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

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Message-ID: <564CC314.1090904@linaro.org>
Date:	Wed, 18 Nov 2015 10:27:32 -0800
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	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 11/18/2015 10:11 AM, Tejun Heo wrote:
> 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 for the explanation. It sounds like a compiler problem.

If you think it is still good to cease the compile warning, maybe we 
could just assign it to an insane value as what Andrew suggested, maybe 
0xdeadbeef.

Thanks,
Yang

>
> Thanks.
>

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