linux-kernel - Re: 3.0-git15 Atomic scheduling in pidmap

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Date:	Wed, 10 Aug 2011 16:53:41 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Josh Boyer <jwboyer@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: 3.0-git15 Atomic scheduling in pidmap_init

On Wed, Aug 10, 2011 at 08:45:29AM -0400, Josh Boyer wrote:
> On Tue, Aug 09, 2011 at 01:35:18PM +0200, Frederic Weisbecker wrote:
> > > But setting rdp->qs_pending to 1 in rcu_init_percpu_data() has no effect
> > > until a grace period starts.  So, if grace periods are prevented from
> 
> Er... really?  Because it gets set and __rcu_pending looks at it
> unconditionally in the case that is calling set_need_resched.  It
> doesn't check if there is anything about a grace period going on or not.

You mean this?

	if (rdp->qs_pending && !rdp->passed_quiesc) {

		/*
		 * If force_quiescent_state() coming soon and this CPU
		 * needs a quiescent state, and this is either RCU-sched
		 * or RCU-bh, force a local reschedule.
		 */
		rdp->n_rp_qs_pending++;
		if (!rdp->preemptible &&
		    ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs) - 1,
				 jiffies))
			set_need_resched();
	}

On boot, if we don't start a grace period, we don't schedule a grace period forcing,
so rsp->jiffies_force_qs is 0.

With ULONG_CMP_LT taking care of (-1 < jiffies) to be valid even with
ulong, then we are fine I guess.
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