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

[Abhinav Singh <singhabhinav9051571833@xxxxxxxxx>]

On 11/30/23 05:22, Danilo Krummrich wrote:
> 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?
Is this description okay
"The reason for using unrcu_pointer(...) instead of rcu_dereference(...)
or rcu_dereference_protected(...) is because, before nv10_fence_emit() 
and nv_04_fence_emit() did not add this fence to the fence context's
pending list, thus channel doesn't need any protection" ?
> > 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
To test rcu related code lockdep must be enabled, it gives any warning 
or error message if we are dealing inappropriately with rcu pointers. So 
I tested this lockdep enabled. I added the test description in this 
patch as well 
https://lore.kernel.org/all/0754e669-8b00-461c-b6fe-79c659bf59a3@xxxxxxxxxx/ 
which is very similar to this patch so I thought I should here as well. 
Is it not relevant here?

Thank You,
Abhinav Singh
> > Signed-off-by: Abhinav Singh <singhabhinav9051571833@xxxxxxxxx>
> > ---
> >   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;