rcu: Abstract the dynticks snapshot operation

This commit is the second step towards full abstraction of all accesses to
the ->dynticks counter, implementing the previously open-coded atomic
add of zero in a new rcu_dynticks_snap() function.  This abstraction will
ease changes o the ->dynticks counter operation.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcu/tree.c

@@ -281,6 +281,17 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
 #endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
 };
 
+/*
+ * Snapshot the ->dynticks counter with full ordering so as to allow
+ * stable comparison of this counter with past and future snapshots.
+ */
+static int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
+{
+	int snap = atomic_add_return(0, &rdtp->dynticks);
+
+	return snap;
+}
+
 /*
  * Do a double-increment of the ->dynticks counter to emulate a
  * momentary idle-CPU quiescent state.
@@ -1049,7 +1060,9 @@ void rcu_nmi_exit(void)
  */
 bool notrace __rcu_is_watching(void)
 {
-	return atomic_read(this_cpu_ptr(&rcu_dynticks.dynticks)) & 0x1;
+	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
+
+	return atomic_read(&rdtp->dynticks) & 0x1;
 }
 
 /**
@@ -1132,7 +1145,7 @@ static int rcu_is_cpu_rrupt_from_idle(void)
 static int dyntick_save_progress_counter(struct rcu_data *rdp,
 					 bool *isidle, unsigned long *maxj)
 {
-	rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);
+	rdp->dynticks_snap = rcu_dynticks_snap(rdp->dynticks);
 	rcu_sysidle_check_cpu(rdp, isidle, maxj);
 	if ((rdp->dynticks_snap & 0x1) == 0) {
 		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
@@ -1157,7 +1170,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
 	int *rcrmp;
 	unsigned int snap;
 
-	curr = (unsigned int)atomic_add_return(0, &rdp->dynticks->dynticks);
+	curr = (unsigned int)rcu_dynticks_snap(rdp->dynticks);
 	snap = (unsigned int)rdp->dynticks_snap;
 
 	/*

kernel/rcu/tree_exp.h

@@ -356,10 +356,9 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 		mask_ofl_test = 0;
 		for_each_leaf_node_possible_cpu(rnp, cpu) {
 			struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
-			struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
 
 			rdp->exp_dynticks_snap =
-				atomic_add_return(0, &rdtp->dynticks);
+				rcu_dynticks_snap(rdp->dynticks);
 			if (raw_smp_processor_id() == cpu ||
 			    !(rdp->exp_dynticks_snap & 0x1) ||
 			    !(rnp->qsmaskinitnext & rdp->grpmask))
@@ -380,12 +379,11 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 		for_each_leaf_node_possible_cpu(rnp, cpu) {
 			unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
 			struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
-			struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
 
 			if (!(mask_ofl_ipi & mask))
 				continue;
 retry_ipi:
-			if (atomic_add_return(0, &rdtp->dynticks) !=
+			if (rcu_dynticks_snap(rdp->dynticks) !=
 			    rdp->exp_dynticks_snap) {
 				mask_ofl_test |= mask;
 				continue;