[PATCH] rust: DamFence: Remove warning about unsignaled drop

[PATCH] rust: DamFence: Remove warning about unsignaled drop

[Philipp Stanner]

DriverFence::drop() contains a pr_warn!() for when the fence drops
unsignaled. This warning gets into the way of canonical teardown in
Rust: when you drop a lock, it unlocks itself, when you drop a KBox, it
frees itself etc. Congruently, users should be able to tear down fences
they don't care about anymore with ECANCELED, which is already the
default error code.

Remove the warning.

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
---
 rust/kernel/dma_buf/dma_fence.rs | 1 -
 1 file changed, 1 deletion(-)

diff --git a/rust/kernel/dma_buf/dma_fence.rs b/rust/kernel/dma_buf/dma_fence.rs
index a1d851656562..2c8001cb9a59 100644
--- a/rust/kernel/dma_buf/dma_fence.rs
+++ b/rust/kernel/dma_buf/dma_fence.rs
@@ -942,7 +942,6 @@ fn drop(&mut self) {
         // SAFETY: `guard` is valid until the `call_rcu()` below.
         let signaled: bool = unsafe { bindings::dma_fence_test_signaled_flag(guard.as_raw()) };
         if !signaled {
-            pr_err!("DriverFence drops unsignaled. Danger of memory corruption!\n");
             // SAFETY: `guard` is valid until the `call_rcu()` below. The fence
             // must not have been signaled yet, which we check directly above.
             unsafe { bindings::dma_fence_set_error(guard.as_raw(), ECANCELED.to_errno()) };
-- 
2.56.0

Re: [PATCH] rust: DamFence: Remove warning about unsignaled drop

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] rust: dma_fence: stale DriverFence rustdoc after warning removal
  in drop()
--

commit 1fa69b16f405cf6ecb2511fccf990063ada22d3a
Author: Philipp Stanner <phasta@kernel.org>

rust: DamFence: Remove warning about unsignaled drop

This commit removes the error message printed when a DriverFence is dropped
before being signaled.

> diff --git a/rust/kernel/dma_buf/dma_fence.rs b/rust/kernel/dma_buf/dma_fence.rs
> index 18a43e1bb442f..a46b6a1f114f1 100644
> --- a/rust/kernel/dma_buf/dma_fence.rs
> +++ b/rust/kernel/dma_buf/dma_fence.rs
> @@ -934,7 +934,6 @@ fn drop(&mut self) {
>          // SAFETY: `guard` is valid until the `call_rcu()` below.
>          let signaled: bool = unsafe { bindings::dma_fence_test_signaled_flag(guard.as_raw()) };
>          if !signaled {
> -            pr_err!("DriverFence drops unsignaled. Danger of memory corruption!\n");
>              // SAFETY: `guard` is valid until the `call_rcu()` below. The fence
>              // must not have been signaled yet, which we check directly above.
>              unsafe { bindings::dma_fence_set_error(guard.as_raw(), ECANCELED.to_errno()) };

[Severity: Low]
Should the rustdoc comment on struct DriverFence also be updated now that
dropping an unsignaled fence is canonical teardown and no longer logs a
message?

rust/kernel/dma_buf/dma_fence.rs:
    /// As a last resort, a [`DriverFence`] will signal itself if it drops
    /// unsignaled and print a warning.
    pub struct DriverFence<'a, T: FenceContextOps> {

Also, the commit subject misspells the component as DamFence, and the
commit message refers to the removed pr_err!() call as pr_warn!().

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261112101015.528127-2-phasta@kernel.org?part=1

Re: [PATCH] rust: DamFence: Remove warning about unsignaled drop

[Christian König]

On 11/12/26 11:10, Philipp Stanner wrote:
> DriverFence::drop() contains a pr_warn!() for when the fence drops
> unsignaled. This warning gets into the way of canonical teardown in
> Rust: when you drop a lock, it unlocks itself, when you drop a KBox, it
> frees itself etc. Congruently, users should be able to tear down fences
> they don't care about anymore with ECANCELED, which is already the
> default error code.

Well a dma_fence should never automatically signal when the last reference drops.

Leaving it unsignaled is the most defensive path we can go. That can lead to deadlocks but it can at least not lead to memory corruption.

Regards,
Christian.

> 
> Remove the warning.
> 
> Suggested-by: Danilo Krummrich <dakr@kernel.org>
> Signed-off-by: Philipp Stanner <phasta@kernel.org>
> ---
>  rust/kernel/dma_buf/dma_fence.rs | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/rust/kernel/dma_buf/dma_fence.rs b/rust/kernel/dma_buf/dma_fence.rs
> index a1d851656562..2c8001cb9a59 100644
> --- a/rust/kernel/dma_buf/dma_fence.rs
> +++ b/rust/kernel/dma_buf/dma_fence.rs
> @@ -942,7 +942,6 @@ fn drop(&mut self) {
>          // SAFETY: `guard` is valid until the `call_rcu()` below.
>          let signaled: bool = unsafe { bindings::dma_fence_test_signaled_flag(guard.as_raw()) };
>          if !signaled {
> -            pr_err!("DriverFence drops unsignaled. Danger of memory corruption!\n");
>              // SAFETY: `guard` is valid until the `call_rcu()` below. The fence
>              // must not have been signaled yet, which we check directly above.
>              unsafe { bindings::dma_fence_set_error(guard.as_raw(), ECANCELED.to_errno()) };