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

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Message-ID: <564CC9FD.3080307@linaro.org>
Date:	Wed, 18 Nov 2015 10:57:01 -0800
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Andrew Morton <akpm@...ux-foundation.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 11/18/2015 10:55 AM, Andrew Morton wrote:
> 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.

It sounds 5.x is smarter :-)
>
>
> --- 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;

Still need v2?

Thanks,
Yang

>
>   		/*
>   		 * Unstable writes are a feature of certain networked
> _
>

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