linux-kernel - Re: [PATCH printk v3 03/15] printk: add missing memory barrier to wake_up

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Message-ID: <Yl/9iDqRL3zKBoSJ@alley>
Date:   Wed, 20 Apr 2022 14:34:50 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v3 03/15] printk: add missing memory barrier to
 wake_up_klogd()

On Wed 2022-04-20 01:52:25, John Ogness wrote:
> It is important that any new records are visible to preparing
> waiters before the waker checks if the wait queue is empty.
> Otherwise it is possible that:
> 
> - there are new records available
> - the waker sees an empty wait queue and does not wake
> - the preparing waiter sees no new records and begins to wait
> 
> This is exactly the problem that the function description of
> waitqueue_active() warns about.
> 
> Use wq_has_sleeper() instead of waitqueue_active() because it
> includes the necessary full memory barrier.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Great catch! Looks good to me.

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr