[PATCH v3 5/4] srcu: Document __srcu_read_{,un}lock_fast() implicit RCU readers

[Paul E. McKenney]

This commit documents the implicit RCU readers that are implied by the
this_cpu_inc() and atomic_long_inc() operations in __srcu_read_lock_fast()
and __srcu_read_unlock_fast().  While in the area, fix the documentation
of the memory pairing of atomic_long_inc() in __srcu_read_lock_fast().

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: <bpf@xxxxxxxxxxxxxxx>

diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 043b5a67ef71e..78e1a7b845ba9 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -245,9 +245,9 @@ static inline struct srcu_ctr __percpu *__srcu_read_lock_fast(struct srcu_struct
 	struct srcu_ctr __percpu *scp = READ_ONCE(ssp->srcu_ctrp);
 
 	if (!IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE))
-		this_cpu_inc(scp->srcu_locks.counter); /* Y */
+		this_cpu_inc(scp->srcu_locks.counter); // Y, and implicit RCU reader.
 	else
-		atomic_long_inc(raw_cpu_ptr(&scp->srcu_locks));  /* Z */
+		atomic_long_inc(raw_cpu_ptr(&scp->srcu_locks));  // Y, and implicit RCU reader.
 	barrier(); /* Avoid leaking the critical section. */
 	return scp;
 }
@@ -271,9 +271,9 @@ static inline void __srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_
 {
 	barrier();  /* Avoid leaking the critical section. */
 	if (!IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE))
-		this_cpu_inc(scp->srcu_unlocks.counter);  /* Z */
+		this_cpu_inc(scp->srcu_unlocks.counter);  // Z, and implicit RCU reader.
 	else
-		atomic_long_inc(raw_cpu_ptr(&scp->srcu_unlocks));  /* Z */
+		atomic_long_inc(raw_cpu_ptr(&scp->srcu_unlocks));  // Z, and implicit RCU reader.
 }
 
 void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);