linux-kernel - Re: Side-effect free printk?

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Message-ID: <1417629958.2902.18.camel@perches.com>
Date:	Wed, 03 Dec 2014 10:05:58 -0800
From:	Joe Perches <joe@...ches.com>
To:	Julia Lawall <julia.lawall@...6.fr>
Cc:	cocci <cocci@...teme.lip6.fr>, Sebastien.Hinderer@...ia.fr,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Side-effect free printk?

On Wed, 2014-12-03 at 19:02 +0100, Julia Lawall wrote:
> 
> On Wed, 3 Dec 2014, Joe Perches wrote:
> 
> > Most all printks uses do not have any side-effects.
> >
> > Some however modify local or global state or perform
> > IO on various ports.
> >
> > Things like:
> >
> > drivers/video/fbdev/sa1100fb.c:	dev_dbg(fbi->dev, "DBAR1: 0x%08x\n", readl_relaxed(fbi->base + DBAR1));
> > drivers/remoteproc/remoteproc_core.c:	dev_err(dev, "handling crash #%u in %s\n", ++rproc->crash_cnt,
> >
> > CONFIG_PRINTK can be set to 'n', but all direct printk
> > calls still evaluate their arguments.
> >
> > These calls can unnecessarily increase code size.
> >
> > Some printk using macros are defined like:
> >
> > #define foo_dbg(fmt, ...)	\
> > do {				\
> > 	if (0)			\
> > 		printk(...);	\
> > } while (0)
> >
> > The compiler can optimize any use away so this can
> > eliminate any side-effect.
> >
> > For the general case, printk arguments that call
> > functions that perform simple calculations should not
> > qualify unless there is some global state change or
> > additional IO.
> >
> > So, with the goal of elimination of side-effects from
> > as many of the printks as possible (and the eventual
> > removal of all of the side-effects), is it possible to
> > use coccinelle to list all printk calls that have
> > side-effects in their arguments?
> >
> > It seems coccinelle would need the entire source tree
> > to do this, so I'm not sure it's possible, but it
> > doesn't hurt to ask...
[]
> When you say "have the entire source tree", do you mean things like:
> 
> printk(..., foo(x));
> 
> where it is not clear whether foo performs a side effect or not?  That
> could indeed be harder to detect.

Yes, exactly.

The ++/-- stuff is trivial.
grep can find those easily enough.


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