On Fri, Oct 04, 2024 at 04:52:24PM +0100, Gary Guo wrote:
> 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.
That's scary, and of course feels wrong on many levels, but:
> @@ -91,13 +95,17 @@ pub(crate) unsafe fn start_unchecked(this: &ARef<Self>) {
> /// C `struct request`. If the operation fails, `this` is returned in the
> /// `Err` variant.
> fn try_set_end(this: ARef<Self>) -> Result<*mut bindings::request, ARef<Self>> {
> - // We can race with `TagSet::tag_to_rq`
> - if let Err(_old) = this.wrapper_ref().refcount().compare_exchange(
> - 2,
> - 0,
> - Ordering::Relaxed,
> - Ordering::Relaxed,
> - ) {
> + // To hand back the ownership, we need the current refcount to be 2.
> + // Since we can race with `TagSet::tag_to_rq`, this needs to atomically reduce
> + // refcount to 0. `Refcount` does not provide a way to do this, so use the underlying
> + // atomics directly.
> + if this
> + .wrapper_ref()
> + .refcount()
> + .as_atomic()
> + .compare_exchange(2, 0, Ordering::Relaxed, Ordering::Relaxed)
> + .is_err()
Why not just call rust_helper_refcount_set()? Or is the issue that you
think you might not be 2 here? And if you HAVE to be 2, why that magic
value (i.e. why not just always be 1 and rely on normal
increment/decrement?)
I know some refcounts are odd in the kernel, but I don't see where the
block layer is caring about 2 as a refcount anywhere, what am I missing?
thanks,
greg k-h
