Re: [PATCH] driver: gpu: Fixing warning directly dereferencing a rcu pointer

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On 11/13/23 09:24, Maarten Lankhorst wrote:
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@xxxxxxxxx>
---
  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.

Yes, before nouveau_fence_emit() did not add this fence to the fence context's
pending list ->channel doesn't need any protection. We can probably just use
unrcu_pointer(), as in [1].

[1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/nouveau/nouveau_fence.c#L210


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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