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

[Boqun Feng]

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.

Regards,
Boqun

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