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

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Message-ID: <20090429194546.GA17021@elte.hu>
Date:	Wed, 29 Apr 2009 21:45:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
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


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> 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.

The potential for duplicates is inherent in Cc: lines to begin with.

> > ( 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;		\
> 							\

hm, this doubles the flag size on 32-bit kernels.

> 	if (!test_and_set_bit(0, &__print_once))	\
> 		printk(x);				\
> })
> 
> How can	two CPUs do the printk(x)?

they cannot. Did i say they do? What i said (or thought to have said 
;-) was that all the print-once APIs need similar treatment - or 
neither should.

	Ingo
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