rcu: Add explicit barrier() to __rcu_read_unlock()

Because preemptible RCU's __rcu_read_unlock() is an external function,
the rough equivalent of an implicit barrier() is inserted by the compiler.
Except that there is a direct call to __rcu_read_unlock() in that same
file, and compilers are getting to the point where they might choose to
inline the fastpath of the __rcu_read_unlock() function.

This commit therefore adds an explicit barrier() to the very beginning
of __rcu_read_unlock().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 2d60377..a32494c 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -393,8 +393,9 @@ void __rcu_read_unlock(void)
 {
        struct task_struct *t = current;
 
+       barrier();  // critical section before exit code.
        if (rcu_preempt_read_exit() == 0) {
-               barrier();  /* critical section before exit code. */
+               barrier();  // critical-section exit before .s check.
                if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
                        rcu_read_unlock_special(t);
        }