linux-kernel - Re: [PATCH] mm: Fix printk format strings

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Message-Id: <20071011001300.7682a93b.akpm@linux-foundation.org>
Date:	Thu, 11 Oct 2007 00:13:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Vegard Nossum" <vegard.nossum@...il.com>
Cc:	"Nick Piggin" <nickpiggin@...oo.com.au>,
	"Hugh Dickins" <hugh@...itas.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Fix printk format strings

On Thu, 11 Oct 2007 09:04:57 +0200 "Vegard Nossum" <vegard.nossum@...il.com> wrote:

> > > -     printk("Active:%lu inactive:%lu dirty:%lu writeback:%lu unstable:%lu\n"
> > > -             " free:%lu slab:%lu mapped:%lu pagetables:%lu bounce:%lu\n",
> > > +     printk("Active:%lu inactive:%lu dirty:%lu writeback:%lu unstable:%lu\n",
> > >               global_page_state(NR_ACTIVE),
> > >               global_page_state(NR_INACTIVE),
> > >               global_page_state(NR_FILE_DIRTY),
> > >               global_page_state(NR_WRITEBACK),
> > > -             global_page_state(NR_UNSTABLE_NFS),
> > > +             global_page_state(NR_UNSTABLE_NFS));
> > > +     printk(" free:%lu slab:%lu mapped:%lu pagetables:%lu bounce:%lu\n",
> > >               global_page_state(NR_FREE_PAGES),
> > >               global_page_state(NR_SLAB_RECLAIMABLE) +
> > >                       global_page_state(NR_SLAB_UNRECLAIMABLE),
> >
> > I don't understand the reason for this change.
> 
> I'm sorry :). It helps to make one line per call only, since this
> allows changing the printk internals for the better by reducing some
> complexity. I believe this is a good thing. I have a patch that
> changes printk, but it assumes that each format string only contains a
> single line. Is this a very bad assumption to make? Or maybe I should
> have sent that change first and made a reference to it?
> 
> But don't you also agree, on the grounds of principle, that a single
> line per call is better style?

Well we do multiple-lines-per-printk in rather a lot of places and it has
two advantages:

- less text size (I expect)

- the printk is "atomic" in that the logically-connected output lines
  won't get tangled up with an intervening printk from another CPU, or from
  an interrupt handler on this CPU.

Those are pretty modest advantages and I guess we could live without them
if we got a significant gain from doing so.  But it'd take quite some
effort hunting down all the callsites, and preventing new ones from
occuring.

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