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

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Date:	Tue, 25 Jun 2013 12:55:56 -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 Mon, Jun 24, 2013 at 01:04:36PM -0400, Steven Rostedt wrote:
 > On Mon, 2013-06-24 at 12:51 -0400, Dave Jones wrote:
 > > On Mon, Jun 24, 2013 at 12:24:39PM -0400, Steven Rostedt wrote:
 > > 
 > >  > > Ah, this is the first victim of my new 'check sanity of nodes during list walks' patch.
 > >  > > It's doing the same prev->next next->prev checking as list_add and friends.
 > >  > > I'm looking at getting it into shape for a 3.12 merge after some other preparatory patches
 > >  > > go into 3.11
 > >  > 
 > >  > OK, and you may need to make an exception for the ring buffer. To do a
 > >  > lockless swap out of the reader page for one of the pages in the buffer,
 > >  > it uses the 2 LSB as flags. Notice the "next=ffff880243288001", that "1"
 > >  > is a flag that states the next page is the "header" page (next to be
 > >  > read). We use cmpxchg to update the pages to handle races between the
 > >  > reader and writer.
 > > 
 > > I just had a plumber come visit to replace my toilet.
 > > I think even he would say "dude, gross" about that hack.
 > 
 > Wow, that hack made you so sick you needed to replace your toilet?
 > 
 > Note, the idea of using the 2 LSB bits of pointers came from -rt. Where
 > we do the same with the rt_mutex owner.

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)

	Dave

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