linux-kernel - Re: [Patch 1/4] Allow arch-specific cleanup before breakpoint unregistration

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Message-ID: <1274886827.21113.15.camel@concordia>
Date:	Thu, 27 May 2010 01:13:47 +1000
From:	Michael Ellerman <michael@...erman.id.au>
To:	David Howells <dhowells@...hat.com>
Cc:	prasad@...ux.vnet.ibm.com, Michael Neuling <mikey@...ling.org>,
	Benjamin Herrenschmidt <benh@....ibm.com>,
	shaggy@...ux.vnet.ibm.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Millton Miller <miltonm@....com>,
	David Gibson <dwg@....ibm.com>,
	"linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Paul Mackerras <paulus@...ba.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [Patch 1/4] Allow arch-specific cleanup before breakpoint
 unregistration

On Wed, 2010-05-26 at 10:54 +0100, David Howells wrote:
> K.Prasad <prasad@...ux.vnet.ibm.com> wrote:
> 
> > > My understanding is weak function definitions must appear in a different C
> > > file than their call sites to work on some toolchains.
> > > 
> > 
> > Atleast, there are quite a few precedents inside the Linux kernel for
> > __weak functions being invoked from the file in which they are defined
> > (arch_hwblk_init, arch_enable_nonboot_cpus_begin and hw_perf_disable to
> > name a few).
> > Moreover the online GCC docs haven't any such constraints mentioned.
> 
> I've seen problems in this area.  gcc sometimes inlines a weak function that's
> in the same file as the call point.

See the functions in kernel/softirq.c for example, and commits 43a256322
and b2e2fe996 - though unhelpfully they don't mention the gcc version. A
bit of googling suggests it was probably "gcc version 4.1.1 20060525
(Red Hat 4.1.1-1)" in that case.

But the example of hw_perf_enable() (which is weak in the same unit),
suggests maybe this isn't a bug many people are hitting in practice
anymore.

Having said that the #define foo foo pattern is reasonably neat and
avoids the problem altogether, see eg. arch_setup_msi_irqs.

cheers

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