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

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Message-ID: <20110719235215.GS11013@ZenIV.linux.org.uk>
Date:	Wed, 20 Jul 2011 00:52:16 +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, neilb@...e.de
Subject: Re: [PATCH] vfs: fix race in rcu lookup of pruned dentry

On Wed, Jul 20, 2011 at 12:45:51AM +0100, Al Viro wrote:

> You know what...  I doubt that you want to mess with ->d_seq checks here.
> It's definitely not Hugh's bug (unless he has bindings somewhere odd) and
> both ->mnt_mountpoint and ->mnt_root are pinned (and we are holding
> vfsmount_lock anyway).  *inode assignment too early is a real bug, indeed,
> and we want to assign nd->seq if we cross mountpoint as both versions do,
> but check just before that is, in the best case, BUG_ON() fodder.  We'd
> just found a vfsmount with ->mnt_mountpoint equal to path->dentry; it *can't*
> be stale, or we have a really nasty problem anyway.

Kudos to neilb for spotting the pointless check, BTW; and no, his theory
that it might be needed since we could race with umount() is wrong - due to
vfsmount_lock being held.
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