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

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Message-ID: <CABb+yY3CBiLpZ1KrD8RFypRgkP9MOzBf1FB+gL2E-qEuSbrj6A@mail.gmail.com>
Date: Tue, 22 Jul 2025 06:50:32 +0000
From: Joonwon Kang <joonwonkang@...gle.com>
To: jassisinghbrar@...il.com, Joonwon Kang <joonwonkang@...gle.com>
Cc: alexey.klimov@....com, jonathanh@...dia.com, linux-kernel@...r.kernel.org, 
	linux-tegra@...r.kernel.org, sudeep.holla@....com, thierry.reding@...il.com
Subject: Re: [PATCH 1/2] mailbox: Use per-thread completion to fix wrong
 completion order

From: Jassi Brar <jassisinghbrar@...il.com>

On Wed, Jun 18, 2025 at 3:04 AM Joonwon Kang <joonwonkang@...gle.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 as many as the number of concurrent sender
>>   threads
>> - A completion is created in 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
>>
>> Link: https://lore.kernel.org/all/1490809381-28869-1-git-send-email-jaswi>nder.singh@...aro.org
>
> Is your issue different from what is described in the Link?
>
> thanks

The issue is the same, but the solution is different.

The previous solution in the Link tried to have per-message completion; each
completion belongs to each slot of the message queue. However, the solution in
this patch tries to have per-thread completion; each completion belongs to each
sender thread and each slot of the message queue has a pointer to that
completion; `struct mbox_message` has the "pointer" field
`struct completion *tx_complete` which points to the completion which is
created in the stack of the sender instead of owning the completion by having
the non-pointer field `struct completion tx_complete`. This way, we could avoid
the window where the same completion is reused by different sender threads
which could lead to a wrong completion order, IMHO.