linux-kernel - Re: [PATCH] fs: ocfs2: move_extents.c: Fix to remove null pointer checks that could never happen

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Date:	Thu, 29 May 2014 17:23:08 -0400
From:	Dave Jones <davej@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>, ocfs2-devel@....oracle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: ocfs2: move_extents.c:  Fix to remove null pointer
 checks that could never happen

On Thu, May 29, 2014 at 02:03:37PM -0700, Andrew Morton wrote:
 > On Tue, 27 May 2014 22:23:51 +0200 Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se> wrote:
 > 
 > > Removal of null pointer checks that could never happen
 > 
 > How do you know it never happens?
 > 
 > > --- a/fs/ocfs2/move_extents.c
 > > +++ b/fs/ocfs2/move_extents.c
 > > @@ -904,9 +904,6 @@ static int ocfs2_move_extents(struct ocfs2_move_extents_context *context)
 > >  	struct buffer_head *di_bh = NULL;
 > >  	struct ocfs2_super *osb = OCFS2_SB(inode->i_sb);
 > >  
 > > -	if (!inode)
 > > -		return -ENOENT;
 > > -
 > 
 > If it's due to assuming that the previous statement would have oopsed
 > then that is mistaken.  Is is sometimes the case that gcc will move the
 > evaluation of inode->i_sb to after the test, so this function can be
 > passed NULL and it will not oops.

'sometimes' ?

You have a lot more faith in gcc than I do. What happens if we decide to
switch to llvm one day ? Can we guarantee every compiler will implement
the same magic ?  This seems fragile as hell to me.

	Dave

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