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@xxxxxxxxxx/
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@xxxxxxxxx>
---
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;
