linux-kernel - Re: [announce] new tree: "fix all build warnings, on all configs"

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Date:	Wed, 22 Oct 2008 11:47:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jörn Engel <joern@...fs.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland Dreier <rdreier@...co.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	David Howells <dhowells@...hat.com>
Subject: Re: [announce] new tree: "fix all build warnings, on all configs"


* Jörn Engel <joern@...fs.org> wrote:

> On Mon, 20 October 2008 21:21:10 +0200, Ingo Molnar wrote:
> >  
> >  /* Alas, no aliases. Too much hassle with bringing module.h everywhere */
> >  #define fops_get(fops) \
> > -	(((fops) && try_module_get((fops)->owner) ? (fops) : NULL))
> > +	(((fops != NULL) && try_module_get((fops)->owner) ? (fops) : NULL))
> >  #define fops_put(fops) \
> > -	do { if (fops) module_put((fops)->owner); } while(0)
> > +	do { if (fops != NULL) module_put((fops)->owner); } while(0)
> 
> This, I would argue, makes the code worse.

Have a look at:

   $ git log -p --grep="NULL noise"

for example:

        for (i = 0; i < MAX_FEB_SIZE; i++)
-               if (tb->FEB[i] != 0)
+               if (tb->FEB[i] != NULL)
                        break;

so checking for != NULL is a valid way of testing a pointer's existence. 
The "if (tb->FEB[i])" is a valid shortcut for the same thing as well.

In this specific case the issue is that the 'fops' parameter can 
occasionally be a constant pointer (turning the test into always-true) 
so the compiler is at least minimally correct at asking the "are you 
sure you want this" question - which we answer in the affirmative via 
the explicit NULL check. But these are really nuances.

	Ingo
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