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Date: Wed, 28 May 2014 21:14:40 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Miklos Szeredi <mszeredi@suse.cz>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: fs/dcache.c - BUG: soft lockup - CPU#5 stuck for 22s!
 [systemd-udevd:1667]
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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?