linux-kernel - Re: [PATCH] v2 Teach RCU that idle task is not quiscent state at boot

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Message-ID: <20090224145035.GA12157@linux.vnet.ibm.com>
Date:	Tue, 24 Feb 2009 06:50:35 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	linux-kernel@...r.kernel.org, vegard.nossum@...il.com,
	mingo@...e.hu, stable@...nel.org, akpm@...ux-foundation.org,
	npiggin@...e.de, penberg@...helsinki.fi
Subject: Re: [PATCH] v2 Teach RCU that idle task is not quiscent state at
	boot

On Mon, Feb 23, 2009 at 09:08:54PM -0800, Paul E. McKenney wrote:
> On Tue, Feb 24, 2009 at 03:02:56PM +1100, Nick Piggin wrote:
> > On Tuesday 24 February 2009 07:43:32 Paul E. McKenney wrote:
> > 
> > > @@ -80,6 +81,10 @@ void wakeme_after_rcu(struct rcu_head  *head)
> > >  void synchronize_rcu(void)
> > >  {
> > >  	struct rcu_synchronize rcu;
> > > +
> > > +	if (num_online_cpus() == 1)
> > > +		return;  /* If UP, synchronize_rcu() is a grace period! */
> > > +
> > 
> > Nice patch... May I just suggest a comment to say that this is
> > also a correctness requirement due to the grace period holdoff
> > for early boot?
> 
> Excellent point!  If Ingo reports that this actually fixes the hang that
> he saw, I will add such a comment.
> 
> (And glad you like the patch!)

Gah!!!  I forgot about preemptable RCU, which the previous patch breaks
completely on single-CPU systems.

A fix is in the works.

							Thanx, Paul
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