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

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Message-ID: <d03c618e-aa4b-423b-9c50-143b8a197cac@amd.com>
Date: Thu, 10 Apr 2025 14:13:34 +0200
From: Christian König <christian.koenig@....com>
To: 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>, Sabrina Dubroca <sd@...asysnail.net>,
 Sumit Semwal <sumit.semwal@...aro.org>
Cc: dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
 stable@...r.kernel.org
Subject: Re: [PATCH 1/3] drm/nouveau: Prevent signaled fences in pending list

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@...r.kernel.org> # 4.10+, precise commit not to be determined
> 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 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