[PATCH v2 tip/core/rcu 4/6] rcu: Abstract extended quiescent state determination

[Paul E. McKenney]

This commit is the fourth step towards full abstraction of all accesses
to the ->dynticks counter, implementing previously open-coded checks and
comparisons in new rcu_dynticks_in_eqs() and rcu_dynticks_in_eqs_since()
functions.  This abstraction will ease changes to the ->dynticks counter
operation.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
---
 include/linux/rcutiny.h  |  6 ++++++
 kernel/rcu/tree.c        | 52 +++++++++++++++++++++++++++++++++++-------------
 kernel/rcu/tree.h        |  2 ++
 kernel/rcu/tree_exp.h    |  6 +++---
 kernel/rcu/tree_plugin.h |  2 +-
 kernel/rcu/tree_trace.c  |  2 +-
 6 files changed, 51 insertions(+), 19 deletions(-)

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index ac81e4063b40..4f9b2fa2173d 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -27,6 +27,12 @@
 
 #include <linux/cache.h>
 
+struct rcu_dynticks;
+static inline int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
+{
+	return 0;
+}
+
 static inline unsigned long get_state_synchronize_rcu(void)
 {
 	return 0;
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 70b01e1983e6..c5362b6c7ed3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -337,10 +337,22 @@ static void rcu_dynticks_eqs_online(void)
 }
 
 /*
+ * Is the current CPU in an extended quiescent state?
+ *
+ * No ordering, as we are sampling CPU-local information.
+ */
+bool rcu_dynticks_curr_cpu_in_eqs(void)
+{
+	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
+
+	return !(atomic_read(&rdtp->dynticks) & 0x1);
+}
+
+/*
  * 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 rcu_dynticks_snap(struct rcu_dynticks *rdtp)
 {
 	int snap = atomic_add_return(0, &rdtp->dynticks);
 
@@ -348,6 +360,25 @@ static int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
 }
 
 /*
+ * Return true if the snapshot returned from rcu_dynticks_snap()
+ * indicates that RCU is in an extended quiescent state.
+ */
+static bool rcu_dynticks_in_eqs(int snap)
+{
+	return !(snap & 0x1);
+}
+
+/*
+ * Return true if the CPU corresponding to the specified rcu_dynticks
+ * structure has spent some time in an extended quiescent state since
+ * rcu_dynticks_snap() returned the specified snapshot.
+ */
+static bool rcu_dynticks_in_eqs_since(struct rcu_dynticks *rdtp, int snap)
+{
+	return snap != rcu_dynticks_snap(rdtp);
+}
+
+/*
  * Do a double-increment of the ->dynticks counter to emulate a
  * momentary idle-CPU quiescent state.
  */
@@ -1045,7 +1076,7 @@ void rcu_nmi_enter(void)
 	 * to be in the outermost NMI handler that interrupted an RCU-idle
 	 * period (observation due to Andy Lutomirski).
 	 */
-	if (!(atomic_read(&rdtp->dynticks) & 0x1)) {
+	if (rcu_dynticks_curr_cpu_in_eqs()) {
 		rcu_dynticks_eqs_exit();
 		incby = 1;
 	}
@@ -1071,7 +1102,7 @@ void rcu_nmi_exit(void)
 	 * to us!)
 	 */
 	WARN_ON_ONCE(rdtp->dynticks_nmi_nesting <= 0);
-	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
+	WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs());
 
 	/*
 	 * If the nesting level is not 1, the CPU wasn't RCU-idle, so
@@ -1097,9 +1128,7 @@ void rcu_nmi_exit(void)
  */
 bool notrace __rcu_is_watching(void)
 {
-	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
-
-	return atomic_read(&rdtp->dynticks) & 0x1;
+	return !rcu_dynticks_curr_cpu_in_eqs();
 }
 
 /**
@@ -1184,7 +1213,7 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp,
 {
 	rdp->dynticks_snap = rcu_dynticks_snap(rdp->dynticks);
 	rcu_sysidle_check_cpu(rdp, isidle, maxj);
-	if ((rdp->dynticks_snap & 0x1) == 0) {
+	if (rcu_dynticks_in_eqs(rdp->dynticks_snap)) {
 		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
 		if (ULONG_CMP_LT(READ_ONCE(rdp->gpnum) + ULONG_MAX / 4,
 				 rdp->mynode->gpnum))
@@ -1203,12 +1232,7 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp,
 static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
 				    bool *isidle, unsigned long *maxj)
 {
-	unsigned int curr;
 	int *rcrmp;
-	unsigned int snap;
-
-	curr = (unsigned int)rcu_dynticks_snap(rdp->dynticks);
-	snap = (unsigned int)rdp->dynticks_snap;
 
 	/*
 	 * If the CPU passed through or entered a dynticks idle phase with
@@ -1218,7 +1242,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
 	 * read-side critical section that started before the beginning
 	 * of the current RCU grace period.
 	 */
-	if ((curr & 0x1) == 0 || UINT_CMP_GE(curr, snap + 2)) {
+	if (rcu_dynticks_in_eqs_since(rdp->dynticks, rdp->dynticks_snap)) {
 		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
 		rdp->dynticks_fqs++;
 		return 1;
@@ -3807,7 +3831,7 @@ rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
 	rdp->grpmask = leaf_node_cpu_bit(rdp->mynode, cpu);
 	rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
 	WARN_ON_ONCE(rdp->dynticks->dynticks_nesting != DYNTICK_TASK_EXIT_IDLE);
-	WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1);
+	WARN_ON_ONCE(rcu_dynticks_in_eqs(rcu_dynticks_snap(rdp->dynticks)));
 	rdp->cpu = cpu;
 	rdp->rsp = rsp;
 	rcu_boot_init_nocb_percpu_data(rdp);
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index fe98dd24adf8..3b953dcf6afc 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -595,6 +595,8 @@ extern struct rcu_state rcu_bh_state;
 extern struct rcu_state rcu_preempt_state;
 #endif /* #ifdef CONFIG_PREEMPT_RCU */
 
+int rcu_dynticks_snap(struct rcu_dynticks *rdtp);
+
 #ifdef CONFIG_RCU_BOOST
 DECLARE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
 DECLARE_PER_CPU(int, rcu_cpu_kthread_cpu);
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 011f626b2fd8..e155a465cf84 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -360,7 +360,7 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 			rdp->exp_dynticks_snap =
 				rcu_dynticks_snap(rdp->dynticks);
 			if (raw_smp_processor_id() == cpu ||
-			    !(rdp->exp_dynticks_snap & 0x1) ||
+			    rcu_dynticks_in_eqs(rdp->exp_dynticks_snap) ||
 			    !(rnp->qsmaskinitnext & rdp->grpmask))
 				mask_ofl_test |= rdp->grpmask;
 		}
@@ -383,8 +383,8 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 			if (!(mask_ofl_ipi & mask))
 				continue;
 retry_ipi:
-			if (rcu_dynticks_snap(rdp->dynticks) !=
-			    rdp->exp_dynticks_snap) {
+			if (rcu_dynticks_in_eqs_since(rdp->dynticks,
+						      rdp->exp_dynticks_snap)) {
 				mask_ofl_test |= mask;
 				continue;
 			}
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 56583e764ebf..652209589adf 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1643,7 +1643,7 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
 	       "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
 	       "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
 	       ticks_value, ticks_title,
-	       atomic_read(&rdtp->dynticks) & 0xfff,
+	       rcu_dynticks_snap(rdtp) & 0xfff,
 	       rdtp->dynticks_nesting, rdtp->dynticks_nmi_nesting,
 	       rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
 	       READ_ONCE(rsp->n_force_qs) - rsp->n_force_qs_gpstart,
diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c
index b1f28972872c..b833cd0a29e8 100644
--- a/kernel/rcu/tree_trace.c
+++ b/kernel/rcu/tree_trace.c
@@ -124,7 +124,7 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
 		   rdp->rcu_qs_ctr_snap == per_cpu(rcu_qs_ctr, rdp->cpu),
 		   rdp->core_needs_qs);
 	seq_printf(m, " dt=%d/%llx/%d df=%lu",
-		   atomic_read(&rdp->dynticks->dynticks),
+		   rcu_dynticks_snap(rdp->dynticks),
 		   rdp->dynticks->dynticks_nesting,
 		   rdp->dynticks->dynticks_nmi_nesting,
 		   rdp->dynticks_fqs);
-- 
2.5.2