linux-kernel - wake_up_all: was: Re: [PATCH printk v1 10/13] printk: add kthread console printers

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Date:   Fri, 18 Feb 2022 10:12:50 +0100
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,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: wake_up_all: was: Re: [PATCH printk v1 10/13] printk: add kthread
 console printers

On Mon 2022-02-07 20:49:20, John Ogness wrote:
> Create a kthread for each console to perform console printing. During
> normal operation (@system_state == SYSTEM_RUNNING), the kthread
> printers are responsible for all printing on their respective
> consoles.
> 
> During non-normal operation, console printing is done as it has been:
> within the context of the printk caller or within irq work triggered
> by the printk caller.
> 
> Console printers synchronize against each other and against console
> lockers by taking the console lock for each message that is printed.
> ---
>  include/linux/console.h |   2 +
>  kernel/printk/printk.c  | 159 +++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 159 insertions(+), 2 deletions(-)
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3371,7 +3526,7 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work)
>  	}
>  
>  	if (pending & PRINTK_PENDING_WAKEUP)
> -		wake_up_interruptible(&log_wait);
> +		wake_up_interruptible_all(&log_wait);

Good catch. I am curious how this worked so far. It looks like only
the first waiter was waken and others had to wait for another new
message.

The typical problem with wake_up_interruptible_all() is that it wakes
many processes that needs to take the same lock and block each
other. There is even a name for this situation but I do not recall it.

My understanding is that wake_up_interruptible() requires that
the waken process wakes the next waiter when done. But I do not see
it guaranteed for @log_wait wait queue.

Best Regards,
Petr