Re: [PATCH] rust: sync: add wait_interruptible_freezable

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Feb 03, 2025 at 02:41:37PM +0100, Alice Ryhl wrote:
> On Mon, Feb 3, 2025 at 2:38 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Mon, Feb 03, 2025 at 12:56:05PM +0100, Alice Ryhl wrote:
> > > On Mon, Feb 3, 2025 at 12:54 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > > >
> > > > On Thu, Jan 30, 2025 at 11:30:44AM +0000, Alice Ryhl wrote:
> > > > > Binder allows you to freeze a process where some of its threads are
> > > > > blocked on the Binder driver. To make this work, we need to pass
> > > > > TASK_FREEZABLE when going to sleep in the appropriate places. Thus, add
> > > > > a new method wait_interruptible_freezable for the condition variable so
> > > > > that sleeps where this is supported can be marked as such.
> > > >
> > > > The constraint on freezable is that you must not hold locks. There is a
> > > > lockdep check for this in the code, but it would probably make sense to
> > > > teach Rust about this constraint as well, hmm?
> > >
> > > Unfortunately, I don't think there's any way to enforce this at
> > > compile time, but I'm definitely happy to add this in the
> > > documentation.
> >
> > Ah, ISTR people talking about teaching Rust about the whole raw_spinlock
> > vs spinlock vs mutex nesting order and figured if it can do that, then

Peter, are you talking about the POC idea I proposed on tracking irqsave
status:

	https://lore.kernel.org/rust-for-linux/20241018055125.2784186-1-boqun.feng@gmail.com/

? I'm working on this right now, however, I don't think this would help
spinlock or mutex nesting? Because there's no global(percpu) status of
acquiring these locks. Am I missing something here?

Regards,
Boqun

> > this should be doable too.
> >
> > But perhaps that never quite happened.
> 
> There isn't too much progress on that front lately, but you are right
> that this work could be extended to support this case too.
> 
> > Yes, documentation would be good. Just in case it isn't obviuos,
> > freezing a task that holds a lock can trivially deadlock vs another task
> > that needs that lock to complete before it too can hit freezable.
> 
> I'll include those details, thanks!
> 
> Alice