[tip/core/rcu,09/10] rcu: Add explicit barrier() to __rcu_read_unlock()
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Message ID 20210304000019.22459-9-paulmck@kernel.org
State In Next
Commit 8572cafecf5095da7e267053196f063e34e641fd
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Series
  • Miscellaneous fixes for v5.13
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Commit Message

Paul E. McKenney March 4, 2021, midnight UTC
From: "Paul E. McKenney" <paulmck@kernel.org>

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>
---
 kernel/rcu/tree_plugin.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Patch
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diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 36212de..d9495de 100644
--- 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);
 	}