linux-kernel - Re: [PATCH] mailbox: forward the hrtimer if not queued and under a lock

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Date:   Thu, 14 Apr 2022 09:30:14 -0500
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Björn Ardö <bjorn.ardo@...s.com>
Cc:     kernel@...s.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mailbox: forward the hrtimer if not queued and under a lock

On Thu, Mar 31, 2022 at 2:01 AM Björn Ardö <bjorn.ardo@...s.com> wrote:
>
> This reverts commit c7dacf5b0f32957b24ef29df1207dc2cd8307743,
> "mailbox: avoid timer start from callback"
>
> The previous commit was reverted since it lead to a race that
> caused the hrtimer to not be started at all. The check for
> hrtimer_active() in msg_submit() will return true if the
> callback function txdone_hrtimer() is currently running. This
> function could return HRTIMER_NORESTART and then the timer
> will not be restarted, and also msg_submit() will not start
> the timer. This will lead to a message actually being submitted
> but no timer will start to check for its compleation.
>
> The original fix that added checking hrtimer_active() was added to
> avoid a warning with hrtimer_forward. Looking in the kernel
> another solution to avoid this warning is to check hrtimer_is_queued()
> before calling hrtimer_forward_now() instead. This however requires a
> lock so the timer is not started by msg_submit() inbetween this check
> and the hrtimer_forward() call.
>
This is a very dense api used by many use-cases, I am not confident
making any changes without confirming its a real issue with the common
code. Please share your client code and traces, that will help me get
a clearer picture.

Thanks.