linux-kernel - Re: [xfs-masters] RFC: Fix f

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Message-ID: <20090102182757.GF28946@ZenIV.linux.org.uk>
Date:	Fri, 2 Jan 2009 18:27:57 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Christoph Hellwig <hch@....de>
Cc:	Jonathan Corbet <corbet@....net>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>, bfields@...ldses.org,
	xfs-masters@....sgi.com, Andi Kleen <andi@...stfloor.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [xfs-masters] RFC: Fix f_flags races without the BKL

On Mon, Dec 29, 2008 at 01:50:50PM +0100, Christoph Hellwig wrote:
> On Mon, Dec 29, 2008 at 04:13:52AM -0700, Jonathan Corbet wrote:
> > Accesses to the f_flags field have always involved a read-modify-write
> > operation, and have always been racy in the absence of the BKL.  The recent
> > BKL-removal work made this problem worse, but it has been there for a very
> > long time.  The race is quite small, and, arguably, has never affected
> > anybody, but it's still worth fixing.
> > 
> > After pondering for a while, I couldn't come up with anything better than a
> > global file->f_flags mutex.  There's no point in bloating struct file with
> > a mutex just for this purpose; it's hard to imagine that there will be any
> > real contention for this lock.
> 
> What speaks against having on in fs_struct so that it's at least not
> globally serialized?

WTF?  References to struct file can be shared by tasks with different
associated fs_struct; how the devil can that ever work?
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