Re: [PATCH v6 6/7] rust: ww_mutex/exec: add high-level API

["Onur Özkan" <work@onurozkan.dev>]

On Sun, 7 Sep 2025 11:38:17 +0300
Onur <work@onurozkan.dev> wrote:

> On Sun, 7 Sep 2025 11:20:06 +0300
> Onur <work@onurozkan.dev> wrote:
> 
> > On Sat, 6 Sep 2025 12:04:34 -0300
> > Daniel Almeida <daniel.almeida@collabora.com> wrote:
> > 
> > > 
> > > 
> > > > On 6 Sep 2025, at 08:13, Onur <work@onurozkan.dev> wrote:
> > > > 
> > > > On Fri, 5 Sep 2025 16:42:09 -0300
> > > > Daniel Almeida <daniel.almeida@collabora.com> wrote:
> > > > 
> > > >> Hi Onur,
> > > >> 
> > > >>> On 3 Sep 2025, at 10:13, Onur Özkan <work@onurozkan.dev>
> > > >>> wrote:
> > > >>> 
> > > >>> `ExecContext` is a helper built on top of ww_mutex
> > > >> 
> > > >> Again, I wonder what people think about this particular name.
> > > >> 
> > > >>> that provides a retrying interface for lock acquisition.
> > > >>> When `EDEADLK` is hit, it drops all held locks, resets
> > > >>> the acquire context and retries the given (by the user)
> > > >>> locking algorithm until it succeeds.
> > > >>> 
> > > >>> The API keeps track of acquired locks, cleans them up
> > > >>> automatically and allows data access to the protected
> > > >>> data through `with_locked()`. The `lock_all()` helper
> > > >>> allows implementing multi-mutex algorithms in a simpler
> > > >>> and less error-prone way while keeping the ww_mutex
> > > >>> semantics.
> > > >>> 
> > > >> 
> > > >> Great, this was exactly what I was looking for! :)
> > > >> 
> > > >>> Signed-off-by: Onur Özkan <work@onurozkan.dev>
> > > >>> ---
> > > >>> rust/kernel/sync/lock/ww_mutex.rs      |   2 +
> > > >>> rust/kernel/sync/lock/ww_mutex/exec.rs | 176
> > > >>> +++++++++++++++++++++++++ 2 files changed, 178 insertions(+)
> > > >>> create mode 100644 rust/kernel/sync/lock/ww_mutex/exec.rs
> > > >>> 
> > > >>> diff --git a/rust/kernel/sync/lock/ww_mutex.rs
> > > >>> b/rust/kernel/sync/lock/ww_mutex.rs index
> > > >>> b415d6deae9b..7de6578513e5 100644 ---
> > > >>> a/rust/kernel/sync/lock/ww_mutex.rs +++
> > > >>> b/rust/kernel/sync/lock/ww_mutex.rs @@ -16,6 +16,8 @@
> > > >>> use core::cell::UnsafeCell;
> > > >>> use core::marker::PhantomData;
> > > >>> 
> > > >>> +pub mod exec;
> > > >>> +
> > > >>> /// Create static [`WwClass`] instances.
> > > >>> ///
> > > >>> /// # Examples
> > > >>> diff --git a/rust/kernel/sync/lock/ww_mutex/exec.rs
> > > >>> b/rust/kernel/sync/lock/ww_mutex/exec.rs new file mode 100644
> > > >>> index 000000000000..2f1fc540f0b8
> > > >>> --- /dev/null
> > > >>> +++ b/rust/kernel/sync/lock/ww_mutex/exec.rs
> > > >>> @@ -0,0 +1,176 @@
> > > >>> +// SPDX-License-Identifier: GPL-2.0
> > > >>> +
> > > >>> +//! A high-level [`WwMutex`] execution helper.
> > > >>> +//!
> > > >>> +//! Provides a retrying lock mechanism on top of [`WwMutex`]
> > > >>> and [`WwAcquireCtx`]. +//! It detects [`EDEADLK`] and handles
> > > >>> it by rolling back and retrying the +//! user-supplied locking
> > > >>> algorithm until success. +
> > > >>> +use crate::prelude::*;
> > > >>> +use crate::sync::lock::ww_mutex::{WwAcquireCtx, WwClass,
> > > >>> WwMutex, WwMutexGuard}; +use core::ptr;
> > > >>> +
> > > >>> +/// High-level execution type for ww_mutex.
> > > >>> +///
> > > >>> +/// Tracks a series of locks acquired under a common
> > > >>> [`WwAcquireCtx`]. +/// It ensures proper cleanup and retry
> > > >>> mechanism on deadlocks and provides +/// type-safe access to
> > > >>> locked data via [`with_locked`]. +///
> > > >>> +/// Typical usage is through [`lock_all`], which retries a
> > > >>> user-supplied +/// locking algorithm until it succeeds without
> > > >>> deadlock. +pub struct ExecContext<'a> {
> > > >>> +    class: &'a WwClass,
> > > >>> +    acquire: Pin<KBox<WwAcquireCtx<'a>>>,
> > > >>> +    taken: KVec<WwMutexGuard<'a, ()>>,
> > > >>> +}
> > > >>> +
> > > >>> +impl<'a> Drop for ExecContext<'a> {
> > > >>> +    fn drop(&mut self) {
> > > >>> +        self.release_all_locks();
> > > >> 
> > > >> If we move this to the acquire context, then we can do away
> > > >> with this drop impl.
> > > >> 
> > > >>> +    }
> > > >>> +}
> > > >>> +
> > > >>> +impl<'a> ExecContext<'a> {
> > > >>> +    /// Creates a new [`ExecContext`] for the given lock
> > > >>> class.
> > > >>> +    ///
> > > >>> +    /// All locks taken through this context must belong to
> > > >>> the same class.
> > > >>> +    ///
> > > >>> +    /// TODO: Add some safety mechanism to ensure classes are
> > > >>> not different.
> > > >> 
> > > >> core::ptr::eq()?
> > > >> 
> > > > 
> > > > I was thinking more of a type-level mechanism to do ensure that.
> > > 
> > > Why?
> > > 
> > 
> > So that wait-wound and wait-die classes don't get mixed up in the
> > same `ExecContext` by using type validation at compile time.
> > 
> > Of course, `core::ptr::eq()` is still useful/required when the
> > classes are of the same type but not exactly the same value. Maybe
> > we can do both.
> > 
> > 
> > Thanks,
> > Onur
> 
> I will also look into whether it's possible to remove the class from
> the mutex and instead derive it from ExecContext and WwAcquireCtx.
> This would fix both issues at once in a better way.

This is not possible due to limitations in the C implementation. :(

A mutex must be initialized with a specific class (or without one if
it's just a regular mutex) and this cannot be changed later.

I noted this to revisit the C side and see if we can make this possible
in the future (once this patch has landed).

Regards,
Onur

> 
>  -Onur 
>