rcu: Protect uses of ->jiffies_stall with ACCESS_ONCE()

Some of the accesses to the rcu_state structure's ->jiffies_stall
field are unprotected. This patch protects them with ACCESS_ONCE().
The following coccinelle script was used to acheive this:
/* coccinelle script to protect uses of ->jiffies_stall with ACCESS_ONCE() */
@@
identifier a;
@@
(
	ACCESS_ONCE(a->jiffies_stall)
|
-	a->jiffies_stall
+	ACCESS_ONCE(a->jiffies_stall)
)

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcu/tiny_plugin.h

@@ -144,7 +144,7 @@ static void check_cpu_stall(struct rcu_ctrlblk *rcp)
 		return;
 	rcp->ticks_this_gp++;
 	j = jiffies;
-	js = rcp->jiffies_stall;
+	js = ACCESS_ONCE(rcp->jiffies_stall);
 	if (*rcp->curtail && ULONG_CMP_GE(j, js)) {
 		pr_err("INFO: %s stall on CPU (%lu ticks this GP) idle=%llx (t=%lu jiffies q=%ld)\n",
 		       rcp->name, rcp->ticks_this_gp, rcu_dynticks_nesting,
@@ -152,17 +152,17 @@ static void check_cpu_stall(struct rcu_ctrlblk *rcp)
 		dump_stack();
 	}
 	if (*rcp->curtail && ULONG_CMP_GE(j, js))
-		rcp->jiffies_stall = jiffies +
+		ACCESS_ONCE(rcp->jiffies_stall) = jiffies +
 			3 * rcu_jiffies_till_stall_check() + 3;
 	else if (ULONG_CMP_GE(j, js))
-		rcp->jiffies_stall = jiffies + rcu_jiffies_till_stall_check();
+		ACCESS_ONCE(rcp->jiffies_stall) = jiffies + rcu_jiffies_till_stall_check();
 }
 
 static void reset_cpu_stall_ticks(struct rcu_ctrlblk *rcp)
 {
 	rcp->ticks_this_gp = 0;
 	rcp->gp_start = jiffies;
-	rcp->jiffies_stall = jiffies + rcu_jiffies_till_stall_check();
+	ACCESS_ONCE(rcp->jiffies_stall) = jiffies + rcu_jiffies_till_stall_check();
 }
 
 static void check_cpu_stalls(void)