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

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Message-ID: <20160323092541.GC5522@pathway.suse.cz>
Date:	Wed, 23 Mar 2016 10:25:41 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	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 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.


> - second, smp_send_stop() does not guarantee that all of the CPUs received
> STOP IPI by the time it returns. on some platforms (ARM, for instance)
> smp_send_stop()

Good to know.

> so I wanted to have in printk some panic indication that once set never
> gets cleared. my proposal was
> 
> void console_panic(void)
> {
> 	printk_sync = false;
> }

Great idea. I think that we want to call this in panic() instead of
in vprintk_emit(). I mean that we should change the global flag only
when we are really going down.

Best Regards,
Petr