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

[Lyude Paul]

On Tue, 2024-10-15 at 13:21 -0700, Boqun Feng wrote:
> On Tue, Oct 15, 2024 at 01:17:37PM -0700, Boqun Feng wrote:
> > On Tue, Oct 15, 2024 at 02:57:11PM +0200, Andreas Hindborg wrote:
> > > Boqun Feng <boqun.feng@xxxxxxxxx> 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.
> > 
> 
> But another problem is that with this signature, `cb` can drop the lock,
> which is not expected, because the lock dropping should be done by
> `first_lock` itself.

I thought we agreed on switching this to &mut though? In which case dropping
the guard doesn't really matter

> 
> Regards,
> Boqun
> 
> > Regards,
> > Boqun
> > 
> > > BR Andreas
> > > 
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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