linux-kernel - Re: [PATCH tip/core/rcu 0/3] Cleanups/bugfixes for large systems and TREE_PREEMPT

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Date:	Sat, 19 Sep 2009 08:55:23 +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
Subject: Re: [PATCH tip/core/rcu 0/3] Cleanups/bugfixes for large systems
	and TREE_PREEMPT_RCU


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

> This set of patches does some cleanups and fixes a bug that prevents 
> TREE_PREEMPT_RCU from running reliably on large systems (more than 64 
> CPUs, or on 32-bit systems, more than 32 CPUs).  The patches are as 
> follows:
> 
> 1.	Add WARN_ON_ONCE() consistency checks to catch bugs.  These
> 	are all on slowpaths, so are appropriate for production use.
> 
> 2.	Apply the results of a code walkthrough of rcutree_plugin.h.
> 	This includes a fix for a theoretical race that could result in
> 	excessively long RCU grace periods, or perhaps even hangs/OOMs.
> 
> 3.	Fix a bug introduced in a bugfix commit #de078d875 that caused
> 	large systems to only partially initialize the fields in the
> 	rcu_node tree.  TREE_RCU doesn't care about any of these fields,
> 	which explains why only TREE_PREEMPT_RCU was broken.
> 
> With these fixes, large systems using TREE_PREEMPT_RCU pass moderate 
> rcutorture runs (but with Josh Triplett's mods that force frequent 
> preemption within RCU read-side critical sections).  I am sure that 
> there are more bugs, but these fixes get things much closer.
> 
>  b/kernel/rcutree.c        |   13 +++++++++----
>  b/kernel/rcutree_plugin.h |   21 ++++++++++++++-------
>  kernel/rcutree.c          |   38 +++++++++-----------------------------
>  kernel/rcutree_plugin.h   |   16 +++++++++-------
>  4 files changed, 41 insertions(+), 47 deletions(-)

Thanks Paul, applied them to tip:core/urgent.

	Ingo
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