* [PATCH] driver: gpu: Fix warning directly dereferencing a rcu pointer
@ 2023-11-26 14:57 Abhinav Singh
2023-11-29 23:52 ` Danilo Krummrich
0 siblings, 1 reply; 3+ messages in thread
From: Abhinav Singh @ 2023-11-26 14:57 UTC (permalink / raw)
To: kherbst, lyude, dakr, airlied, daniel
Cc: dri-devel, nouveau, linux-kernel-mentees, Abhinav Singh
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.
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/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;
--
2.39.2
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH] driver: gpu: Fix warning directly dereferencing a rcu pointer
2023-11-26 14:57 [PATCH] driver: gpu: Fix warning directly dereferencing a rcu pointer Abhinav Singh
@ 2023-11-29 23:52 ` Danilo Krummrich
2023-12-03 8:11 ` Abhinav Singh
0 siblings, 1 reply; 3+ messages in thread
From: Danilo Krummrich @ 2023-11-29 23:52 UTC (permalink / raw)
To: Abhinav Singh
Cc: kherbst, lyude, airlied, daniel, dri-devel, nouveau,
linux-kernel-mentees
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?
>
> 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
>
> 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;
>
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH] driver: gpu: Fix warning directly dereferencing a rcu pointer
2023-11-29 23:52 ` Danilo Krummrich
@ 2023-12-03 8:11 ` Abhinav Singh
0 siblings, 0 replies; 3+ messages in thread
From: Abhinav Singh @ 2023-12-03 8:11 UTC (permalink / raw)
To: Danilo Krummrich
Cc: kherbst, lyude, airlied, daniel, dri-devel, nouveau,
linux-kernel-mentees
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;
>
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2023-12-03 8:12 UTC | newest]
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2023-11-26 14:57 [PATCH] driver: gpu: Fix warning directly dereferencing a rcu pointer Abhinav Singh
2023-11-29 23:52 ` Danilo Krummrich
2023-12-03 8:11 ` Abhinav Singh
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