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

[Danilo Krummrich]

Hi Abhinav,

Thanks for sending this follow-up patch.

On 11/26/23 15:57, Abhinav Singh wrote:

Fix 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. To get a normal
(non __rcu tagged pointer) from a __rcu tagged pointer we are using the
function unrcu_pointer(...). The non __rcu tagged pointer then can be
dereferenced just like a normal pointer.

Can you please add a brief explanation why unrcu_pointer() is fine here?

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.

How is that relevant for this patch?

- Danilo

Signed-off-by: Abhinav Singh <singhabhinav9051571...@gmail.com>
---
  drivers/gpu/drm/nouveau/nv10_fence.c | 2 +-
  drivers/gpu/drm/nouveau/nv84_fence.c | 2 +-
  2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nv10_fence.c 
b/drivers/gpu/drm/nouveau/nv10_fence.c
index c6a0db5b9e21..845b64c079ed 100644
--- a/drivers/gpu/drm/nouveau/nv10_fence.c
+++ b/drivers/gpu/drm/nouveau/nv10_fence.c
@@ -32,7 +32,7 @@
  int
  nv10_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_MTHD(push, NV06E, SET_REFERENCE, fence->base.seqno);
diff --git a/drivers/gpu/drm/nouveau/nv84_fence.c 
b/drivers/gpu/drm/nouveau/nv84_fence.c
index 812b8c62eeba..d42e72e23dec 100644
--- a/drivers/gpu/drm/nouveau/nv84_fence.c
+++ b/drivers/gpu/drm/nouveau/nv84_fence.c
@@ -85,7 +85,7 @@ nv84_fence_chid(struct nouveau_channel *chan)
  static int
  nv84_fence_emit(struct nouveau_fence *fence)
  {
-       struct nouveau_channel *chan = fence->channel;
+       struct nouveau_channel *chan = unrcu_pointer(fence->channel);
        struct nv84_fence_chan *fctx = chan->fence;
        u64 addr = fctx->vma->addr + nv84_fence_chid(chan) * 16;