From: Arnd Bergmann <arnd@arndb.de>
To: linaro-kernel@lists.linaro.org
Cc: Andrew Morton <akpm@linux-foundation.org>, Yang Shi <yang.shi@linaro.org>,
        tj@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] writeback: initialize m_dirty to avoid compile warning
Date: Wed, 18 Nov 2015 10:53:08 +0100
Message-ID: <9389290.7xT8jb43sX@wuerfel>
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On Tuesday 17 November 2015 15:38:55 Andrew Morton wrote:
> On Fri, 13 Nov 2015 10:26:41 -0800 Yang Shi <yang.shi@linaro.org> wrote:
> 
> > When building kernel with gcc 5.2, the below warning is raised:
> > 
> > mm/page-writeback.c: In function 'balance_dirty_pages.isra.10':
> > mm/page-writeback.c:1545:17: warning: 'm_dirty' may be used uninitialized in this function [-Wmaybe-uninitialized]
> >    unsigned long m_dirty, m_thresh, m_bg_thresh;
> > 
> > The m_dirty{thresh, bg_thresh} are initialized in the block of "if (mdtc)",
> > so if mdts is null, they won't be initialized before being used.
> > Initialize m_dirty to zero, also initialize m_thresh and m_bg_thresh to keep
> > consistency.
> > 
> > They are used later by if condition:
> > !mdtc || m_dirty <= dirty_freerun_ceiling(m_thresh, m_bg_thresh)
> > 
> > If mdtc is null, dirty_freerun_ceiling will not be called at all, so the
> > initialization will not change any behavior other than just ceasing the compile
> > warning.
> 
> Geeze I hate that warning.  gcc really could be a bit smarter about it
> and this is such a case.
> 
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -1542,7 +1542,7 @@ static void balance_dirty_pages(struct address_space *mapping,
> >       for (;;) {
> >               unsigned long now = jiffies;
> >               unsigned long dirty, thresh, bg_thresh;
> > -             unsigned long m_dirty, m_thresh, m_bg_thresh;
> > +             unsigned long m_dirty = 0, m_thresh = 0, m_bg_thresh = 0;
> >  
> >               /*
> >                * Unstable writes are a feature of certain networked
> 
> Adding runtime overhead to suppress a compile-time warning is Just
> Wrong.
> 
> With gcc-4.4.4 the above patch actually reduces page-writeback.o's
> .text by 36 bytes, lol.  With gcc-4.8.4 the patch saves 19 bytes.  No
> idea what's going on there...

I've done tons of build tests and never got the warning for the variables
other than m_dirty, and that one also just with very few configurations
(e.g. ARM omap2plus_defconfig).
 
How about initializing only m_dirty but not the others?

	Arnd
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