rcu: Protect ->gp_flags accesses with ACCESS_ONCE()

A number of ->gp_flags accesses don't have ACCESS_ONCE(), but all of
the can race against other loads or stores.  This commit therefore
applies ACCESS_ONCE() to the unprotected ->gp_flags accesses.
Reported-by: Alexey Roytman <alexey.roytman@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcu/tree.c

@@ -1403,12 +1403,12 @@ static int rcu_gp_init(struct rcu_state *rsp)
 	rcu_bind_gp_kthread();
 	raw_spin_lock_irq(&rnp->lock);
 	smp_mb__after_unlock_lock();
-	if (rsp->gp_flags == 0) {
+	if (!ACCESS_ONCE(rsp->gp_flags)) {
 		/* Spurious wakeup, tell caller to go back to sleep.  */
 		raw_spin_unlock_irq(&rnp->lock);
 		return 0;
 	}
-	rsp->gp_flags = 0; /* Clear all flags: New grace period. */
+	ACCESS_ONCE(rsp->gp_flags) = 0; /* Clear all flags: New grace period. */
 
 	if (WARN_ON_ONCE(rcu_gp_in_progress(rsp))) {
 		/*
@@ -1501,7 +1501,7 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
 	if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
 		raw_spin_lock_irq(&rnp->lock);
 		smp_mb__after_unlock_lock();
-		rsp->gp_flags &= ~RCU_GP_FLAG_FQS;
+		ACCESS_ONCE(rsp->gp_flags) &= ~RCU_GP_FLAG_FQS;
 		raw_spin_unlock_irq(&rnp->lock);
 	}
 	return fqs_state;
@@ -1566,7 +1566,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	rdp = this_cpu_ptr(rsp->rda);
 	rcu_advance_cbs(rsp, rnp, rdp);  /* Reduce false positives below. */
 	if (cpu_needs_another_gp(rsp, rdp)) {
-		rsp->gp_flags = RCU_GP_FLAG_INIT;
+		ACCESS_ONCE(rsp->gp_flags) = RCU_GP_FLAG_INIT;
 		trace_rcu_grace_period(rsp->name,
 				       ACCESS_ONCE(rsp->gpnum),
 				       TPS("newreq"));
@@ -1695,7 +1695,7 @@ rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
 		 */
 		return;
 	}
-	rsp->gp_flags = RCU_GP_FLAG_INIT;
+	ACCESS_ONCE(rsp->gp_flags) = RCU_GP_FLAG_INIT;
 	trace_rcu_grace_period(rsp->name, ACCESS_ONCE(rsp->gpnum),
 			       TPS("newreq"));
 
@@ -2320,7 +2320,7 @@ static void force_quiescent_state(struct rcu_state *rsp)
 		raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
 		return;  /* Someone beat us to it. */
 	}
-	rsp->gp_flags |= RCU_GP_FLAG_FQS;
+	ACCESS_ONCE(rsp->gp_flags) |= RCU_GP_FLAG_FQS;
 	raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
 	wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
 }