On Sat, Aug 09, 2025 at 05:21:49PM +0900, Alexandre Courbot wrote:

On Thu Jul 24, 2025 at 8:32 AM JST, Gary Guo wrote:

quoted

From: Gary Guo <gary@garyguo.net>

Currently there's a custom reference counting in `block::mq`, which uses
`AtomicU64` Rust atomics, and this type doesn't exist on some 32-bit
architectures. We cannot just change it to use 32-bit atomics, because
doing so will make it vulnerable to refcount overflow. So switch it to
use the kernel refcount `kernel::sync::Refcount` instead.

There is an operation needed by `block::mq`, atomically decreasing
refcount from 2 to 0, which is not available through refcount.h, so
I exposed `Refcount::as_atomic` which allows accessing the refcount
directly.

Tested-by: David Gow <redacted>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/kernel/block/mq/operations.rs |  7 ++--
 rust/kernel/block/mq/request.rs    | 63 ++++++++----------------------
 rust/kernel/sync/refcount.rs       | 14 +++++++
 3 files changed, 34 insertions(+), 50 deletions(-)

diff --git a/rust/kernel/block/mq/operations.rs b/rust/kernel/block/mq/operations.rs
index c2b98f507bcbd..c0f95a9419c4e 100644
--- a/rust/kernel/block/mq/operations.rs
+++ b/rust/kernel/block/mq/operations.rs

@@ -10,9 +10,10 @@
     block::mq::Request,
     error::{from_result, Result},
     prelude::*,
+    sync::Refcount,
     types::ARef,
 };
-use core::{marker::PhantomData, sync::atomic::AtomicU64, sync::atomic::Ordering};
+use core::marker::PhantomData;
 
 /// Implement this trait to interface blk-mq as block devices.
 ///

@@ -78,7 +79,7 @@ impl<T: Operations> OperationsVTable<T> {
         let request = unsafe { &*(*bd).rq.cast::<Request<T>>() };
 
         // One refcount for the ARef, one for being in flight
-        request.wrapper_ref().refcount().store(2, Ordering::Relaxed);
+        request.wrapper_ref().refcount().set(2);
 
         // SAFETY:
         //  - We own a refcount that we took above. We pass that to `ARef`.

@@ -187,7 +188,7 @@ impl<T: Operations> OperationsVTable<T> {
 
             // SAFETY: The refcount field is allocated but not initialized, so
             // it is valid for writes.
-            unsafe { RequestDataWrapper::refcount_ptr(pdu.as_ptr()).write(AtomicU64::new(0)) };
+            unsafe { RequestDataWrapper::refcount_ptr(pdu.as_ptr()).write(Refcount::new(0)) };

Ah, so that's why `0` is allowed as a valid value for `Refcount::new`.
Seeing the use that is made of atomics here, I wonder if `Refcount` is a
good choice, or if we could adapt the code to use them as expected. I am
not familiar enough with this part of the code to give informed advice
unfortunately.

But at the very least, I think the constructor should not be made unsafe
due to account for one particular user. How about doing a

Hmm.. a refcount being 0 is not unsafe I would say, it means no one
references the object (usually in a particular type of referencing). In
general, one should use `Arc` and be done with that, but if you were to
have different types of referencing to a same object, then one of the
refcounts may end up being 0.

`Refcount::new(1)` immediately followed by a `set(0)` so other users are
not tricked into creating an invalid Refcount?

This I will call bad code, as it would introduce further confusion
because the `new(1)` is pretty pointless imo.

Regards,
Boqun