linux-kernel - Re: [PATCH tip/core/rcu 0/3] rcu: prevent hangs by simplifying rcu_barrier/CPU-hotplug, fix lockdep complaint

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Message-ID: <20091007061448.GA21673@elte.hu>
Date:	Wed, 7 Oct 2009 08:14:48 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com
Subject: Re: [PATCH tip/core/rcu 0/3] rcu: prevent hangs by simplifying
	rcu_barrier/CPU-hotplug, fix lockdep complaint


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> This patchset fixes a couple of issues with TREE_PREEMPT_RCU:
> 
> 1.	The current implementation of rcu_barrier() waits for any
> 	prior CPU-hotplug operation to complete.  This code path has
> 	resulted in some hangs, so first move this code from rcupdate.c
> 	to rcutree.c and then use a simpler algorithm that avoids the
> 	waiting, eliminating the possibility of such hangs.
> 
> 2.	The lockdep facility complains from time to time due to the
> 	fact that certain combinations of CPU-hotplug operations can
> 	cause the TREE_PREEMPT_RCU code to acquire the root rcu_node
> 	structure's lock while holding a leaf rcu_node structure's
> 	lock.  This patch places the root rcu_node structure's lock
> 	into its own lockdep class to prevent this false positive.
> 
> Ingo, please rewind tip/core/rcu to commit #135c8aea before applying this
> series, as these patches really need to go into 2.6.32.  Reworked and
> retested rcutiny (for 2.6.33, as Linus requested) will follow in a
> few days.  Or more patches for more bug fixes, as the case may be.  ;-)
> 
>  b/kernel/rcupdate.c       |  120 -----------------------------------
>  b/kernel/rcutree.c        |  120 +++++++++++++++++++++++++++++++++++
>  b/kernel/rcutree.h        |   11 ++-
>  b/kernel/rcutree_plugin.h |   34 ++++++++++
>  b/kernel/rcutree_trace.c  |    5 -
>  kernel/rcutree.c          |  156 ++++++++++++++++++++++++----------------------
>  6 files changed, 248 insertions(+), 198 deletions(-)

Applied, thanks Paul!

I dropped these commits for now:

 3ffea79: rcu: Make hot-unplugged CPU relinquish its own RCU callbacks
 eddd962: rcu: Move rcu_barrier() to rcutree, make lightweight rcu_barrier() for rcutiny
 a39e7d5: rcu-tiny: The Bloatwatch Edition, v6

We can apply rcu-tiny for .33 again, once the dust has settled.

	Ingo
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