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

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Message-ID: <20130625173451.GB17050@redhat.com>
Date:	Tue, 25 Jun 2013 13:34:51 -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 01:29:54PM -0400, Steven Rostedt wrote:
 > On Tue, 2013-06-25 at 13:21 -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).
 > 
 > Ah you didn't handle the bit set case. I just noticed "00" in
 > 00ffff88023c6cdd. To test this, you really need to do a "next & ~3", to
 > clear the pointer.
 > 
 > Perhaps its best to have just a "raw_list_for_each" that doesn't do any
 > check, and have the ring buffer use that instead. The
 > rb_head_page_deactivate() is usually followed by an integrity check
 > anyway.

I think that's probably the best way forward. The ring buffer code does
so many weird things with list heads that it's almost it's own ADT.

	Dave

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