[RFC PATCH] rust: block: Use 32-bit atomics

[Gary Guo]

On Thu,  5 Sep 2024 14:12:14 +0800
David Gow <davidgow at google.com> wrote:

> Not all architectures have core::sync::atomic::AtomicU64 available. In
> particular, 32-bit x86 doesn't support it. AtomicU32 is available
> everywhere, so use that instead.

Switching to 32-bit directly makes it vulnerable to counter overflow
issue. If 32-bit atomics are to be used for this, saturation logic must
be implemented to deal with it.

Ideally we should just use `refcount_t` instead of custom atomic ops.
Although it appears that the rust block driver needs a cmpxchg which
refcount_t doesn't provide.

> 
> Hopefully we can add AtomicU64 to Rust-for-Linux more broadly, so this
> won't be an issue, but it's not supported in core from upstream Rust:
> https://doc.rust-lang.org/std/sync/atomic/#portability

Kernel has a 64-bit atomic implementation which is backed by spinlocks
if the architecture doesn't support it. Although, I think for the
purpose in rust block, it's unlikely to be necessary.

Best,
Gary

> 
> This can be tested on 32-bit x86 UML via:
> ./tools/testing/kunit/kunit.py run --make_options LLVM=1 --kconfig_add CONFIG_RUST=y --kconfig_add CONFIG_64BIT=n --kconfig_add CONFIG_FORTIFY_SOURCE=n
> 
> Fixes: 3253aba3408a ("rust: block: introduce `kernel::block::mq` module")
> Signed-off-by: David Gow <davidgow at google.com>
> ---
> 
> Hi all,
> 
> I encountered this build error with Rust/UML since the kernel::block::mq
> stuff landed. I'm not 100% sure just swapping AtomicU64 with AtomicU32
> is correct -- please correct me if not -- but this does at least get the
> Rust/UML/x86-32 builds here compiling and running again.
> 
> (And gives me more encouragement to go to the Rust atomics talk at
> Plumbers.)
> 
> Cheers,
> -- David
> 
> ---
>  rust/kernel/block/mq/operations.rs |  4 ++--
>  rust/kernel/block/mq/request.rs    | 12 ++++++------
>  2 files changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/rust/kernel/block/mq/operations.rs b/rust/kernel/block/mq/operations.rs
> index 9ba7fdfeb4b2..c31c36af6bc4 100644
> --- a/rust/kernel/block/mq/operations.rs
> +++ b/rust/kernel/block/mq/operations.rs
> @@ -11,7 +11,7 @@
>      error::{from_result, Result},
>      types::ARef,
>  };
> -use core::{marker::PhantomData, sync::atomic::AtomicU64, sync::atomic::Ordering};
> +use core::{marker::PhantomData, sync::atomic::AtomicU32, sync::atomic::Ordering};
>  
>  /// Implement this trait to interface blk-mq as block devices.
>  ///
> @@ -186,7 +186,7 @@ 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(AtomicU32::new(0)) };
>  
>              Ok(0)
>          })
> diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
> index a0e22827f3f4..418256dcd45b 100644
> --- a/rust/kernel/block/mq/request.rs
> +++ b/rust/kernel/block/mq/request.rs
> @@ -13,7 +13,7 @@
>  use core::{
>      marker::PhantomData,
>      ptr::{addr_of_mut, NonNull},
> -    sync::atomic::{AtomicU64, Ordering},
> +    sync::atomic::{AtomicU32, Ordering},