linux-kernel - Re: [PATCH v2 03/13] rust: lock: introduce `Mutex`

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Date:   Wed, 5 Apr 2023 22:49:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <walmeida@...rosoft.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v2 03/13] rust: lock: introduce `Mutex`

On Wed, Apr 05, 2023 at 05:40:39PM -0300, Wedson Almeida Filho wrote:
> On Wed, Apr 05, 2023 at 10:29:32PM +0200, Peter Zijlstra wrote:
> > On Wed, Apr 05, 2023 at 05:21:44PM -0300, Wedson Almeida Filho wrote:
> > > On Wed, Apr 05, 2023 at 09:18:26PM +0200, Peter Zijlstra wrote:
> > > > On Wed, Apr 05, 2023 at 08:04:22PM +0200, Greg KH wrote:
> > > > > On Wed, Apr 05, 2023 at 08:03:11PM +0200, Greg KH wrote:
> > > > > > On Wed, Apr 05, 2023 at 02:51:01PM -0300, Wedson Almeida Filho wrote:
> > > > > > > +void rust_helper_mutex_lock(struct mutex *lock)
> > > > > > > +{
> > > > > > > +	mutex_lock(lock);
> > > > > > > +}
> > > > > > > +EXPORT_SYMBOL_GPL(rust_helper_mutex_lock);
> > > > > > > +
> > > > > > 
> > > > > > No need to ever unlock a mutex?
> > > > > 
> > > > > Oh nevermind, mutex_lock() is a macro, mutex_unlock() is not...
> > > > 
> > > > Yeah, so I despise all these stupid helpers... but I suppose it's the
> > > > best they could come up with to interface the languages :/
> > > > 
> > > > The only hope is that the thing can do cross-language LTO or something
> > > > to re-inline stuff.
> > > 
> > > One thing we could to do improve the situation is to convert some of the
> > > existing macros into inline functions on the header files.
> > > 
> > > We can't do it for all cases (e.g., cases like mutex_init that declare a new
> > > static variable when lockdep is enabled) but mutex_lock is just a function
> > > when lockdep is disabled, and just calls mutex_lock_nested() when it is enabled.
> > > 
> > > How do you feel about this?
> > > 
> > > -#define mutex_lock(lock) mutex_lock_nested(lock, 0)
> > > +static inline void mutex_lock(struct mutex *lock)
> > > +{
> > > +       mutex_lock_nested(lock, 0);
> > > +}
> > 
> > Can rust actually parse C headers and inline C functions ? I thought the
> > whole problem was that it can only call C ABI symbols (which inline
> > functions are not).
> 
> Rust can't. We use a tool called bindgen to read C header files and generate
> equivalent Rust (extern "C") declarations for functions. The tool has been
> enhanced recently (https://github.com/rust-lang/rust-bindgen/pull/2335) to
> handle static inline functions by automatically generating helpers like the one
> above (in addition to the Rust decls).

Oh man, that's sad, I was hoping it would write the equivalent inline
function in rust.

> So the situation is improved in that we don't need to manually write (and
> commit) the helpers. It may improve further in the future if we get better
> integration of the languages.

But yeah, feel free to convert macros to inline functions where the
difference is moot. There is indeed no real reason for mutex_lock() to
not be an inline function in that case.