linux-kernel - Re: [PATCH v2] driver: gpu: Fixing warning directly dereferencing a rcu pointer

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Date:   Tue, 14 Nov 2023 00:25:24 +0530
From:   Abhinav Singh <singhabhinav9051571833@...il.com>
To:     Danilo Krummrich <dakr@...hat.com>, kherbst@...hat.com,
        lyude@...hat.com, airlied@...il.com, daniel@...ll.ch
Cc:     dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v2] driver: gpu: Fixing warning directly dereferencing a
 rcu pointer

On 11/14/23 00:19, Danilo Krummrich wrote:
> Hi,
> 
> thanks for sending a v2.
> 
> On 11/13/23 19:42, Abhinav Singh wrote:
>> 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.
> 
> Better use imperative here, e.g. "Fix a sparse warning ...".
> 
> Wouldn't ask you to send a v3 for that alone...
> 
>>
>> 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.
> 
> ...but this doesn't seem accurate anymore as well.
> 
> - Danilo
> 
>>
>> 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@...il.com>
>> ---
>> v1 -> v2 : Replaced the rcu_dereference(...) with unrcu_pointer(...) and
>> also removed the rcu locking and unlocking function call.
>>
>>   drivers/gpu/drm/nouveau/nv04_fence.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/nouveau/nv04_fence.c 
>> b/drivers/gpu/drm/nouveau/nv04_fence.c
>> index 5b71a5a5cd85..cdbc75e3d1f6 100644
>> --- a/drivers/gpu/drm/nouveau/nv04_fence.c
>> +++ b/drivers/gpu/drm/nouveau/nv04_fence.c
>> @@ -39,7 +39,7 @@ struct nv04_fence_priv {
>>   static int
>>   nv04_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_NVSQ(push, NV_SW, 0x0150, fence->base.seqno);
> 
Hi maintainers thanks a lot for reviewing this patch.
I think I should fix my mistake by sending in another patch so that the 
code changes and description matches. So should I send another patch ?

Thank You,
Abhinav Singh