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

Christian König
2025-04-10T12:13:42Z

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.
>
> 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);

See the code above that:

if (fence->base.ops == &nouveau_fence_ops_legacy ||
fence->base.ops == &nouveau_fence_ops_uevent) {
....

Nouveau first tests if it's one of it's own fences, and if yes does some special handling. E.g. checking the fence status bits etc...

So this dma_fence_is_signaled() is for all non-nouveau fences and then not touching the internal flags is perfectly correct as far as I can see.

Regards,
Christian.

> }
>
> static long