linux-kernel - Re: [RFC][PATCH v6 1/2] printk: Make printk() completely async

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Date:	Wed, 23 Mar 2016 14:20:11 +0100
From:	Jan Kara <jack@...e.cz>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>, Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org,
	Byungchul Park <byungchul.park@....com>,
	Jan Kara <jack@...e.cz>
Subject: Re: [RFC][PATCH v6 1/2] printk: Make printk() completely async

On Wed 23-03-16 10:25:41, Petr Mladek wrote:
> On Wed 2016-03-23 10:24:43, Sergey Senozhatsky wrote:
> > On (03/22/16 17:36), Petr Mladek wrote:
> > > > -	/* cpu currently holding logbuf_lock in this function */
> > > > -	static unsigned int logbuf_cpu = UINT_MAX;
> > > > +	bool in_panic = console_loglevel == CONSOLE_LOGLEVEL_MOTORMOUTH;
> > > 
> > > I am just looking at the printk in NMI patchset and I will need to
> > > deal with the panic state as well. I am not sure if this detection
> > > is secure.
> > > 
> > > This console level is set also by kdb_show_stack()
> > > and kdb_dumpregs(). I am not sure how this kdb stuff works
> > > and if it affects normal kernel but...
> > > 
> > > Anyway, it seems that many locations detects the panic situation
> > > via the variable oops_in_progress. It has another advantage
> > > that it can be easily checked and we would not need any extra
> > > variable here.
> > 
> > oops_in_progress is not my favorite global. and we can't rely on it
> > in async printk.
> > 
> > in panic() we have
> > 
> >  console_verbose();
> >  bust_spinlocks(1); 		<< sets to one
> > 
> > 	pr_emerg("Kernel panic - not syncing: %s\n", buf);
> > 	smp_send_stop();
> > 
> >  bust_spinlocks(0);		<< sets it back to zero
> > 
> > 	console_flush_on_panic();
> > 
> > there are several issues here.
> > - first, panic_cpu does not see oops_in_progress right after bust_spinlocks(0).
> > thus all printk issued from panic_cpu can go via async printk.
> 
> I though that it actually could be an advantage. console_verbore() is
> called also by oops_begin() and it does not need to be fatal. But you
> are right that it does not need to be the righ approach.

If we oops, I want printk to be sync regardless whether the machine is able
to live afterwards or not. You never know in advance... That's why I've
chosen the console_verbose() trigger and I still think it is better than
oops_in_progress or special console_panic() trigger.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR