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
