Re: [PATCH v3 1/2] mailbox: Use per-thread completion to fix wrong completion order

Jassi Brar
2026-04-03T16:20:18Z

On Fri, Apr 3, 2026 at 9:51 AM Joonwon Kang <joonwonkang@google.com> wrote:
>
> > On Thu, Apr 2, 2026 at 12:07 PM Joonwon Kang <joonwonkang@google.com> wrote:
> > >
> > > Previously, a sender thread in mbox_send_message() could be woken up at
> > > a wrong time in blocking mode. It is because there was only a single
> > > completion for a channel whereas messages from multiple threads could be
> > > sent in any order; since the shared completion could be signalled in any
> > > order, it could wake up a wrong sender thread.
> > >
> > > This commit resolves the false wake-up issue with the following changes:
> > > - Completions are created just as many as the number of concurrent sender
> > > threads
> > > - A completion is created on a sender thread's stack
> > > - Each slot of the message queue, i.e. `msg_data`, contains a pointer to
> > > its target completion
> > > - tx_tick() signals the completion of the currently active slot of the
> > > message queue
> > >
> > I think I reviewed it already or is this happening on
> > one-channel-one-client usage? Because mailbox api does not support
> > channels shared among multiple clients.
>
> Yes, this patch is handling the one-channel-one-client usage but when that
> single channel is shared between multiple threads.

hmm.... how is this not single-channel-multiple-clients ?
A channel is returned as an opaque token to the clients, if that
client shares that with other threads - they will race.
It is the job of the original client to serialize its threads' access
to the channel.

> From my understanding, the
> discussion back then ended with how to circumvent the issue rather than whether
> we will eventually solve this in the mailbox framework or not, and if yes, how
> we will, and if not, why.

It will be interesting to see how many current clients actually need
to share channels. If there are enough, it makes sense to implement
some helper api
on top of existing code, instead of changing its nature totally.

Thanks
Jassi