rcu: Move rcu_report_exp_rdp() setting of ->cpu_no_qs.b.exp under lock

This commit reduces the state space of rcu_report_exp_rdp() by moving
the setting of ->cpu_no_qs.b.exp under the rcu_node structure's ->lock.
The lock isn't really all that important here, given that this per-CPU
field is supposed to be written only by its CPU, but the disabling of
interrupts excludes things like rcu_exp_handler(), which also can write
to this same field.  Avoiding this sort of interleaved access reduces
the state space.

Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Signed-off-by: Uladzislau Rezki (Sony) <[email protected]>

Author: paulmckrcu

kernel/rcu/tree_exp.h

@@ -260,8 +260,8 @@ static void rcu_report_exp_rdp(struct rcu_data *rdp)
 	unsigned long flags;
 	struct rcu_node *rnp = rdp->mynode;
 
-	WRITE_ONCE(rdp->cpu_no_qs.b.exp, false);
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
+	WRITE_ONCE(rdp->cpu_no_qs.b.exp, false);
 	rcu_report_exp_cpu_mult(rnp, flags, rdp->grpmask, true);
 }