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SubjectRe: [PATCH] driver: gpu: Fixing warning directly dereferencing a rcu pointer
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Hey,

Den 2023-11-13 kl. 09:10, skrev Abhinav Singh:
> 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.
>
> 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.
>
> 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@gmail.com>
> ---
> drivers/gpu/drm/nouveau/nv04_fence.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nv04_fence.c b/drivers/gpu/drm/nouveau/nv04_fence.c
> index 5b71a5a5cd85..e62bad1ac720 100644
> --- a/drivers/gpu/drm/nouveau/nv04_fence.c
> +++ b/drivers/gpu/drm/nouveau/nv04_fence.c
> @@ -39,7 +39,9 @@ struct nv04_fence_priv {
> static int
> nv04_fence_emit(struct nouveau_fence *fence)
> {
> - struct nvif_push *push = fence->channel->chan.push;
> + rcu_read_lock();
> + struct nvif_push *push = rcu_dereference(fence->channel)->chan.push;
> + rcu_read_unlock();
> int ret = PUSH_WAIT(push, 2);
> if (ret == 0) {
> PUSH_NVSQ(push, NV_SW, 0x0150, fence->base.seqno);

I'm not an expert in nouveau fence channel lifetime, but I'm pretty sure
this should probably be a rcu_dereference_protected, since a fence
likely can't lose its channel before its command to signal is submitted.

But in case it's not, I would at least advise to check for
fence->channel lifetime before submitting a patch like this. At least
the original code warned about it not being 100% correct.

Cheers,

~Maarten

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