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

Andrew Morton
2009-04-29T19:54:46Z

On Wed, 29 Apr 2009 21:45:46 +0200
Ingo Molnar <mingo@elte.hu> wrote:

>
> * 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.

But if someone does reply-to-all to linux-next and linux-kernel, and
the linux-next email gets redirected to linux-kernel, badness might
happen. Bearing in mind the screwiness of the mail clients whcih some
people use..

> > > ( 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.

Well yeah. I was wondering whether __print_once should be a char
anyway. Will that hurt text size on any arch? Will gcc dtrt with such
things? It might go and 4-byte align the chars anwyay.