linux-kernel - Re: [PATCH 1/2] fs: optimize mpage

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Message-ID: <20100604083031.GE31073@ZenIV.linux.org.uk>
Date:	Fri, 4 Jun 2010 09:30:31 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Borislav Petkov <bp@...en8.de>, Changli Gao <xiaosuo@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] fs: optimize mpage_readpage()

On Fri, Jun 04, 2010 at 10:13:22AM +0200, Borislav Petkov wrote:
> > > > -	bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
> > > > +	bio = do_mpage_readpage(NULL, page, 1, &last_block_in_bio,

> Right, the uninitialized warning above happens when you remove the NULL
> assignment, i.e.
> 
> struct bio *bio;
> 
> ...
> 
> bio = do_mpage_readpage(bio, ...)

WTF?  His patch does *NOT* leave you with bio = do_mpage_readpage(bio, ...),
it replaces that with bio = do_mpage_readpage(NULL, ...).

Which variant has produced a warning?

> But(!), in the mpage_readpage(), bio _absolutely_ has to be NULL because
> it is checked if being so later in do_mpage_readpage(), so this one is a
> complete different story.
> 
> To cut a long story short, you're correct, gcc is b0rked when warning
> about passing addresses of variables to functions which only write to
> them.

To make it even shorter, you've misapplied the patch.  Correct?
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