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

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Message-ID: <20140528201440.GB18016@ZenIV.linux.org.uk>
Date:	Wed, 28 May 2014 21:14:40 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 Wed, May 28, 2014 at 12:43:24PM -0700, Linus Torvalds wrote:

> It does require that the dentry shrinking code always hold the RCU
> lock for reading, because others may actually be doing the final
> dput() while the thing is on the shrinking list (and holding the RCU
> lock is what protects the entry from actually being free'd).
> 
> NOTE! I don't claim that this fixes anything, but I do think that it
> makes that whole cross-thread complexity of that DCACHE_MAY_FREE go
> away. I think it's easier to understand, and it removes code in the
> process. Comments?

Unless I'm badly misreading your patch, you are calling dentry_kill()
with rcu_read_lock() held.  And that can trigger all sorts of interesting
things, starting with iput() and tons of disk IO...

What am I missing here?
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