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

Ingo Molnar
2009-04-29T19:46:21Z

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

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