linux-kernel - Re: [PATCH v5 2/3] implement ww_mutex abstraction for the Rust tree

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Message-Id: <DAU0ELV91E2Q.35FZOII18W44J@kernel.org>
Date: Mon, 23 Jun 2025 17:14:37 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>
Cc: Onur Özkan <work@...rozkan.dev>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <ojeda@...nel.org>, <alex.gaynor@...il.com>, <gary@...yguo.net>,
 <a.hindborg@...nel.org>, <aliceryhl@...gle.com>, <tmgross@...ch.edu>,
 <dakr@...nel.org>, <peterz@...radead.org>, <mingo@...hat.com>,
 <will@...nel.org>, <longman@...hat.com>, <felipe_life@...e.com>,
 <daniel@...lak.dev>, <bjorn3_gh@...tonmail.com>
Subject: Re: [PATCH v5 2/3] implement ww_mutex abstraction for the Rust tree

On Mon Jun 23, 2025 at 4:47 PM CEST, Boqun Feng wrote:
> On Mon, Jun 23, 2025 at 03:44:58PM +0200, Benno Lossin wrote:
>> I didn't have a concrete API in mind, but after having read the
>> abstractions more, would this make sense?
>> 
>>     let ctx: &WwAcquireCtx = ...;
>>     let m1: &WwMutex<T> = ...;
>>     let m2: &WwMutex<Foo> = ...;
>> 
>>     let (t, foo, foo2) = ctx
>>         .begin()
>>         .lock(m1)
>>         .lock(m2)
>>         .lock_with(|(t, foo)| &*foo.other)
>>         .finish();
>> 
>
> Cute!
>
> However, each `.lock()` will need to be polymorphic over a tuple of
> locks that are already held, right? Otherwise I don't see how
> `.lock_with()` knows it's already held two locks. That sounds like a
> challenge for implementation.

I think it's doable if we have 

    impl WwActiveCtx {
        fn begin(&self) -> WwActiveCtx<'_, ()>;
    }

    struct WwActiveCtx<'a, Locks> {
        locks: Locks,
        _ctx: PhantomData<&'a WwAcquireCtx>,
    }

    impl<'a, Locks> WwActiveCtx<'a, Locks>
    where
        Locks: Tuple
    {
        fn lock<'b, T>(
            self,
            lock: &'b WwMutex<T>,
        ) -> WwActiveCtx<'a, Locks::Append<WwMutexGuard<'b, T>>>;

        fn lock_with<'b, T>(
            self,
            get_lock: impl FnOnce(&Locks) -> &'b WwMutex<T>,
        ) -> WwActiveCtx<'a, Locks::Append<WwMutexGuard<'b, T>>>;
        // I'm not 100% sure that the lifetimes will work out...

        fn finish(self) -> Locks;
    }

    trait Tuple {
        type Append<T>;

        fn append<T>(self, value: T) -> Self::Append<T>;
    }

    impl Tuple for () {
        type Append<T> = (T,);

        fn append<T>(self, value: T) -> Self::Append<T> {
            (value,)
        }
    }
    
    impl<T1> Tuple for (T1,) {
        type Append<T> = (T1, T);

        fn append<T>(self, value: T) -> Self::Append<T> {
            (self.0, value,)
        }
    }

    impl<T1, T2> Tuple for (T1, T2) {
        type Append<T> = (T1, T2, T);

        fn append<T>(self, value: T) -> Self::Append<T> {
            (self.0, self.1, value,)
        }
    }

    /* these can easily be generated by a macro */

> We also need to take into consideration that the user want to drop any
> lock in the sequence? E.g. the user acquires a, b and c, and then drop
> b, and then acquires d. Which I think is possible for ww_mutex.

Hmm what about adding this to the above idea?:

    impl<'a, Locks> WwActiveCtx<'a, Locks>
    where
        Locks: Tuple
    {
        fn custom<L2>(self, action: impl FnOnce(Locks) -> L2) -> WwActiveCtx<'a, L2>;
    }

Then you can do:

    let (a, c, d) = ctx.begin()
        .lock(a)
        .lock(b)
        .lock(c)
        .custom(|(a, _, c)| (a, c))
        .lock(d)
        .finish();

>>     let _: &mut T = t;
>>     let _: &mut Foo = foo;
>>     let _: &mut Foo = foo2;

Ah these will actually be `WwMutexGuard<'_, ...>`, but that should be
expected.

---
Cheers,
Benno