rcu: apply TINY_PREEMPT_RCU read-side speedup to TREE_PREEMPT_RCU

Replace one of the ACCESS_ONCE() calls in each of __rcu_read_lock()
and __rcu_read_unlock() with barrier() as suggested by Steve Rostedt in
order to avoid the potential compiler-optimization-induced bug noted by
Mathieu Desnoyers.

Located-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 87f60f06b18e..e9e0bc74ff37 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -201,7 +201,7 @@ static void rcu_preempt_note_context_switch(int cpu)
  */
 void __rcu_read_lock(void)
 {
-        ACCESS_ONCE(current->rcu_read_lock_nesting)++;
+        current->rcu_read_lock_nesting++;
         barrier();  /* needed if we ever invoke rcu_read_lock in rcutree.c */
 }
 EXPORT_SYMBOL_GPL(__rcu_read_lock);
@@ -344,7 +344,9 @@ void __rcu_read_unlock(void)
         struct task_struct *t = current;
 
         barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
-        if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
+        --t->rcu_read_lock_nesting;
+        barrier();  /* decrement before load of ->rcu_read_unlock_special */
+        if (t->rcu_read_lock_nesting == 0 &&
             unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
                 rcu_read_unlock_special(t);
 #ifdef CONFIG_PROVE_LOCKING