From: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
To: Abhinav Singh <singhabhinav9051571833@gmail.com>,
kherbst@redhat.com, lyude@redhat.com, dakr@redhat.com,
airlied@gmail.com, daniel@ffwll.ch
Cc: nouveau@lists.freedesktop.org,
linux-kernel-mentees@lists.linuxfoundation.org,
linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org
Subject: Re: [Nouveau] [PATCH] driver: gpu: Fixing warning directly dereferencing a rcu pointer
Date: Mon, 13 Nov 2023 09:24:50 +0100 [thread overview]
Message-ID: <f229d1ed-b71e-434a-acca-06ee81826260@linux.intel.com> (raw)
In-Reply-To: <20231113081040.2947143-1-singhabhinav9051571833@gmail.com>
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
next prev parent reply other threads:[~2023-11-13 8:25 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-13 8:10 [PATCH] driver: gpu: Fixing warning directly dereferencing a rcu pointer Abhinav Singh
2023-11-13 8:24 ` Maarten Lankhorst [this message]
2023-11-13 17:10 ` [Nouveau] " Danilo Krummrich
2023-11-13 18:37 ` [PATCH v2] drivers: gpu: Fixing warning directly dereferencing a rcu pointer v2 Abhinav Singh
2023-11-13 18:42 ` [PATCH v2] driver: gpu: Fixing warning directly dereferencing a rcu pointer Abhinav Singh
2023-11-13 18:49 ` [Nouveau] " Danilo Krummrich
2023-11-13 18:55 ` Abhinav Singh
2023-11-13 19:00 ` [Nouveau] " Danilo Krummrich
2023-11-13 19:13 ` [PATCH v3] " Abhinav Singh
2023-11-13 19:31 ` Abhinav Singh
2023-11-21 1:20 ` [Nouveau] " Danilo Krummrich
2023-11-21 8:59 ` Abhinav Singh
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