rcu: exp: Fix "must hold exp_mutex" comments for QS reporting functions

Since commit d9a3da0699b2 ("rcu: Add expedited grace-period support
for preemptible RCU"), there are comments for some funtions in
rcu_report_exp_rnp()'s call-chain saying that exp_mutex or its
predecessors needs to be held.

However, exp_mutex and its predecessors were used only to synchronize
between GPs, and it is clear that all variables visited by those functions
are under the protection of rcu_node's ->lock. Moreover, those functions
are currently called without held exp_mutex, and seems that doesn't
introduce any trouble.

So this patch fixes this problem by updating the comments to match the
current code.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Fixes: d9a3da0699b2 ("rcu: Add expedited grace-period support for preemptible RCU")
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>

kernel/rcu/tree_exp.h

@@ -154,7 +154,7 @@ static void __maybe_unused sync_exp_reset_tree(struct rcu_state *rsp)
  * for the current expedited grace period.  Works only for preemptible
  * RCU -- other RCU implementation use other means.
  *
- * Caller must hold the rcu_state's exp_mutex.
+ * Caller must hold the specificed rcu_node structure's ->lock
  */
 static bool sync_rcu_preempt_exp_done(struct rcu_node *rnp)
 {
@@ -170,8 +170,7 @@ static bool sync_rcu_preempt_exp_done(struct rcu_node *rnp)
  * recursively up the tree.  (Calm down, calm down, we do the recursion
  * iteratively!)
  *
- * Caller must hold the rcu_state's exp_mutex and the specified rcu_node
- * structure's ->lock.
+ * Caller must hold the specified rcu_node structure's ->lock.
  */
 static void __rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
 				 bool wake, unsigned long flags)
@@ -207,8 +206,6 @@ static void __rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
 /*
  * Report expedited quiescent state for specified node.  This is a
  * lock-acquisition wrapper function for __rcu_report_exp_rnp().
- *
- * Caller must hold the rcu_state's exp_mutex.
  */
 static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
 					      struct rcu_node *rnp, bool wake)
@@ -221,8 +218,7 @@ static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
 
 /*
  * Report expedited quiescent state for multiple CPUs, all covered by the
- * specified leaf rcu_node structure.  Caller must hold the rcu_state's
- * exp_mutex.
+ * specified leaf rcu_node structure.
  */
 static void rcu_report_exp_cpu_mult(struct rcu_state *rsp, struct rcu_node *rnp,
 				    unsigned long mask, bool wake)