linux-kernel - Re: [PATCH v9 1/2] binder: frozen notification

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Message-ID: <ZpZ_jva0L5DorZPh@google.com>
Date: Tue, 16 Jul 2024 14:11:26 +0000
From: Carlos Llamas <cmllamas@...gle.com>
To: Yu-Ting Tseng <yutingtseng@...gle.com>
Cc: tkjos@...gle.com, gregkh@...uxfoundation.org, arve@...roid.com,
	maco@...roid.com, joel@...lfernandes.org, brauner@...nel.org,
	surenb@...gle.com, aliceryhl@...gle.com, kernel-team@...roid.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 1/2] binder: frozen notification

On Tue, Jul 09, 2024 at 12:00:47AM -0700, Yu-Ting Tseng wrote:
> Frozen processes present a significant challenge in binder transactions.
> When a process is frozen, it cannot, by design, accept and/or respond to
> binder transactions. As a result, the sender needs to adjust its
> behavior, such as postponing transactions until the peer process
> unfreezes. However, there is currently no way to subscribe to these
> state change events, making it impossible to implement frozen-aware
> behaviors efficiently.
> 
> Introduce a binder API for subscribing to frozen state change events.
> This allows programs to react to changes in peer process state,
> mitigating issues related to binder transactions sent to frozen
> processes.
> 
> Implementation details:
> For a given binder_ref, the state of frozen notification can be one of
> the followings:
> 1. Userspace doesn't want a notification. binder_ref->freeze is null.
> 2. Userspace wants a notification but none is in flight.
>    list_empty(&binder_ref->freeze->work.entry) = true
> 3. A notification is in flight and waiting to be read by userspace.
>    binder_ref_freeze.sent is false.
> 4. A notification was read by userspace and kernel is waiting for an ack.
>    binder_ref_freeze.sent is true.
> 
> When a notification is in flight, new state change events are coalesced into
> the existing binder_ref_freeze struct. If userspace hasn't picked up the
> notification yet, the driver simply rewrites the state. Otherwise, the
> notification is flagged as requiring a resend, which will be performed
> once userspace acks the original notification that's inflight.
> 
> See https://r.android.com/3070045 for how userspace is going to use this
> feature.
> 
> Signed-off-by: Yu-Ting Tseng <yutingtseng@...gle.com>
> ---

Thanks,

Acked-by: Carlos Llamas <cmllamas@...gle.com>