linux-kernel - Re: [PATCH] v3 Teach RCU that idle task is not quiscent state at boot

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Message-ID: <20090225044356.GD7241@linux.vnet.ibm.com>
Date:	Tue, 24 Feb 2009 20:43:56 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	linux-kernel@...r.kernel.org, vegard.nossum@...il.com,
	mingo@...e.hu, stable@...nel.org, akpm@...ux-foundation.org,
	penberg@...helsinki.fi
Subject: Re: [PATCH] v3 Teach RCU that idle task is not quiscent state at
	boot

On Wed, Feb 25, 2009 at 05:12:59AM +0100, Nick Piggin wrote:
> Looks OK to me. It's a bit "theoretically" displeasing that
> you still treat synchronize_rcu as a gp even after we spawn
> some kernel threads.
> 
> ... but it probably isn't worth trying to rearrange the boot
> code just to make this look better. I can't see any actual
> problems that could arise, as the threads should not have
> had a chance to run yet.

And if rcuclassic or rcutree on a UP system, even if threads have had
a chance to run, it is OK to treat synchronize_rcu() as a grace period,
since it has the opportunity to block.

> I wonder if you couldn't put in a WARN_ON(nr_context_switches() > 0)
> in rcu_idle_now_means_idle() just in case?

Makes a lot of sense -- updated and am firing off the tests!

							Thanx, Paul

> On Tue, Feb 24, 2009 at 04:29:37PM -0800, Paul E. McKenney wrote:
> > This patch fixes a bug located by Vegard Nossum with the aid of
> > kmemcheck, updated based on review comments from Nick Piggin,
> > Ingo Molnar, and Andrew Morton.
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