linux-kernel - Re: [tip:core/rcu] rcu: Add second diagnostic check for a possible CPU-hotplug race

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Message-ID: <20090809192345.GA19433@linux.vnet.ibm.com>
Date:	Sun, 9 Aug 2009 12:23:45 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/rcu] rcu: Add second diagnostic check for a possible
	CPU-hotplug race

On Sun, Aug 09, 2009 at 11:30:10AM -0700, Paul E. McKenney wrote:
> On Sun, Aug 09, 2009 at 01:10:08PM +0200, Ingo Molnar wrote:
> > 
> > * Ingo Molnar <mingo@...e.hu> wrote:
> > 
> > > > rcu: Add second diagnostic check for a possible CPU-hotplug race
> > > 
> > > has a build problem too:
> > > 
> > >  kernel/built-in.o: In function `rcu_cpu_notified':
> > >  (.text+0x1d787): undefined reference to `cpu_notified'
> > >  kernel/built-in.o: In function `rcu_init':
> > >  (.init.text+0x1174): undefined reference to `cpu_notified'
> > >  kernel/built-in.o: In function `rcu_init':
> > >  (.init.text+0x11a8): undefined reference to `cpu_notified'
> > > 
> > > Maybe we should simplify all those Kconfig rules? It's a maze.
> 
> Or I could fix up my scripts so that I once again test the relevant
> combinations -before- I submit the patch.  :-/

That said, I could imagine the following simplifications:

o	CONFIG_SMP always implies CONFIG_HOTPLUG_CPU, eliminating the
	need to test CONFIG_SMP&&!CONFIG_HOTPLUG_CPU.

o	Make CONFIG_NO_HZ unconditional.  (Now, -that- should get a
	reaction!  For one thing, I am not sure that all architectures
	support CONFIG_NO_HZ.)

o	Eliminate the combination CONFIG_PREEMPT && CONFIG_TREE_RCU.
	In other words, make a preemptable kernel imply preemptable RCU
	and vice versa.

o	After sufficient testing, eliminate CONFIG_PREEMPT_RCU in favor
	of CONFIG_TREE_PREEMPT_RCU (currently under development).

o	After sufficient testing, make !CONFIG_SMP imply
	CONFIG_TINY_RCU.  This might require a preemptable variant of
	CONFIG_TINY_RCU.

There are probably quite a few similar simplifications.

							Thanx, Paul
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