linux-kernel - Re: [PATCH 5/5] ring-buffer: fix printk output

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Message-Id: <20090429080907.efb8ba2f.akpm@linux-foundation.org>
Date:	Wed, 29 Apr 2009 08:09:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Stephen Rothwell <sfr@...b.auug.org.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 5/5] ring-buffer: fix printk output

On Wed, 29 Apr 2009 11:56:25 +0200 Ingo Molnar <mingo@...e.hu> wrote:

> 
> > > My larger point remains, about possibly embedding linux-next 
> > > into lkml. I couldnt think of a single linux-next mail that isnt 
> > > relevant to lkml. It's all about commits that are destined for 
> > > upstream in 0-2.5 months.
> > 
> > Sure, I'd be OK with zapping the linux-next list.
> 
> Another, less drastic solution would be to keep it as an _alias_ 
> list. All mails posted to it also go to lkml, but it would still be 
> subscribe-able separately.

That would work, although I wonder about the potential for duplicates
turning up somewhere.

> ( This has come up before and this would be useful for a number of 
>   other things - such as tracing/instrumentation. Someone who is 
>   only interested in instrumentation related discussions could 
>   subscribe to that list. )
> 
> > > > printk_once() is racy on smp and preempt btw ;)
> > > 
> > > Like WARN_ONCE() and WARN_ON_ONCE(). It's really an "oh crap" 
> > > facility, not for normal kernel messages.
> > > 
> > > Do we want to complicate them with locking and preemption - or 
> > > should we just concentrate on getting the "oh crap" message out 
> > > to the syslog (before it's possibly too late to get anything 
> > > out)?
> > > 
> > > I have no strong opinion about it - but i tend to like the 
> > > simpler method most. printk + stack dumps themselves arent 
> > > atomic to begin with.
> > 
> > Well, it's hardly likely to be a problem.  otoh, if two CPUs _do_ 
> > hit the thing at the same time, the resulting output will be all 
> > messed up and we'd really like to see it.
> > 
> > Easily fixed with test_and_set_bit()?
> 
> but if two CPUs hit it at once then the printk+stack-dump itself is 
> already mixed up. So if we do any atomicity it should be done for 
> all the print-once APIs. (note, lockdep does such message-atomicity 
> already, in its own facility)

Confused.

<gets distracted by FW_BUG and friends.  ytf are they in kernel.h?>

#define printk_once(x...) ({				\
	static unsigned long __print_once;		\
							\
	if (!test_and_set_bit(0, &__print_once))	\
		printk(x);				\
})

How can	two CPUs do the printk(x)?
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