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

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Date:	Sat, 25 Aug 2012 17:12:33 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Michael Cree <mcree@...on.net.nz>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Zankel <chris@...kel.net>,
	"3.2.x.." <stable@...r.kernel.org>,
	Chen Liqin <liqin.chen@...plusct.com>,
	Lennox Wu <lennox.wu@...il.com>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Helge Deller <deller@....de>,
	Parisc <linux-parisc@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Koichi Yasutake <yasutake.koichi@...panasonic.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	m68k <linux-m68k@...ts.linux-m68k.org>,
	Hirokazu Takata <takata@...ux-m32r.org>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	Mikael Starvik <starvik@...s.com>,
	Jesper Nilsson <jesper.nilsson@...s.com>,
	Cris <linux-cris-kernel@...s.com>,
	Richard Henderson <rth@...ddle.net>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Matt Turner <mattst88@...il.com>,
	alpha <linux-alpha@...r.kernel.org>
Subject: Re: [PATCH 00/11] rcu: Add missing RCU idle APIs on idle loop v2

On Sun, Aug 26, 2012 at 09:15:27AM +1200, Michael Cree wrote:
> 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.

OK, fair enough.

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

Both Alpha patches should apply as-is back to 3.3, and should also fix
the problem.  Could you please check this on the versions of interest?

If you are seeing something like this on 3.2 or earlier, you have some
other problem that needs to be tracked down separately.

							Thanx, Paul

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