rcu: Avoid unneeded function call in rcu_read_unlock()

Since commit aa40c13 ("rcu: Report QS for outermost PREEMPT=n
rcu_read_unlock() for strict GPs") the function rcu_read_unlock_strict()
is invoked by the inlined rcu_read_unlock() function.  However,
rcu_read_unlock_strict() is an empty function in production kernels,
which are built with CONFIG_RCU_STRICT_GRACE_PERIOD=n.

There is a mention of rcu_read_unlock_strict() in the BPF verifier,
but this is in a deny-list, meaning that BPF does not care whether
rcu_read_unlock_strict() is ever called.

This commit therefore provides a slight performance improvement
by hoisting the check of CONFIG_RCU_STRICT_GRACE_PERIOD from
rcu_read_unlock_strict() into rcu_read_unlock(), thus avoiding the
pointless call to an empty function.

Cc: Alexei Starovoitov <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

Author: Waiman-Long

include/linux/rcupdate.h

@@ -71,7 +71,8 @@ static inline void __rcu_read_lock(void)
 static inline void __rcu_read_unlock(void)
 {
 	preempt_enable();
-	rcu_read_unlock_strict();
+	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
+		rcu_read_unlock_strict();
 }
 
 static inline int rcu_preempt_depth(void)

kernel/rcu/tree_plugin.h

@@ -814,8 +814,7 @@ void rcu_read_unlock_strict(void)
 {
 	struct rcu_data *rdp;
 
-	if (!IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ||
-	   irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
+	if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
 		return;
 	rdp = this_cpu_ptr(&rcu_data);
 	rcu_report_qs_rdp(rdp);