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

[Danilo Krummrich <dakr@redhat.com>]

On 11/13/23 09:24, Maarten Lankhorst wrote:
> 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@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.

Yes, before nouveau_fence_emit() did not add this fence to the fence context's
pending list ->channel doesn't need any protection. We can probably just use
unrcu_pointer(), as in [1].

[1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/nouveau/nouveau_fence.c#L210

> 
> 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
>