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 13:51:07 +0300
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Al Viro <viro@...IV.linux.org.uk>
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 Thu, May 29, 2014 at 06:34:44AM +0100, Al Viro wrote:
> On Thu, May 29, 2014 at 04:52:33AM +0100, Al Viro wrote:
> > On Thu, May 29, 2014 at 04:11:49AM +0100, Al Viro wrote:
> > > On Wed, May 28, 2014 at 07:39:54PM +0100, Al Viro wrote:
> > > 
> > > > OK, the warnings about averting your eyes very much apply; the thing below
> > > > definitely needs more massage before it becomes acceptable (and no, it's
> > > > not a single commit; I'm not that insane), but it changes behaviour in the
> > > > way described above.  Could you check if the livelock persists with it?
> > > > No trace-generating code in there, so the logs should be compact enough...
> > > 
> > > Here's an updated patch, hopefully slightly less vomit-inducing.  Should
> > > give the same behaviour as the previous one...  Again, it's a cumulative
> > > diff - I'm still massaging the splitup here.
> > 
> > BTW, it still leaves the "proceed to parent" case in shrink_dentry_list();
> > in theory, it's also vulnerable to the same livelock.  Can be dealt pretty
> > much the same way; I'd rather leave that one for right after -final, though,
> > if the already posted variant turns out to be sufficient...
> 
> ... which is (presumably) dealt with the incremental I'd just sent to Linus;
> seeing what kind of dumb mistakes I'm making, I'd better call it quits for
> tonight - it's 1:30am here and I didn't have anywhere near enough sleep
> yesterday.  I'd appeciate if you could test the patch immediately
> upthread (from Message-ID: <20140529031149.GE18016@...IV.linux.org.uk>)
> and see if it helps.  There's an incremental on top of it (from
> Message-ID: <20140529052621.GH18016@...IV.linux.org.uk>) that might or
> might not be a good idea.

Thanks for the patch.

I tested patch <20140529031149.GE18016@...IV.linux.org.uk> and it seems
to improve things. After first plug/unplug I can see similar behaviour
but after a while it recovered. I did several iterations of plug/unplug
afterwards and didn't see the livelock to trigger.

dmesg is attached.

I'm going to try your incremental patch now.

View attachment "dmesg.out" of type "text/plain" (157997 bytes)