linux-kernel - Re: [PATCH] tty/vt: Add console_lock check to vt_console

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Date:   Tue, 30 Aug 2022 19:07:29 +0200
From:   Sam Ravnborg <sam@...nborg.org>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Xuezhi Zhang <zhangxuezhi1@...lpad.com>,
        Yangxi Xiang <xyangxi5@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        nick black <dankamongmen@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>
Subject: Re: [PATCH] tty/vt: Add console_lock check to vt_console_print()

Hi Daniel,

On Tue, Aug 30, 2022 at 04:49:45PM +0200, Daniel Vetter wrote:
> I'm scratching my head why we have this printing_lock. Digging through
> historical git trees shows that:
> - Added in 1.1.73, and I found absolutely no reason why.
> - Converted to atomic bitops in 2.1.125pre2, I guess as part of SMP
>   enabling/bugfixes.
> - Converted to a proper spinlock in b0940003f25d ("vt: bitlock fix")
>   because the hand-rolled atomic version lacked necessary memory
>   barriers.
> 
> Digging around in lore for that time period did also not shed further
> light.
> 
> The only reason I think this might still be relevant today is that (to
> my understanding at least, ymmv) during an oops we might be printing
> without console_lock held. See console_flush_on_panic() and the
> comments in there - we flush out the console buffers irrespective of
> whether we managed to acquire the right locks.
> 
> The strange thing is that this reason is fairly recent, because the
> console flushing was historically done without oops_in_progress set.
> This only changed in c7c3f05e341a ("panic: avoid deadlocks in
> re-entrant console drivers"), which removed the call to
> bust_spinlocks(0) (which decrements oops_in_progress again) before
> flushing out the console (which back then was open coded as a
> console_trylock/unlock pair).
> 
> Note that this entire mess should be properly fixed in the
> printk/console layer, and not inflicted on each implementation.
> 
> For now just document what's going on and check that in all other
> cases callers obey the locking rules.
> 
> v2: WARN_CONSOLE_UNLOCKED already checks for oops_in_progress
> (something else that should be fixed I guess), hence remove the
> open-coded check I've had.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jiri Slaby <jirislaby@...nel.org>
> Cc: "Ilpo Järvinen" <ilpo.jarvinen@...ux.intel.com>
> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
> Cc: Xuezhi Zhang <zhangxuezhi1@...lpad.com>
> Cc: Yangxi Xiang <xyangxi5@...il.com>
> Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Cc: nick black <dankamongmen@...il.com>
> Cc: Petr Mladek <pmladek@...e.com>
> Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: John Ogness <john.ogness@...utronix.de>
> Cc: Sam Ravnborg <sam@...nborg.org>

It is always good to warn in case assumptions do not hold.
And thanks for the comment.

Reviewed-by: Sam Ravnborg <sam@...nborg.org>

Hmm, I prefer to start comments with upper-case, but in vt.c there is no
specific style.

	Sam

> --
> Note that this applies on top of my earlier vt patch:
> 
> https://lore.kernel.org/lkml/20220826202419.198535-1-daniel.vetter@ffwll.ch/
> 
> Expect more, I'm digging around in here a bit ...
> -Daniel
> ---
>  drivers/tty/vt/vt.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 4d29e4a17db7..a6be32798fad 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -3083,7 +3083,9 @@ static void vt_console_print(struct console *co, const char *b, unsigned count)
>  	ushort start_x, cnt;
>  	int kmsg_console;
>  
> -	/* console busy or not yet initialized */
> +	WARN_CONSOLE_UNLOCKED();
> +
> +	/* this protects against concurrent oops only */
>  	if (!spin_trylock(&printing_lock))
>  		return;
>  
> -- 
> 2.37.2