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 13:42:15 +0300
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	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 01:02:23PM -0700, Linus Torvalds wrote:
> On Wed, May 28, 2014 at 12:43 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >  - the shrinker list logic depends on the actual freeing of the dentry
> > to be delayed until the RCU grace period (already true for RCU-lookup
> > dentries)
> 
> Side note: I just unconditionally removed the immediate __d_free()
> case, but it could as well have become
> 
> -       if (!(dentry->d_flags & DCACHE_RCUACCESS))
> +       if (!(dentry->d_flags & (DCACHE_RCUACCESS|DCACHE_SHRINK_LIST))
>                 __d_free(&dentry->d_u.d_rcu);
>         else
>                 call_rcu(&dentry->d_u.d_rcu, __d_free);
> 
> instead.
> 
> 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.

I checked my .config and CONFIG_PREEMPT_RCU is not set. Attached my
.config just in case.

View attachment ".config" of type "text/plain" (114051 bytes)