linux-kernel - Re: [PATCH V3] implement `ww_mutex` abstraction for the Rust tree

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Message-ID: <20250619145334.GM1613376@noisy.programming.kicks-ass.net>
Date: Thu, 19 Jun 2025 16:53:34 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Onur <work@...rozkan.dev>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org, mingo@...hat.com, will@...nel.org,
	boqun.feng@...il.com, longman@...hat.com, ojeda@...nel.org,
	alex.gaynor@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	lossin@...nel.org, a.hindborg@...nel.org, tmgross@...ch.edu,
	dakr@...nel.org, thatslyude@...il.com
Subject: Re: [PATCH V3] implement `ww_mutex` abstraction for the Rust tree

On Thu, Jun 19, 2025 at 04:44:01PM +0200, Alice Ryhl wrote:
> On Thu, Jun 19, 2025 at 4:33 PM Onur <work@...rozkan.dev> wrote:
> >
> > On Thu, 19 Jun 2025 16:14:01 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > > On Thu, Jun 19, 2025 at 05:06:56PM +0300, Onur Özkan wrote:
> > > > +bool rust_helper_ww_mutex_is_locked(struct ww_mutex *lock)
> > > > +{
> > > > +   return ww_mutex_is_locked(lock);
> > > > +}
> > >
> > > Do we really need this? In general I dislike all the _is_locked()
> > > functions and would ideally like to remove them.
> > >
> > > Pretty much the only useful pattern for any of the _is_locked()
> > > functions is:
> > >
> > >   WARN_ON_ONCE(!foo_is_locked(&foo));
> > >
> > > Any other use is dodgy as heck.
> > >
> > >
> >
> > It's an abstraction of `ww_mutex_is_locked`. Since this is an
> > abstraction module, as long as `ww_mutex_is_locked` exists I think
> > we should keep it. FWIW it's also quite useful for tests.
> 
> We're not just adding copies of all of the C methods - instead we
> focus on the things we have a use-case for. If you're using them in
> tests, then that could make sense, but otherwise you shouldn't add
> them.

It might make sense to include the assert in the method. That is,
instead of providing .is_locked() that returns a boolean, have a void
method .assert_is_locked() that traps if not locked.