[PATCH 2/5] Add memory barriers and comments to rcu_check_callbacks()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

Add comments to the logic that infers quiescent states when interrupting
from either user mode or the idle loop.  Also add a memory barrier: it
appears that James Huang was in fact onto something, as the scheduler
is much less synchronization-happy than it once was, so we can no longer
rely on its memory barriers in all cases.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reported-by: James Huang <jamesclhuang@yahoo.com>
---

 rcuclassic.c |   30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff -urpNa -X dontdiff linux-2.6.25-C1-call_rcu_sched/kernel/rcuclassic.c linux-2.6.25-C2-rcuclassic-fixes/kernel/rcuclassic.c
--- linux-2.6.25-C1-call_rcu_sched/kernel/rcuclassic.c	2008-04-16 19:49:44.000000000 -0700
+++ linux-2.6.25-C2-rcuclassic-fixes/kernel/rcuclassic.c	2008-04-21 10:50:12.000000000 -0700
@@ -502,10 +502,38 @@ void rcu_check_callbacks(int cpu, int us
 	if (user ||
 	    (idle_cpu(cpu) && !in_softirq() &&
 				hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
+
+		/*
+		 * Get here if this CPU took its interrupt from user
+		 * mode or from the idle loop, and if this is not a
+		 * nested interrupt.  In this case, the CPU is in
+		 * a quiescent state, so count it.
+		 *
+		 * Also do a memory barrier.  This is needed to handle
+		 * the case where writes from a preempt-disable section
+		 * of code get reordered into schedule() by this CPU's
+		 * write buffer.  The memory barrier makes sure that
+		 * the rcu_qsctr_inc() and rcu_bh_qsctr_inc() are see
+		 * by other CPUs to happen after any such write.
+		 */
+
+		smp_mb();  /* See above block comment. */
 		rcu_qsctr_inc(cpu);
 		rcu_bh_qsctr_inc(cpu);
-	} else if (!in_softirq())
+
+	} else if (!in_softirq()) {
+
+		/*
+		 * Get here if this CPU did not take its interrupt from
+		 * softirq, in other words, if it is not interrupting
+		 * a rcu_bh read-side critical section.  This is an _bh
+		 * critical section, so count it.  The memory barrier
+		 * is needed for the same reason as is the above one.
+		 */
+
+		smp_mb();  /* See above block comment. */
 		rcu_bh_qsctr_inc(cpu);
+	}
 	raise_rcu_softirq();
 }