am 10.04.25 um 11:24 schrieb Philipp Stanner:
> > Nouveau currently relies on the assumption that dma_fences will
> > only
> > ever get signaled through nouveau_fence_signal(), which takes care
> > of
> > removing a signaled fence from the list nouveau_fence_chan.pending.
> >
> > This self-imposed rule is violated in nouveau_fence_done(), where
> > dma_fence_is_signaled() (somewhat surprisingly, considering its
> > name)
> > can signal the fence without removing it from the list. This
> > enables
> > accesses to already signaled fences through the list, which is a
> > bug.
> >
> > In particular, it can race with nouveau_fence_context_kill(), which
> > would then attempt to set an error code on an already signaled
> > fence,
> > which is illegal.
> >
> > In nouveau_fence_done(), the call to nouveau_fence_update() already
> > ensures to signal all ready fences. Thus, the signaling potentially
> > performed by dma_fence_is_signaled() is actually not necessary.
>
> Ah, I now got what you are trying to do here! But that won't help.
>
> The problem is it is perfectly valid for somebody external (e.g.
> other driver, TTM etc...) to call dma_fence_is_signaled() on a
> nouveau fence.
>
> This will then in turn still signal the fence and leave it on the
> pending list and creating the problem you have.

Good to hear – precisely that then is the use case for a dma_fence
callback! ^_^ It guarantees that, no matter who signals a fence, no
matter at what place, a certain action will always be performed.

I can't think of any other mechanism which could guarantee that a
signaled fence immediately gets removed from nouveau's pending list,
other than the callbacks.

But seriously, I don't think that anyone does this currently, nor do I
think that anyone could get away with doing it without the entire
computer burning down.

P.

>
> Regards,
> Christian.
>
> >
> > Replace the call to dma_fence_is_signaled() with
> > nouveau_fence_base_is_signaled().
> >
> > Cc: <stable@vger.kernel.org> # 4.10+, precise commit not to be
> > determined
> > Signed-off-by: Philipp Stanner <phasta@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 7cc84472cece..33535987d8ed 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
> > @@ -274,7 +274,7 @@ nouveau_fence_done(struct nouveau_fence *fence)
> >   nvif_event_block(&fctx->event);
> >   spin_unlock_irqrestore(&fctx->lock, flags);
> >   }
> > - return dma_fence_is_signaled(&fence->base);
> > + return test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence-
> > >base.flags);
> >  }
> >
> >  static long
>