Abhinav Singh
2023-Nov-13 08:10 UTC
[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 at 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); -- 2.39.2
Maarten Lankhorst
2023-Nov-13 08:24 UTC
[Nouveau] [PATCH] driver: gpu: Fixing warning directly dereferencing a rcu pointer
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 at 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