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

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Date:   Mon, 13 Nov 2023 20:00:33 +0100
From:   Danilo Krummrich <dakr@...hat.com>
To:     Abhinav Singh <singhabhinav9051571833@...il.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 11/13/23 19:55, Abhinav Singh wrote:
> 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 ?

Yes, please send a v3.

> 
> Thank You,
> Abhinav Singh
>