linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <20141202204812.GP25340@linux.vnet.ibm.com>
Date:	Tue, 2 Dec 2014 12:48:12 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Tue, Dec 02, 2014 at 03:30:44PM -0500, Dave Jones wrote:
> On Mon, Dec 01, 2014 at 12:36:34PM -0800, Linus Torvalds wrote:
>  > On Mon, Dec 1, 2014 at 12:28 PM, Dâniel Fraga <fragabr@...il.com> wrote:
>  > >
>  > >         Hi Paul. Please, I'd like the patch, because without
>  > > preemption, I'm unable to trigger this bug.
>  > 
>  > Ok, that's already interesting information. And yes, it would probably
>  > be interesting to see if CONFIG_PREEMPT=y but !CONFIG_TREE_PREEMPT_RCU
>  > then solves it too, to narrow it down to one but not the other..
> 
> That combination doesn't seem possible. TREE_PREEMPT_RCU is the only
> possible choice if PREEMPT=y

Indeed, getting that combination requires a Kconfig patch, which I
supplied below.  Not for mainline, debugging only.

							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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