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

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Date:   Mon, 3 Apr 2023 07:04:43 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Wedson Almeida Filho <wedsonaf@...il.com>,
        rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...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 03/13] rust: lock: introduce `Mutex`

On Mon, Apr 03, 2023 at 10:20:52AM +0200, Peter Zijlstra wrote:
> On Thu, Mar 30, 2023 at 11:47:12AM -0700, Boqun Feng wrote:
> > On Thu, Mar 30, 2023 at 03:01:08PM +0200, Peter Zijlstra wrote:
> > > On Thu, Mar 30, 2023 at 01:39:44AM -0300, Wedson Almeida Filho wrote:
> > > > From: Wedson Almeida Filho <walmeida@...rosoft.com>
> > > > 
> > > > This is the `struct mutex` lock backend and allows Rust code to use the
> > > > kernel mutex idiomatically.
> > > 
> > > What, if anything, are the plans to support the various lockdep
> > > annotations? Idem for the spinlock thing in the other patch I suppose.
> > 
> > FWIW:
> > 
> > *	At the init stage, SpinLock and Mutex in Rust use initializers
> > 	that are aware of the lockdep, so everything (lockdep_map and
> > 	lock_class) is all set up.
> > 
> > *	At acquire or release time, Rust locks just use ffi to call C
> > 	functions that have lockdep annotations in them, so lockdep
> > 	should just work.
> > 
> 
> ffi is what the C++ world calls RAII ?
> 

ffi is foreign function interface, it means calling a C function from
Rust. Sorry if I make things confusing ;-)

> But yes, I got that far, but I wondered about things like
> spin_lock_nested(&foo, SINGLE_DEPTH_NESTING) and other such 'advanced'
> annotations.
> 

Right, I haven't really thought through them, but I think it's easy to
add them later (famous later words).

> Surely we're going to be needing them at some point. I suppose you can
> do the single depth nesting one with a special guard type (or whatever
> you call that in the rust world) ?

or a different method for Lock:

	impl Lock { // implementation block for type `Lock`
	//                 v function is called via a.lock_nested(SINGLE_DEPTH_NESTING), a is a Lock
	    fn lock_nested(&self, level: i32) -> Guard<..> {
	//  ^ defines a function           ^ returns a guard

	        ..  
	    }
	}

since Rust side just uses the same function to unlock as C side, so a
normal Guard type suffices, because we don't treat nested lock
differently at unlock time. But if we were to add some more checking at
compile time, we could have a slight different Guard or something.

Regards,
Boqun