linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <547e0947.c332e00a.23bf.ffffa8bd@mx.google.com>
Date:	Tue, 2 Dec 2014 16:47:31 -0200
From:	Dâniel Fraga <fragabr@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Tue, 2 Dec 2014 10:42:02 -0800
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> I was actually suggesting something a bit different.  Instead of bisecting
> by release, bisect by code.  The procedure is as follows:
> 
> 1.	I figure out some reliable way of making RCU allow preemption to
> 	be disabled for some RCU read-side critical sections, but not for
> 	others.  I send you the patch, which has rcu_read_lock_test()
> 	as well as rcu_read_lock().
> 
> 2.	You build a kernel without my Kconfig hack, with my patch from
> 	#1 above, and build a kernel with CONFIG_PREEMPT=y (which of
> 	course implies CONFIG_TREE_PREEMPT_RCU=y, given that you are
> 	building without my Kconfig hack).
> 
> 3.	You make a list of all the rcu_read_lock() uses in the kernel
> 	(or ask me to provide it).  You change the rcu_read_lock()
> 	calls in the first half of this list to rcu_read_lock_test().
> 
> 	If the kernel locks up as easily with this change as it did
> 	in a stock CONFIG_PREEMPT=y CONFIG_TREE_PREEMPT_RCU=y kernel,
> 	change half of the remaining rcu_read_lock() calls to
> 	rcu_read_lock_test().  If the kernel is much more resistant
> 	to lockup, change half of the rcu_read_lock_test() calls
> 	back to rcu_read_lock().

	Ok Paul, I want to do everything I can to help you debug this.

	So can you provide me the list you mentioned at point 3 (or
tell me how can I get it)? If you guide me through this, I can do
whatever you need. Thanks!

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