[RFC PATCH v5 3/6] rust: DmaFence: Don't drop on signal

[Philipp Stanner <phasta@kernel.org>]

The original design for DmaFence intended to comply as strictly as
possible with the "dma_fence contract", which, according to some, says
that a fence must only be signaled once. Thus, the implementation was
made so that a DriverFence drops on signal.

While extending JobQueue's capabilities, it was discovered that this
characteristic causes - unnecessary - trouble: fence-data must be
dropped in an RCU-deferred manner because the C backend demands that.
However, fences are often signaled in atomic context (e.g., interrupt
handler) where you don't want to drop, but signal as quickly as
possible.

Actually, there is no real hard reason to enforce that a fence can only
get signaled once, because the C backend is robust against multiple
signal attempts.

What is decisive is that a) all fences signal and b) they only drop no
earlier than 1 RCU grace period after signaling.

Don't drop a DriverFence on signal().

Signed-off-by: Philipp Stanner <phasta@kernel.org>
---
 rust/kernel/dma_buf/dma_fence.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/rust/kernel/dma_buf/dma_fence.rs b/rust/kernel/dma_buf/dma_fence.rs
index 0028124e6aa6..b67121c7abd4 100644
--- a/rust/kernel/dma_buf/dma_fence.rs
+++ b/rust/kernel/dma_buf/dma_fence.rs
@@ -806,7 +806,7 @@ pub fn as_fence(&self) -> &Fence {
     }
 
     /// Signal the fence. This will invoke all registered callbacks.
-    pub fn signal(self, res: Result) {
+    pub fn signal(&self, res: Result) {
         let fence = self.as_fence().lock();
 
         // SAFETY: `fence` is valid because `self` is valid. The lock must be
-- 
2.55.0