1. Patchew
  2. linux
  3. View series

[PATCH v3] driver: gpu: Fixing warning directly dereferencing a rcu pointer

Abhinav Singh posted 1 patch 2 years, 1 month ago 
drivers/gpu/drm/nouveau/nv04_fence.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
[PATCH v3] driver: gpu: Fixing warning directly dereferencing a rcu pointer
Posted by Abhinav Singh 2 years, 1 month ago 
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. 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.

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>
---
v1 -> v2 : Replaced the rcu_dereference(...) with unrcu_pointer(...) and
           also removed the rcu locking and unlocking function call.
v2 -> v3 : Changed the description of the patch to match it with the actual
	   implementation.

 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);
-- 
2.39.2
Re: [PATCH v3] driver: gpu: Fixing warning directly dereferencing a rcu pointer
Posted by Danilo Krummrich 2 years, 1 month ago 
On 11/13/23 20:13, 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.
> 
> 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.
> 
> 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>

Applied, thanks!

There are a few more such occurrences. [1][2] Plan to fix them as well?

[1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/nouveau/nv10_fence.c#L35
[2] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/nouveau/nv84_fence.c#L88

> ---
> v1 -> v2 : Replaced the rcu_dereference(...) with unrcu_pointer(...) and
>             also removed the rcu locking and unlocking function call.
> v2 -> v3 : Changed the description of the patch to match it with the actual
> 	   implementation.
> 
>   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);
Re: [PATCH v3] driver: gpu: Fixing warning directly dereferencing a rcu pointer
Posted by Abhinav Singh 2 years, 1 month ago 
On 11/21/23 06:50, Danilo Krummrich wrote:
> On 11/13/23 20:13, 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.
>>
>> 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.
>>
>> 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>
> 
> Applied, thanks!
> 
> There are a few more such occurrences. [1][2] Plan to fix them as well?
> 
> [1] 
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/nouveau/nv10_fence.c#L35
> [2] 
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/nouveau/nv84_fence.c#L88
> 
>> ---
>> v1 -> v2 : Replaced the rcu_dereference(...) with unrcu_pointer(...) and
>>             also removed the rcu locking and unlocking function call.
>> v2 -> v3 : Changed the description of the patch to match it with the 
>> actual
>>        implementation.
>>
>>   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);
> 
Thanks a lot for merging this.
Yeah sure I will submit the patch for the issues soon.

Regards,
Abhinav Singh
Re: [PATCH v3] driver: gpu: Fixing warning directly dereferencing a rcu pointer
Posted by Abhinav Singh 2 years, 1 month ago 
On 11/14/23 00:43, 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.
> 
> 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.
> 
> 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>
> ---
> v1 -> v2 : Replaced the rcu_dereference(...) with unrcu_pointer(...) and
>             also removed the rcu locking and unlocking function call.
> v2 -> v3 : Changed the description of the patch to match it with the actual
> 	   implementation.
> 
>   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, just for the sake of my own confirmation, the patch is merge ready 
right? once the CI runs successfully it will be merged right?


Thank You,
Abhinav Singh

Patchew 2.1-devel-b67e4c2

© 2016 - 2025 Red Hat, Inc.