linux-kernel - Re: [PATCH] console: prevent registered consoles from dumping old kernel message over again

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Date:	Thu, 20 Jan 2011 09:44:58 +0800
From:	Feng Tang <feng.tang@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"gregkh@...e.de" <gregkh@...e.de>,
	"alan@...ux.intel.com" <alan@...ux.intel.com>
Subject: Re: [PATCH] console: prevent registered consoles from dumping old
 kernel message over again

On Thu, 20 Jan 2011 08:11:35 +0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 12 Jan 2011 14:48:46 +0800
> Feng Tang <feng.tang@...el.com> wrote:
> 
> > For a platform with many consoles like:
> >  "console=tty1 console=ttyMFD2 console=ttyS0 earlyprintk=mrst"
> > 
> > Each time when the non "selected_console" (tty1 and ttyMFD2 here)
> > get registered, the existing kernel message will be printed out on
> > registered consoles again, the "mrst" early console will get some
> > same message for 3 times, and "tty1" will get some for twice.
> > 
> > So try to temporarily disable registered console's printing when
> > dump the existing kernel messages to the new console.
> > 
> > Signed-off-by: Feng Tang <feng.tang@...el.com>
> > ---
> >  kernel/printk.c |   13 +++++++++++++
> >  1 files changed, 13 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/printk.c b/kernel/printk.c
> > index f64b899..ba7186b 100644
> > --- a/kernel/printk.c
> > +++ b/kernel/printk.c
> > @@ -1245,6 +1245,7 @@ void register_console(struct console *newcon)
> >  	int i;
> >  	unsigned long flags;
> >  	struct console *bcon = NULL;
> > +	struct console *con;
> >  
> >  	/*
> >  	 * before we register a new CON_BOOT console, make sure we
> > don't @@ -1357,8 +1358,20 @@ void register_console(struct console
> > *newcon) spin_lock_irqsave(&logbuf_lock, flags);
> >  		con_start = log_start;
> >  		spin_unlock_irqrestore(&logbuf_lock, flags);
> > +
> > +		/*
> > +		 * Disable the existing consoles temporarily to
> > prevent it from
> > +		 * printing out kernel log again.
> > +		 */
> > +		for_each_console(con)
> > +			if (con != newcon)
> > +				con->flags &= ~CON_ENABLED;
> 
> Is it racy?  If some CPU does an unrelated printk in the middle of all
> this, might that get lost?  Perhaps not, given the way we pass the
> start/end indexes into log_buf[].

The printk message should be ok, as they will fall into the log_buf[] anyway.

> 
> >  	}
> >  	release_console_sem();
> > +
> > +	for_each_console(con)
> > +		con->flags |= CON_ENABLED;
> 
> If a console previously had CON_ENABLED unset, this will bogusly set
> it.  That will (at least) defeat the intent of a previous
> console_stop().
> 
> How's about something like this?

Yes, it makes sense, I should add some check. And this new
"exclusive_console" should be better as its name shows what it is for.

Thanks,
Feng

> 
> --- a/kernel/printk.c~a
> +++ a/kernel/printk.c
> @@ -113,6 +113,11 @@ static unsigned con_start;	/* Index into
>  static unsigned log_end;	/* Index into log_buf:
> most-recently-written-char + 1 */ 
>  /*
> + * If exclusive_console is non-NULL then only this console is to be
> printed to.
> + */
> +static struct console *exclusive_console;
> +
> +/*
>   *	Array of consoles built from command line options
> (console=) */
>  struct console_cmdline
> @@ -460,6 +465,8 @@ static void __call_console_drivers(unsig
>  	struct console *con;
>  
>  	for_each_console(con) {
> +		if (exclusive_console && con != exclusive_console)
> +			continue;
>  		if ((con->flags & CON_ENABLED) && con->write &&
>  				(cpu_online(smp_processor_id()) ||
>  				(con->flags & CON_ANYTIME)))
> @@ -1358,8 +1365,15 @@ void register_console(struct console *ne
>  		spin_lock_irqsave(&logbuf_lock, flags);
>  		con_start = log_start;
>  		spin_unlock_irqrestore(&logbuf_lock, flags);
> +		/*
> +		 * We're about to replay the log buffer.  Only do
> this to the
> +		 * just-registered console to avoid excessive
> message spam to
> +		 * the already-registered consoles.
> +		 */
> +		exclusive_console = newcon;
>  	}
>  	release_console_sem();
> +	exclusive_console = NULL;
>  	console_sysfs_notify();
>  
>  	/*
> _
> 
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