linux-kernel - Re: [PATCH] intel-iommu: fix build with CONFIG_BRANCH

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Message-ID: <20090407121457.GC11641@elte.hu>
Date:	Tue, 7 Apr 2009 14:14:57 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	torvalds@...ux-foundation.org, iommu@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] intel-iommu: fix build with CONFIG_BRANCH_TRACER=y


* David Woodhouse <dwmw2@...radead.org> wrote:

> On Tue, 2009-04-07 at 11:02 +0200, Ingo Molnar wrote:
> > for_each_active_iommu() and for_each_iommu() uses some tricky
> > C that is weird and borderline valid but does not allow the
> > macro evaluation trick used by the branch tracer/profiler:
> > 
> >    drivers/pci/intr_remapping.c:680:28: error: macro "if" passed 2
> > arguments, but takes just 1
> > 
> > Switch it to a braced group statement.
> 
> It doesn't even need the braces -- just putting simple parens 
> around the comma-expression would have sufficed (that was Andrew's 
> attempt).
> 
> But it's a workaround, not a fix -- if we're going to #define if() 
> then we should damn well make it transparent, and not have to work 
> around breakage in arbitrary places.

Well, i consider it a feature that it flags weird if (x, y) 
constructs: and yes, these iterators you introduced, while they are 
legit C, definitely count as 'weird'. If regular code was doing it, 
not a loop abstraction, i'd call it non-obvious and borderline 
broken straight away.

We should _never ever_ put comma statements into if () constructs 
without a _really_ good reason - and if yes, we can flag that we 
know what we are doing, via extra parentheses.

> Hence the patch I sent to Linus last night, which fixes it in the 
> _right_ place:
> 	http://marc.info/?l=linux-kernel&m=123907505308502&w=2

I dont think that's the right fix for the reason above - all the 
places where the branch-profiler 'broke' the build before were 
genuinely weird pieces of code that needed fixing.

	Ingo
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