linux-kernel - Re: fs/dcache.c - BUG: soft lockup - CPU#5 stuck for 22s! [systemd-udevd:1667]

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Date:	Thu, 29 May 2014 12:14:51 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: fs/dcache.c - BUG: soft lockup - CPU#5 stuck for 22s! [systemd-udevd:1667]

On Thu, May 29, 2014 at 11:52 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Grrr...  Sadly, that's not good enough.  Leaking rcu_read_lock() on
> success is trivial, but there's more serious problem: suppose dentries
> involved get moved before we get to locking what we thought was parent.
> We end up taking ->d_lock on two dentries that might be nowhere near each
> other in the tree, with obvious nasty implications.  Would be _very_ hard
> to reproduce ;-/

Yeah, I don't think you can reproduce that, but I guess renaming
directories into each other (two renames needed) could trigger an ABBA
deadlock by changing the topological order of dentry/parent.

I suspect there's no way in hell that tiny race will ever happen in
practice, but let's not risk it.

And your solution (to re-check after just taking the parent lock)
seems sufficient and sane, since dentry_lock_for_move() will always
take the parent lock(s) before we move a dentry.

So that looks good to me.

              Linus
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