Re: [PATCH] rust: sync: add wait_interruptible_freezable

[Alice Ryhl]

On Mon, Feb 3, 2025 at 2:38 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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
> 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