linux-kernel - Re: [PATCH] kbuild, sparseirq: work around GCC bug with _

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Message-ID: <18773.43656.189438.155690@cargo.ozlabs.ibm.com>
Date:	Sat, 27 Dec 2008 15:09:44 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Sam Ravnborg <sam@...nborg.org>, Yinghai Lu <yinghai@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-next@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	mel@....ul.ie
Subject: Re: [PATCH] kbuild, sparseirq: work around GCC bug with __weak aliases

Ingo Molnar writes:

> * Ingo Molnar <mingo@...e.hu> wrote:
> 
> > > I recall David Howells had a similar issue with the bootparamter patch 
> > > set. The workaround he used was to add a barrier(); call in the weak 
> > > function to avoid the inline.
> > 
> > could we add some extra attribute to __weak that would have a similar 
> > effect? Something like __attribute__((noinline)), or something silly 
> > like __attribute__((deprecated)) - just to keep gcc from screwing up 
> > __weak functions? Perhaps adding a section attribute would have a 
> > similar effect? (putting weak definitions into an extra section is 
> > probably helpful anyway)
> 
> I've applied the patch below to tip/irq/sparseirq - could someone with an 
> affected GCC version please check whether this solves the crash?

I recall from discussions earlier that noinline doesn't fix the
problem, and I just tested a similar case and verified that adding
noinline doesn't stop some versions of gcc from inlining them.  The
empty weak functions in kernel/perf_counter.c were getting inlined by
the cross-gcc (gcc 4.1.1) I use for compiling powerpc kernels on my
laptop, and adding noinline doesn't help there.

Paul.
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