linux-kernel - Re: + mm-mmapc-replace-find_vma_prepare-with-clearer-find_vma

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Message-ID: <alpine.DEB.2.00.1209111843010.19285@chino.kir.corp.google.com>
Date:	Tue, 11 Sep 2012 18:44:55 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Hugh Dickins <hughd@...gle.com>
cc:	linux-kernel@...r.kernel.org, mm-commits@...r.kernel.org,
	bhalevy@...ian.com, dhillf@...il.com
Subject: Re: + mm-mmapc-replace-find_vma_prepare-with-clearer-find_vma_links.patch
 added to -mm tree

On Tue, 11 Sep 2012, Hugh Dickins wrote:

> > > This does revert 2.6.27's dfe195fb79e88 ("mm: fix uninitialized variables
> > > for find_vma_prepare callers"), but it looks like gcc 4.3.0 was one of
> > > those releases too eager to shout about uninitialized variables: only
> > > copy_vma() warns with 4.5.1 and 4.7.1, which a BUG on error silences.
> > > 
> > 
> > That trick doesn't work if CONFIG_BUG=n.
> > 
> > mm/mmap.c: In function 'copy_vma':
> > mm/mmap.c:2344: warning: 'prev' may be used uninitialized in this function
> > mm/mmap.c:2345: warning: 'rb_link' may be used uninitialized in this function
> > mm/mmap.c:2345: warning: 'rb_parent' may be used uninitialized in this function
> 
> Hmm, right: not an option I ever choose, and I hadn't given it a thought.
> 
> But do we care?  If this introduced the only such warning, I would care.

It now has the distinction of being the only warning for CONFIG_BUG=n in 
mm/*.

> But that seems to be far from the case - building my usual config without
> CONFIG_BUG gives me 36 warnings, of uninitialized and unused and
> control reaches end of non-void varieties.
> 

What's so special about copy_vma() that we can't fix it all up to use 
uninitialized_var() so this is handled the proper way?  If CONFIG_BUG 
exists, then we should support it, right?
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