linux-kernel - Re: [PATCH] writeback: initialize m

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Message-ID: <20151118181142.GC11496@mtj.duckdns.org>
Date:	Wed, 18 Nov 2015 13:11:42 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Yang Shi <yang.shi@...aro.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH] writeback: initialize m_dirty to avoid compile warning

Hello,

On Tue, Nov 17, 2015 at 03:38:55PM -0800, Andrew Morton wrote:
> > --- 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...
> 
> 
> And initializing locals in the above fashion can hide real bugs -
> looky:

This was the main reason the code was structured the way it is.  If
cgroup writeback is not enabled, any derefs of mdtc variables should
trigger warnings.  Ugh... I don't know.  Compiler really should be
able to tell this much.

Thanks.

-- 
tejun
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