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


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