linux-kernel - Re: frequent softlockups with 3.10rc6.

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Date:	Tue, 25 Jun 2013 13:26:09 -0400
From:	Dave Jones <davej@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrey Vagin <avagin@...nvz.org>
Subject: Re: frequent softlockups with 3.10rc6.

On Tue, Jun 25, 2013 at 01:21:30PM -0400, Steven Rostedt wrote:
 > On Tue, 2013-06-25 at 12:55 -0400, Dave Jones wrote:
 > 
 > > While I've been spinning wheels trying to reproduce that softlockup bug,
 > > On another machine I've been refining my list-walk debug patch.
 > > I added an ugly "ok, the ringbuffer is playing games with lower two bits" special case.
 > > 
 > > But what the hell is going on here ?
 > > 
 > > next->prev should be prev (ffff88023c6cdd18), but was 00ffff88023c6cdd. (next=ffff880243288001).
 > > 
 > > (trace comes from the same ringbuffer code)
 > 
 > What's the above saying?  ffff880243288000->prev == 00ffff88023c6cdd but
 > it should have been ffff88023c6cdd18? That is: ffff88023c6cdd18->next ==
 > ffff880243288001?

It's saying something has done >>8 on a pointer, and stuck it in a list head.

 > Not sure how that would mess up. The ring-buffer code has lots of
 > integrity checks to make sure nothing like this breaks.

My integrity checks can beat up your integrity checks.

	Dav

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