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

Joonwon Kang
2026-04-17T08:41:48Z

> 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

Hi Jassi, can we continue discussing this matter? We can start from the recent
comments from me.

Thanks,
Joonwon Kang