linux-kernel - Re: [PATCH printk v4 13/15] printk: add kthread console printers

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Date:   Fri, 22 Apr 2022 09:48:51 +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,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v4 13/15] printk: add kthread console printers

On Thu 2022-04-21 23:28:48, 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 irqwork triggered
> by the printk caller, referred to as direct printing.
> 
> Since threaded console printers are responsible for all printing
> during normal operation, this also includes messages generated via
> deferred printk calls. If direct printing is in effect during a
> deferred printk call, the queued irqwork will perform the direct
> printing. To make it clear that this is the only time that the
> irqwork will perform direct printing, rename the flag
> PRINTK_PENDING_OUTPUT to PRINTK_PENDING_DIRECT_OUTPUT.
> 
> Threaded console printers synchronize against each other and against
> console lockers by taking the console lock for each message that is
> printed.
> 
> Note that the kthread printers do not care about direct printing.
> They will always try to print if new records are available. They can
> be blocked by direct printing, but will be woken again once direct
> printing is finished.
> 
> Console unregistration is a bit tricky because the associated
> kthread printer cannot be stopped while the console lock is held.
> A policy is implemented that states: whichever task clears
> con->thread (under the console lock) is responsible for stopping
> the kthread. unregister_console() will clear con->thread while
> the console lock is held and then stop the kthread after releasing
> the console lock.
> 
> For consoles that have implemented the exit() callback, the kthread
> is stopped before exit() is called.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Looks good to me.

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

Best Regards,
Petr