linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Tue, 2 Dec 2014 08:58:35 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Dâniel Fraga <fragabr@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Tue, Dec 02, 2014 at 04:40:37PM +0800, Lai Jiangshan wrote:
> On 12/02/2014 03:14 AM, Paul E. McKenney wrote:
> > On Sun, Nov 30, 2014 at 11:02:43PM -0200, Dâniel Fraga wrote:
> >> On Sun, 30 Nov 2014 16:21:19 -0800
> >> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> >>
> >>> Maybe you'll have to turn off RCU_CPU_STALL_VERBOSE first.
> >>>
> >>> Although I think you should be able to just edit the .config file,
> >>> delete the like that says
> >>>
> >>>     CONFIG_TREE_PREEMPT_RCU=y
> >>>
> >>> and then just do a "make oldconfig", and then verify that
> >>> TREE_PREEMPT_RCU hasn't been re-enabled by some dependency. But it
> >>> shouldn't have, and that "make oldconfig" should get rid of anything
> >>> that depends on TREE_PREEMPT_RCU.
> >> 	
> >> 	Ok, I did exactly that, but CONFIG_TREE_PREEMPT_RCU is
> >> re-enabled. I talked with Pranith Kumar and he suggested I could just
> >> disable preemption (No Forced Preemption (Server)) and that's the only
> >> way to disable CONFIG_TREE_PREEMPT_RCU.
> > 
> > If it would help to have !CONFIG_TREE_PREEMPT_RCU with CONFIG_PREEMPT=y,
> 
> It is needed at lest for testing.
> 
> CONFIG_TREE_PREEMPT_RCU=y with CONFIG_PREEMPT=n is needed for testing too.
> 
> Please enable them (or enable them under CONFIG_RCU_TRACE=y)

It is a really easy edit to Kconfig, but I don't want people using it
in production because I really don't need the extra test scenarios.
So I am happy to provide the patch below as needed, but not willing to
submit it to mainline without a lot more justification.  Because if it
appears in mainline, people will start using it in production, whether
I am doing proper testing of it or not.  :-/

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/init/Kconfig b/init/Kconfig
index 903505e66d1d..2cf71fcd514f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -469,7 +469,7 @@ choice
 
 config TREE_RCU
 	bool "Tree-based hierarchical RCU"
-	depends on !PREEMPT && SMP
+	depends on SMP
 	select IRQ_WORK
 	help
 	  This option selects the RCU implementation that is

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