On Thu, 5 Sep 2024 14:12:14 +0800 David Gow <davidgow@xxxxxxxxxx> 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@xxxxxxxxxx> > --- > > 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},
