linux-kernel - [PATCH tip/core/rcu 09/10] rcu: Add explicit barrier() to __rcu_read

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Message-Id: <20210304000019.22459-9-paulmck@kernel.org>
Date:   Wed,  3 Mar 2021 16:00:18 -0800
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 09/10] rcu: Add explicit barrier() to __rcu_read_unlock()

From: "Paul E. McKenney" <paulmck@...nel.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@...nel.org>
---
 kernel/rcu/tree_plugin.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

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);
 	}
-- 
2.9.5