linux-kernel - Re: [PATCH v2 1/3] rust_binder: freeze_notif

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Message-ID: <aOeaVMHC2SWzg_Fe@google.com>
Date: Thu, 9 Oct 2025 11:19:48 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Carlos Llamas <cmllamas@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Arve Hjønnevåg" <arve@...roid.com>, Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joelagnelf@...dia.com>, Christian Brauner <brauner@...nel.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 1/3] rust_binder: freeze_notif_done should resend if
 wrong state

On Wed, Oct 08, 2025 at 04:52:54PM +0000, Carlos Llamas wrote:
> On Wed, Oct 08, 2025 at 06:41:20PM +0200, Alice Ryhl wrote:
> > On Wed, Oct 8, 2025 at 6:38 PM Carlos Llamas <cmllamas@...gle.com> wrote:
> > >
> > > On Wed, Oct 08, 2025 at 06:34:54PM +0200, Alice Ryhl wrote:
> > > > On Wed, Oct 8, 2025 at 6:32 PM Carlos Llamas <cmllamas@...gle.com> wrote:
> > > > >
> > > > > On Tue, Oct 07, 2025 at 09:39:51AM +0000, Alice Ryhl wrote:
> > > > > > Consider the following scenario:
> > > > > > 1. A freeze notification is delivered to thread 1.
> > > > > > 2. The process becomes frozen or unfrozen.
> > > > > > 3. The message for step 2 is delivered to thread 2 and ignored because
> > > > > >    there is already a pending notification from step 1.
> > > > > > 4. Thread 1 acknowledges the notification from step 1.
> > > > > > In this case, step 4 should ensure that the message ignored in step 3 is
> > > > > > resent as it can now be delivered.
> > > > >
> > > > > hmmm, I wonder what happens with 3 threads involved where the state goes
> > > > > back to the (unconsumed) initial freeze notification. Userspace will
> > > > > probably see two separate notifications of the same state?
> > > >
> > > > The way I implemented it, the work items report the current state when
> > > > the work item is *executed*, and they do nothing if there's no change
> > > > since last notification.
> > >
> > > Oh I see, then that means the 2nd and 3rd notifications would do nothing
> > > as the state went back to the last notification, correct?
> > 
> > Yeah.
> > 
> > If the state flips quickly, userspace might not get told about that if
> > it's too slow to receive the update, but that's no different from C
> > Binder.
> 
> I believe the difference is C binder doesn't report the current state at
> the time of consuming the notification. So I'm thinking that it would
> report two notifications regardless of the state, even if they are both
> the same. Oh well.

Yeah, I guess if it only toggles once, then C Binder will report that.
On the other hand, if it toggles multiple times, then C Binder might not
report it that many times.

We could make Rust Binder faithfully report the callback the right
number of times in the face of toggling, but it seems not worth it.

Alice