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

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Date:   Mon, 13 Nov 2023 13:40:40 +0530
From:   Abhinav Singh <singhabhinav9051571833@...il.com>
To:     kherbst@...hat.com, lyude@...hat.com, dakr@...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,
        Abhinav Singh <singhabhinav9051571833@...il.com>
Subject: [PATCH] driver: gpu: Fixing warning directly dereferencing a rcu pointer

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@...il.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);
-- 
2.39.2