Re: [PATCH] drm/nouveau: Prevent signalled fences in pending list

["Christian König" <christian.koenig@amd.com>]

Am 27.03.25 um 09:42 schrieb Philipp Stanner:
> Nouveau currently relies on the assumption that dma_fences will only
> ever get signalled through nouveau_fence_signal(), which takes care of
> removing a signalled fence from the list nouveau_fence_chan.pending.
>
> This self-imposed rule is violated in nouveau_fence_done(), where
> dma_fence_is_signaled() can signal the fence without removing it from
> the list. This enables accesses to already signalled fences through the
> list, which is a bug.
>
> Furthermore, it must always be possible to use standard dma_fence
> methods an a dma_fence and observe valid behavior. The canonical way of
> ensuring that signalling a fence has additional effects is to add those
> effects to a callback and register it on the fence.

Good catch.

>
> Move the code from nouveau_fence_signal() into a dma_fence callback.
> Register that callback when creating the fence.

But that's a really ugly approach.

Either nouveau shouldn't implement the signaled callback or make sure that when returning true from the signaled callback the fence is also removed from the pending list.

>
> Cc: <stable@vger.kernel.org> # 4.10+
> Fixes: f54d1867005c ("dma-buf: Rename struct fence to dma_fence")
> Signed-off-by: Philipp Stanner <phasta@kernel.org>
> ---
> I'm not entirely sure what Fixes-Tag is appropriate. The last time the
> line causing the signalled fence in the list was touched is the commit
> listed above.

Yeah, that's most likely not correct. My educated guess is that the bug was always there just never discovered.

Regards,
Christian.

> ---
>  drivers/gpu/drm/nouveau/nouveau_fence.c | 41 ++++++++++++++++---------
>  drivers/gpu/drm/nouveau/nouveau_fence.h |  1 +
>  2 files changed, 27 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c
> index 7cc84472cece..b2c2241a8803 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
> @@ -50,24 +50,22 @@ nouveau_fctx(struct nouveau_fence *fence)
>  	return container_of(fence->base.lock, struct nouveau_fence_chan, lock);
>  }
>  
> -static int
> -nouveau_fence_signal(struct nouveau_fence *fence)
> +static void
> +nouveau_fence_cleanup_cb(struct dma_fence *dfence, struct dma_fence_cb *cb)
>  {
> -	int drop = 0;
> +	struct nouveau_fence_chan *fctx;
> +	struct nouveau_fence *fence;
> +
> +	fence = container_of(dfence, struct nouveau_fence, base);
> +	fctx = nouveau_fctx(fence);
>  
> -	dma_fence_signal_locked(&fence->base);
>  	list_del(&fence->head);
>  	rcu_assign_pointer(fence->channel, NULL);
>  
> -	if (test_bit(DMA_FENCE_FLAG_USER_BITS, &fence->base.flags)) {
> -		struct nouveau_fence_chan *fctx = nouveau_fctx(fence);
> -
> -		if (!--fctx->notify_ref)
> -			drop = 1;
> -	}
> +	if (test_bit(DMA_FENCE_FLAG_USER_BITS, &fence->base.flags))
> +		--fctx->notify_ref;
>  
>  	dma_fence_put(&fence->base);
> -	return drop;
>  }
>  
>  static struct nouveau_fence *
> @@ -93,7 +91,8 @@ nouveau_fence_context_kill(struct nouveau_fence_chan *fctx, int error)
>  		if (error)
>  			dma_fence_set_error(&fence->base, error);
>  
> -		if (nouveau_fence_signal(fence))
> +		dma_fence_signal_locked(&fence->base);
> +		if (fctx->notify_ref == 0)
>  			nvif_event_block(&fctx->event);
>  	}
>  	fctx->killed = 1;
> @@ -131,7 +130,6 @@ static int
>  nouveau_fence_update(struct nouveau_channel *chan, struct nouveau_fence_chan *fctx)
>  {
>  	struct nouveau_fence *fence;
> -	int drop = 0;
>  	u32 seq = fctx->read(chan);
>  
>  	while (!list_empty(&fctx->pending)) {
> @@ -140,10 +138,10 @@ nouveau_fence_update(struct nouveau_channel *chan, struct nouveau_fence_chan *fc
>  		if ((int)(seq - fence->base.seqno) < 0)
>  			break;
>  
> -		drop |= nouveau_fence_signal(fence);
> +		dma_fence_signal_locked(&fence->base);
>  	}
>  
> -	return drop;
> +	return fctx->notify_ref == 0 ? 1 : 0;
>  }
>  
>  static void
> @@ -235,6 +233,19 @@ nouveau_fence_emit(struct nouveau_fence *fence)
>  			       &fctx->lock, fctx->context, ++fctx->sequence);
>  	kref_get(&fctx->fence_ref);
>  
> +	fence->cb.func = nouveau_fence_cleanup_cb;
> +	/* Adding a callback runs into __dma_fence_enable_signaling(), which will
> +	 * ultimately run into nouveau_fence_no_signaling(), where a WARN_ON
> +	 * would fire because the refcount can be dropped there.
> +	 *
> +	 * Increment the refcount here temporarily to work around that.
> +	 */
> +	dma_fence_get(&fence->base);
> +	ret = dma_fence_add_callback(&fence->base, &fence->cb, nouveau_fence_cleanup_cb);
> +	dma_fence_put(&fence->base);
> +	if (ret)
> +		return ret;
> +
>  	ret = fctx->emit(fence);
>  	if (!ret) {
>  		dma_fence_get(&fence->base);
> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.h b/drivers/gpu/drm/nouveau/nouveau_fence.h
> index 8bc065acfe35..e6b2df7fdc42 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_fence.h
> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.h
> @@ -10,6 +10,7 @@ struct nouveau_bo;
>  
>  struct nouveau_fence {
>  	struct dma_fence base;
> +	struct dma_fence_cb cb;
>  
>  	struct list_head head;
>