linux-kernel - Re: [RFC PATCH v1 19/25] printk: introduce emergency messages

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Date:   Tue, 12 Mar 2019 11:51:28 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 19/25] printk: introduce emergency messages

On (03/11/19 13:04), John Ogness wrote:
> > Great catch!
> 
> Yes, thanks!
> 
> > I think that it is doable to guard the list using RCU.
> 
> I think it would be enough to take the prb_cpulock when modifying the
> console linked list. That will keep printk_emergency() out until the
> list has been updated. (registering/unregistering consoles is not
> something that happens often.)

console_sem can be a bit more than just registering/unregistering.
Especially when it comes to VT, fbcon and ioctls.

$ git grep console_lock drivers/tty/ | wc -l
82

$ git grep console_lock drivers/video/fbdev/ | wc -l
80

	-ss