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

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Message-ID: <1440535866.2670.164.camel@perches.com>
Date:	Tue, 25 Aug 2015 13:51:06 -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 13:39 -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.

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