Re: [PATCH 00/11] rcu: Add missing RCU idle APIs on idle loop v2

From: Michael Cree <mcree@orcon.net.nz>
To: paulmck@linux.vnet.ibm.com
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
	Ben Hutchings <ben@decadent.org.uk>,
	LKML <linux-kernel@vger.kernel.org>,
	Chris Zankel <chris@zankel.net>,
	"3.2.x.." <stable@vger.kernel.org>,
	Chen Liqin <liqin.chen@sunplusct.com>,
	Lennox Wu <lennox.wu@gmail.com>,
	"James E.J. Bottomley" <jejb@parisc-linux.org>,
	Helge Deller <deller@gmx.de>,
	Parisc <linux-parisc@vger.kernel.org>,
	David Howells <dhowells@redhat.com>,
	Koichi Yasutake <yasutake.koichi@jp.panasonic.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	m68k <linux-m68k@vger.kernel.org>,
	Hirokazu Takata <takata@linux-m32r.org>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Mikael Starvik <starvik@axis.com>,
	Jesper Nilsson <jesper.nilsson@axis.com>,
	Cris <linux-cris-kernel@axis.com>,
	Richard Henderson <rth@twiddle.net>,
	Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
	Matt Turner <mattst88@gmail.com>,
	alpha <linux-alpha@vger.kernel.org>
Subject: Re: [PATCH 00/11] rcu: Add missing RCU idle APIs on idle loop v2
Date: Sun, 26 Aug 2012 09:15:27 +1200	[thread overview]
Message-ID: <5039406F.9040604@orcon.net.nz> (raw)
In-Reply-To: <20120825161806.GD3436@linux.vnet.ibm.com>

On 26/08/12 04:18, Paul E. McKenney wrote:
> On Sat, Aug 25, 2012 at 03:16:49PM +0200, Frederic Weisbecker wrote:
>> On Fri, Aug 24, 2012 at 08:50:47PM -0700, Paul E. McKenney wrote:
>>> On Sat, Aug 25, 2012 at 02:19:14AM +0100, Ben Hutchings wrote:
>>>> On Fri, 2012-08-24 at 14:26 -0700, Paul E. McKenney wrote:
>>>>> On Thu, Aug 23, 2012 at 04:58:24PM +0200, Frederic Weisbecker wrote:
>>>>>> Hi,
>>>>>>
>>>>>> Changes since v1:
>>>>>>
>>>>>> - Fixed preempt handling in alpha idle loop
>>>>>> - added ack from Geert
>>>>>> - fixed stable email address, sorry :-/
>>>>>>
>>>>>> This time I built tested everywhere but: h8300 (compiler internal error),
>>>>>> and mn10300, parisc, score (cross compilers not available in
>>>>>> ftp://ftp.kernel.org/pub/tools/crosstool/files/bin/x86_64/4.6.3/)
>>>>>>
>>>>>> For testing, you can pull from:
>>>>>>
>>>>>> git://github.com/fweisbec/linux-dynticks.git
>>>>>> 	rcu/idle-fix-v2 
>>>>>>
>>>>>> Thanks.
>>>>>
>>>>> I have queued these on -rcu branch rcu/idle:
>>>>>
>>>>> 	git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
>>>>>
>>>>> This problem has been in place since 3.3, so it is hard to argue that
>>>>> it is a regression for this merge window.  I have therefore queued it
>>>>> for 3.7.
>>>>
>>>> I don't follow that; I would expect any serious bug fix (serious enough
>>>> for a stable update) to be acceptable for 3.6 at this point.
>>>
>>> OK, if any of the arch maintainers wishes to submit the patch to 3.6,
>>> they are free to do so -- just let me know and I will drop the patch from
>>> my tree.
>>>
>>> That said, all this does is cause spurious warnings to be printed, so
>>> not sure it really qualifies as serious.  But I am happy to leave that
>>> decision with the individual arch maintainers -- it is their arch,
>>> after all, so their decision.
>>
>> Couldn't that cause hung tasks due to long lasting synchronize_rcu() ?
> 
> In theory, definitely.  In practice, they haven't been running into it,
> or they would be reporting hangs.

I am hereby reporting that RCU CPU stall warnings and hung tasks are
being experienced on SMP kernels built for generic Alpha. This problem
dates back quite a few kernel releases. The discussed patches appear to
fix the problem.

A backport to the 3.2 kernel, of at least the Alpha patches, would be
very much appreciated! :-)

Cheers
Michael.