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

[Onur <work@onurozkan.dev>]

On Mon, 07 Jul 2025 17:31:10 +0200
"Benno Lossin" <lossin@kernel.org> wrote:

> On Mon Jul 7, 2025 at 3:39 PM CEST, Onur wrote:
> > On Mon, 23 Jun 2025 17:14:37 +0200
> > "Benno Lossin" <lossin@kernel.org> wrote:
> >
> >> > 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();
> >
> >
> > Instead of `begin` and `custom`, why not something like this:
> >
> > 	let (a, c, d) = ctx.init()
> > 	    .lock(a)
> >             .lock(b)
> >             .lock(c)
> >             .unlock(b)
> >             .lock(d)
> >             .finish();
> >
> > Instead of `begin`, `init` would be better naming to imply `fini`
> > on the C side, and `unlock` instead of `custom` would make the
> > intent clearer when dropping locks mid chain.
> 
> I don't think that this `unlock` operation will work. How would you
> implement it?


We could link mutexes to locks using some unique value, so that we can
access locks by passing mutexes (though that sounds a bit odd).

Another option would be to unlock by the index, e.g.,:

	let (a, c) = ctx.init()
	    .lock(a)
            .lock(b)
            .unlock::<1>()
            .lock(c)
            .finish();

The index approach would require us to write something very similar
to `Tuple` (with macro obviously) you proposed sometime ago.

We could also just go back to your `custom` but find a better name
for it (such as `retain`).

Regards,
Onur