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

[Joonwon Kang <joonwonkang@google.com>]

> 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.

They will race because of the current blocking mode implementation. With this
patch, they should not race as it handles the known racing point. So, I think
it will be important to decide whether to support multi-threads in blocking
mode or not.

> It is the job of the original client to serialize its threads' access
> to the channel.

I can see the disparity with the non-blocking mode here. Currently, the client
does not need to serialize its threads' access to the channel in non-blocking
mode whereas it needs to in blocking mode. It would be nice if the client does
not need to in both modes, but it may also depend on the necessity as you said.

> > 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.

I agree that we may need research on the current uses of channels and the
necessity of shared channels. However, it may require non-trivial amount of
time since it requires thorough understanding of the context of every client
driver. At this point, I think we at least need a clear documentation in terms
of multi-threads support as we have none now. Since it is obvious that
multi-threads is not supported for now, I can create another patch to add this
to the API doc to be clear. How do you think?

Thanks,
Joonwon Kang