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Date: Tue, 18 Dec 2012 01:57:05 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Andy Lutomirski <luto@amacapital.net>
Cc: linux-kernel@vger.kernel.org,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>
Subject: Re: Are there u32 atomic bitops? (or dealing w/ i_flags)
Message-ID: <20121218015705.GH4939@ZenIV.linux.org.uk>
References: <CALCETrXs15M-cJRGD_R7fXBxOXR=Pyqsh2dVC78DgShxwn60nA@mail.gmail.com>
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On Mon, Dec 17, 2012 at 05:10:21PM -0800, Andy Lutomirski wrote:
> I want to change inode->i_flags access to be atomic -- there are some
> locking oddities right now, I think, and I want to use a new inode
> flag to signal mtime updates from page_mkwrite.  The problem is that
> i_flags is an unsigned int, and making it an unsigned long seems like
> a waste, but there aren't any u32 atomic bitops.

... and atomic accesses cost more.  A lot more on some architectures.
FWIW, atomic_t *is* 32bit on 32bit architectures, which still doesn't
make it a good idea.