linux-kernel - Re: [PATCH] rust: sync: add wait_interruptible

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Message-ID: <CAH5fLgjVyCSTyZqS7uSzWMTKvrxPL55PKUyM-H0RrnuZiU9VEg@mail.gmail.com>
Date: Mon, 3 Feb 2025 14:41:37 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, 
	Waiman Long <longman@...hat.com>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: sync: add wait_interruptible_freezable

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