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

[Abhinav Singh <singhabhinav9051571833@gmail.com>]

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@redhat.com/ 
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@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;
>