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

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Message-ID: <20140528141937.GZ18016@ZenIV.linux.org.uk>
Date:	Wed, 28 May 2014 15:19:37 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Wed, May 28, 2014 at 04:11:36PM +0300, Mika Westerberg wrote:

> I added this on top of your previus patch and unfortunately the livelock
> is still there :-(
> 
> >From the previous log, it looks like udev is doing exactly same operation
> (check_submounts_and_drop) on 4 of the CPUs in paraller:

4 threads trying to resolve some pathnames in that subtree for some reason.
All of them hitting ->d_revalidate() on the spot - basically, the first
invalidated directory...

OK, it's not ->i_lock, it's ->d_lock on parent being grabbed after that on
child, while d_walk() keeps taking them in opposite order.  Hmm...

In principle we could do the following:
	* split dentry_kill() into the part that is taking locks and
the rest of it.
	* in case of trylock failure have shrink_dentry_list() do
read_seqlock_excl(&rename_lock) (which will stabilize ->d_parent) and
take ->d_lock in the right order, drop rename_lock and call __dentry_kill().

AFAICS, that would kill the livelock for good.  We still have ->i_lock
trylock failures to deal with, but those are less of a problem - d_walk()
won't step on ->i_lock at all.  I'm going to grab a couple of hours of sleep
and try to put together something along those lines...
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