linux-kernel - Re: [PATCH] cred: Remove unnecessary kdebug atomic reads

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Date:	Tue, 25 Aug 2015 14:10:03 -0700
From:	Joe Perches <joe@...ches.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Howells <dhowells@...hat.com>,
	James Morris <jmorris@...ei.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Julia Lawall <julia.lawall@...6.fr>
Subject: Re: [PATCH] cred: Remove unnecessary kdebug atomic reads

On Tue, 2015-08-25 at 13:56 -0700, Andrew Morton wrote:
> On Tue, 25 Aug 2015 13:51:06 -0700 Joe Perches <joe@...ches.com> wrote:
> 
> > On Tue, 2015-08-25 at 13:39 -0700, Andrew Morton wrote:
> > > On Tue, 25 Aug 2015 09:53:51 -0700 Joe Perches <joe@...ches.com> wrote:
> > > > commit e0e817392b9a ("CRED: Add some configurable debugging [try #6]")
> > > > added the kdebug mechanism to this file back in 2009.
> > > > 
> > > > The kdebug macro calls no_printk which always evaluates arguments.
> > > > 
> > > > Most of the kdebug uses have an unnecessary call of
> > > > 	atomic_read(&cred->usage)
> > > > 
> > > > Make the kdebug macro do nothing by defining it with
> > > > 	do { if (0) no_printk(...); } while (0)
> > > > when not enabled.
> > []
> > > Did you consider doing this within no_printk()?
> > 
> > Yes.
> > 
> > > That would break code
> > > which is relying on side-effects in the evaluation of a printk arg but
> > > that's pretty weird and I bet there isn't (and won't be) such code.
> > 
> > I'll bet you there is more than a little and I don't want to
> > experiment with it unconditionally.
> > 
> > All printks would need to be evaluated for that side-effect.
> > 
> > Safer would be to create a new no_eval_printk macro and convert
> > the no_printk uses over to that as appropriate and possibly create
> > a CONFIG_ option to use no_eval_printk instead of no_printk/printk
> > and let the adventurous find the side-effects.
> > 
> > Maybe a coccinelle script can be written to find all the locations
> > with evaluated non-constant expression arguments with side-effects.
> 
> wimp.

twice shy...

Coccinelle isn't very good at calling tree analysis, so it'd
be a difficult thing for it to do well anyway.

btw; I seems to recall suggestions around the same thing when
no_printk was moved from subsystems to kernel.h by David Howells
in 2010.

> That duplicated printk in cred.c is nasty.  We could do this?
> 
> #if 0
> #define __kdebug printk
> #else
> #define __kdebug if (0) no_printk
> #endif

You could, but what's there is a very common idiom and what
you suggest is unsafe for if/else


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