Re: [PATCH v3 1/3] rust: Introduce irq module

[Lyude Paul <lyude@redhat.com>]

On Thu, 2024-08-15 at 09:02 -0700, Boqun Feng wrote:
> On Thu, Aug 15, 2024 at 06:40:28AM +0000, Benno Lossin wrote:
> [...]
> > > > > > 
> > > > > > I haven't found a problem with `&IrqDisabled` as the closure parameter,
> > > > > > but I may miss something.
> > > > > 
> > > > > We could also use `&'a IrqDisabled` instead of `IrqDisabled<'a>` (note
> > > > > the first one doesn't have a lifetime). But there is no behavioral
> > > > > difference between the two. Originally the intended API was to use `&'a
> > > > > IrqDisabled<'a>` as the closure parameter and `IrqDisabled<'a>` in
> > > > > functions that require irqs being disabled. As long as we decide on a
> > > > > consistent type, I don't mind either (since then we can avoid
> > > > > reborrowing).
> > > > > 
> > > > > > So the key ask from me is: it looks like we are on the same page that
> > > > > > when `cb` returns, the IRQ should be in the same disabled state as when
> > > > > > it gets called. So how do we express this "requirement" then? Type
> > > > > > sytem, comments, safety comments?
> > > > > 
> > > > > I don't think that expressing this in the type system makes sense, since
> > > > > the type that we select (`&'a IrqDisabled` or `IrqDisabled<'a>`) will be
> > > > > `Copy`. And thus you can just produce as many of those as you want.
> > > > > 
> > > 
> > > Hmm.. on a second thought, `Copy` doesn't affect what I'm proposing
> > > here, yes one could have as many `IrqDisabled<'a>` as one wants, but
> > > making `cb` returns a `(IrqDisabled<'a>, T)` means the `cb` has to prove
> > > at least one of the `IrqDisabled<'a>` exists, i.e. it must prove the irq
> > > is still disabled, which the requirement of `with_irqs_disabled`, right?
> > 
> > Yes, but that doesn't do anything. If the token is `Copy`, then we are
> > not allowed to have the following API:
> > 
> >     fn enable_irq(irq: IrqDisabled<'_>);
> > 
> > Since if the token is `Copy`, you can just copy it, call the function
> > and still return an `IrqDisabled<'a>` to satisfy the closure. It only
> > adds verbosity IMO.
> > 
> 
> OK, so I think I'm more clear on this, basically, we are all on the same
> page that `cb` of `with_irqs_disabled()` should have the same irq
> disable state before and after the call. And my proposal of putting this
> into type system seems not worthwhile. However, I think that aligns with
> something else I also want to propose: users should be allowed to change
> the interrupt state inside `cb`, as long as 1) the state is recovered at
> last and 2) not other soundness or invalid context issues. Basically, we
> give the users as much freedom as possible. 
> 
> So two things I want to make it clear in the document of
> `with_irqs_diabled()`:
> 
> 1.	Users need to make sure the irq state remains the same when `cb`
> 	returns.
> 2.	It's up to the users whether the irq is entirely disabled inside
> 	`cb`, but users have to do it correctly.
> 
> Thoughts? Lyude, I think #2 is different than what you have in mind, but
> we actually make have users for this. Thoughts?
> 
> FYI the following is not uncommon in kernel:
> 
> 	local_irq_save(flags);
> 	while (todo) {
> 		todo = do_sth();
> 
> 		if (too_long) {
> 			local_irq_restore(flags);
> 			if (!irqs_disabled())
> 				sleep();
> 			local_irq_save(flags);
> 		}
> 	}
> 	local_irq_restore(flags);
> 
> (of course, usually it makes more sense with local_irq_disable() and
> local_irq_enable() here).

The type system approach is slightly more complicated, but I'm now realizing
it is probably the correct solution actually. Thanks for pointing that out!

So: Functions like wait_event_lock_interruptible_irq() work because they drop
the spinlock in question before re-enabling interrupts, then re-disable
interrupts and re-acquire the lock before checking the condition. This is
where a soundness issue with my current series lies.

For the sake of explanation, let's pretend we have an imaginary rust function
"irqs_on_and_sleep(irq: IrqDisabled<'_>)" that re-enables IRQs explicitly,
sleeps, then turns them back on. This leads to a soundness issue if we have
IrqDisabled be `Copy`:

with_irqs_disabled(|irq| {
  let some_guard = some_spinlockirq.lock_with(irq);
  // ^ Let's call this type Guard<'1, …>

  irqs_on_and_sleep(irq);
  // ^ because `irq` is just copied here, the lifetime '1 doesn't end here.
  // Since we re-enabled interrupts while holding a SpinLockIrq, we would
  // potentially deadlock here.

  some_function(some_guard.some_data);
});

So - I'm thinking we might want to make it so that IrqDisabled does not have
`Copy` - and that resources acquired with it should share the lifetime of an
immutable reference to it. Let's now pretend `.lock_with()` takes an &'1
IrqDisabled, and the irqs_on_and_sleep() function from before returns an
IrqDisabled.

with_irqs_disabled(|irq| { // <- still passed by value here
  let some_guard = some_spinlockirq.lock_with(&irq); // <- Guard<'1, …>

  let irq = irqs_on_and_sleep(irq); // The lifetime of '1 ends here

  some_function(some_guard.some_data);
  // Success! ^ this fails to compile, as '1 no longer lives long enough
  // for the guard to still be usable.
  // Deadlock averted :)
)}

Then if we were to add bindings for things like
wait_event_lock_interruptible_irq() - we could have those take both the
IrqDisabled token and the Guard<'1, …> by value - and then return them
afterwards. Which I believe would fix the soundness issue :)

How does that sound to everyone?

> 
> Regards,
> Boqun
> 
> > > Or you're saying there could exist an `IrqDisabled<'a>` but the
> > > interrupts are enabled?
> > 
> > No.
> > 
> > ---
> > Cheers,
> > Benno
> > 
> 

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

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