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From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: Update of file offset on write() etc. is non-atomic with I/O
Date: Tue, 4 Mar 2014 00:23:43 +0000
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Cc: George Spelvin <linux@horizon.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
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On Mon, Mar 03, 2014 at 03:59:36PM -0800, Linus Torvalds wrote:

> I doubt it's worth caring about. Even when passing things in memory,
> the end result isn't that much worse than the fget_light() model that
> passes just one of the two fields in memory.

I'm not sure if that's the right approach, TBH.  I wonder if something
like
static inline struct fd fdget(int fd)
{
	unsigned long v = __fdget(fd);
	return (struct fd){(struct file *)(v & ~1), v & 1};
}
would not be a better starting point, with __fdget(fd) being
{
        struct files_struct *files = current->files;
        struct file *file;
        if (atomic_read(&files->count) == 1) {
                file = __fcheck_files(files, fd);
                if (file && (file->f_mode & FMODE_PATH))
                        return 0;
		return (unsigned long)file;
        } else {
                file = __fget(fd, FMODE_PATH);
                return file ? 1 | (unsigned long)file : 0;
        }
}