linux-kernel - Re: [PATCH] vfs: fix race in rcu lookup of pruned dentry

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Message-ID: <20110718194703.GI11013@ZenIV.linux.org.uk>
Date:	Mon, 18 Jul 2011 20:47:03 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: fix race in rcu lookup of pruned dentry

On Mon, Jul 18, 2011 at 12:33:28PM -0700, Linus Torvalds wrote:

> Which just makes me suspect that it's *another* case of bug in the
> dentry_kill() sequence. I think the dentry_rcuwalk_barrier() thing is
> crap: it doesn't *cover* the d_inode change with the write lock, it
> just puts that "barrier" there, which is bogus. You can get a
> successful read of the changed inode without ever failing the read
> lock sequence!

Huh?  We do __d_drop() in there, and do that before we start messing
with ->d_inode.  And barrier here will make sure that anything that
might have found this sucker prior to that __d_drop() will fail
->d_seq check - it bumps ->d_seq by 2...

As long as ->d_inode change happens under ->d_lock and ->d_seq is
bumped along with that change, we should be fine, no?
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