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>

1 files changed, 16 insertions, 3 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 14e283c351f6..805d55ee0b2a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -282,6 +282,17 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
 };
 
 /*
+ * 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;
 
         /*