linux-kernel - Re: [PATCH v6 3/3] rust: sync: Add SpinLockIrq

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Message-ID: <Zw7N4RKzWAS9qi4I@Boquns-Mac-mini.local>
Date: Tue, 15 Oct 2024 13:17:37 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Lyude Paul <lyude@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
	rust-for-linux@...r.kernel.org, Danilo Krummrich <dakr@...hat.com>,
	airlied@...hat.com, Ingo Molnar <mingo@...hat.com>,
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	Benno Lossin <benno.lossin@...ton.me>,
	Daniel Almeida <daniel.almeida@...labora.com>,
	Gary Guo <gary@...yguo.net>, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Martin Rodriguez Reboredo <yakoyoku@...il.com>,
	Valentin Obst <kernel@...entinobst.de>
Subject: Re: [PATCH v6 3/3] rust: sync: Add SpinLockIrq

On Tue, Oct 15, 2024 at 02:57:11PM +0200, Andreas Hindborg wrote:
> Boqun Feng <boqun.feng@...il.com> writes:
> 
> > On Sat, Oct 05, 2024 at 02:19:38PM -0400, Lyude Paul wrote:
> >> On Fri, 2024-10-04 at 14:48 -0400, Lyude Paul wrote:
> >> > 
> >> > FWIW: I agree we want things to map C closely wherever we can, but part of the
> >> > reason of having rust in the kernel at all is to take advantage of the
> >> > features it provides us that aren't in C - so there's always going to be
> >> > differences in some places. This being said though, I'm more then happy to
> >> > minimize those as much as possible and explore ways to figure out how to make
> >> > it so that correctly using these interfaces is as obvious and not-error prone
> >> > as possible. The last thing I want is to encourage bad patterns in drivers
> >> > that maintainers have to deal with the headaches of for ages to come,
> >> > especially when rust should be able to help with this as opposed to harm :).
> >> 
> >> I was thinking about this a bit more today and I realized I might actually
> >> have a better solution that I think would actually map a lot closer to the C
> >> primitives and I feel a bit silly it didn't occur to me before.
> >> 
> >> What if instead of with_interrupts_disabled, we extended Lock so that types
> >> like SpinLockIrq that require a context like IrqDisabled can require the use
> >> of two new methods:
> >> 
> >> * first_lock<R>(&self, cb: impl for<'a> FnOnce(Guard<'a, T, B>, B::Context<'a>) -> R) -> R
> >
> > I think you really want to use a `&mut T` instead of `Guard<'a, T, B>`,
> > otherwise people can do:
> >
> > 	let g = lock1.first_lock(|guard, _ctx| { guard });
> > 	// here the lock is held, but the interrupts might be enabled.
> 
> Is it impossible to limit the lifetime of the guard such that it cannot
> be returned from `first_lock`?
> 

I was wrong saying the original doesn't work, because it has a
`for<'a>`, that means `'a` is lifetime of the closure, which cannot
outlive the return value `R`. So this signature might be valid.

Regards,
Boqun

> BR Andreas
>