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

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Date:	Wed, 18 Nov 2015 10:55:17 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Shi, Yang" <yang.shi@...aro.org>
Cc:	Tejun Heo <tj@...nel.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

On Wed, 18 Nov 2015 10:39:23 -0800 "Shi, Yang" <yang.shi@...aro.org> wrote:

> On 11/18/2015 10:33 AM, Tejun Heo wrote:
> > Hello,
> >
> > On Wed, Nov 18, 2015 at 10:27:32AM -0800, Shi, Yang wrote:
> >>> 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 for the explanation. It sounds like a compiler problem.
> >>
> >> If you think it is still good to cease the compile warning, maybe we could
> >
> > If this is gonna be a problem with new gcc versions, I don't think we
> > have any other options. :(
> >
> >> just assign it to an insane value as what Andrew suggested, maybe
> >> 0xdeadbeef.
> >
> > I'd just keep it at zero.  Whatever we do, the effect is gonna be
> > difficult to track down - it's not gonna blow up in an obvious way.
> > Can you please add a comment tho explaining that this is to work
> > around compiler deficiency?
> 
> Sure.
> 
> Other than this, in v2, I will just initialize m_dirty since compiler 
> just reports it is uninitialized.

gcc-4.4.4 and gcc-4.8.4 warn about all three variables.


--- a/mm/page-writeback.c~writeback-initialize-m_dirty-to-avoid-compile-warning-fix
+++ a/mm/page-writeback.c
@@ -1542,7 +1542,9 @@ static void balance_dirty_pages(struct a
 	for (;;) {
 		unsigned long now = jiffies;
 		unsigned long dirty, thresh, bg_thresh;
-		unsigned long m_dirty = 0, m_thresh = 0, m_bg_thresh = 0;
+		unsigned long m_dirty = 0;	/* stop bogus uninit warnings */
+		unsigned long m_thresh = 0;
+		unsigned long m_bg_thresh = 0;
 
 		/*
 		 * Unstable writes are a feature of certain networked
_

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