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Date: Wed, 28 May 2014 21:25:05 +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!
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On Wed, May 28, 2014 at 01:02:23PM -0700, Linus Torvalds wrote:

> Anyway, one reason I looked at this is that most of the threads in
> Mika's NMI watchdog traces were stuck on _raw_spin_lock() in the whole
> d_walk() thing, and I had a *really* hard time convincing myself that
> this was all safe without the RCU lock. I'm wondering if Mika perhaps
> has CONFIG_PREEMPT_RCU set, which means that spinlocks (or the
> rename_lock sequence lock) do not end up being RCU-safe points.

d_walk() covers its arse with ->d_lock (and it wants the starting point
to be pinned, obviously).  So AFAICS RCU is not a problem.