linux-kernel - Re: [RFC PATCH -v4 00/14] fsnotify, dnotify, and inotify

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Message-ID: <20081222232040.GP28946@ZenIV.linux.org.uk>
Date:	Mon, 22 Dec 2008 23:20:40 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"C. Scott Ananian" <cscott@...top.org>
Cc:	Eric Paris <eparis@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH -v4 00/14] fsnotify, dnotify, and inotify

On Mon, Dec 22, 2008 at 06:08:08PM -0500, C. Scott Ananian wrote:

> That's not correct, as /proc/self/fd/<num> and the getcwd syscall make
> clear.  struct inode has a i_dentry member, and via its d_parent links
> you can reconstruct the path, as __d_path in fs/dcache.c does.

Think for a minute

a) you might have any number of links to given inode, *including* *zero*
b) any subset of these links may be in dcache (including empty)
c) any number of bindings might exist to file in question *or* to its
ancestors (again, including zero).
d) none of the relative paths from fs root to file has to be visible
as a path leading to file in question from anywhere (again, think of
bindings)
e) all of that can change before information reaches userland.

> inotify *almost* lets us do that last thing (though the code
> duplication pains me) but is too racy for reliable use. Give me a
> kernel interface without races and I'll call it a good start.  If you
> can save me the trouble of duplicating all of the filesystem's
> directory information in my userspace database in order to handle
> directory moves, I'll actually grin a little.

_WHAT_ interface without races?  Anything along the lines of "somebody had
done something to <pathname>" is racy by definition.  Simply because the
next operation might have changed the mapping from pathnames to files.

What are you actually trying to do?
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