linux-kernel - Re: [PATCH v2] driver: gpu: Fixing warning directly dereferencing a rcu pointer

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Message-ID: <373d97fc-0612-40da-ae9d-6702aa4483ba@redhat.com>
Date:   Mon, 13 Nov 2023 19:49:20 +0100
From:   Danilo Krummrich <dakr@...hat.com>
To:     Abhinav Singh <singhabhinav9051571833@...il.com>,
        kherbst@...hat.com, lyude@...hat.com, airlied@...il.com,
        daniel@...ll.ch
Cc:     dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v2] driver: gpu: Fixing warning directly dereferencing a
 rcu pointer

Hi,

thanks for sending a v2.

On 11/13/23 19:42, Abhinav Singh wrote:
> This patch fixes a sparse warning with this message
> "warning:dereference of noderef expression". In this context it means we
> are dereferencing a __rcu tagged pointer directly.

Better use imperative here, e.g. "Fix a sparse warning ...".

Wouldn't ask you to send a v3 for that alone...

> 
> We should not be directly dereferencing a rcu pointer, rather we should
> be using rcu helper function rcu_dereferece() inside rcu read critical
> section to get a normal pointer which can be dereferenced.

...but this doesn't seem accurate anymore as well.

- Danilo

> 
> I tested with qemu with this command
> qemu-system-x86_64 \
> 	-m 2G \
> 	-smp 2 \
> 	-kernel bzImage \
> 	-append "console=ttyS0 root=/dev/sda earlyprintk=serial net.ifnames=0" \
> 	-drive file=bullseye.img,format=raw \
> 	-net user,host=10.0.2.10,hostfwd=tcp:127.0.0.1:10021-:22 \
> 	-net nic,model=e1000 \
> 	-enable-kvm \
> 	-nographic \
> 	-pidfile vm.pid \
> 	2>&1 | tee vm.log
> with lockdep enabled.
> 
> Signed-off-by: Abhinav Singh <singhabhinav9051571833@...il.com>
> ---
> v1 -> v2 : Replaced the rcu_dereference(...) with unrcu_pointer(...) and
> also removed the rcu locking and unlocking function call.
> 
>   drivers/gpu/drm/nouveau/nv04_fence.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/nv04_fence.c b/drivers/gpu/drm/nouveau/nv04_fence.c
> index 5b71a5a5cd85..cdbc75e3d1f6 100644
> --- a/drivers/gpu/drm/nouveau/nv04_fence.c
> +++ b/drivers/gpu/drm/nouveau/nv04_fence.c
> @@ -39,7 +39,7 @@ struct nv04_fence_priv {
>   static int
>   nv04_fence_emit(struct nouveau_fence *fence)
>   {
> -	struct nvif_push *push = fence->channel->chan.push;
> +	struct nvif_push *push = unrcu_pointer(fence->channel)->chan.push;
>   	int ret = PUSH_WAIT(push, 2);
>   	if (ret == 0) {
>   		PUSH_NVSQ(push, NV_SW, 0x0150, fence->base.seqno);