linux-kernel - [RFC PATCH 05/21] list: Comment missing WRITE_ONCE() in __list

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Message-Id: <20200324153643.15527-6-will@kernel.org>
Date:   Tue, 24 Mar 2020 15:36:27 +0000
From:   Will Deacon <will@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     Will Deacon <will@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Maddie Stone <maddiestone@...gle.com>,
        Marco Elver <elver@...gle.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com,
        kernel-hardening@...ts.openwall.com
Subject: [RFC PATCH 05/21] list: Comment missing WRITE_ONCE() in __list_del()

Although __list_del() is called from the RCU list implementation, it
omits WRITE_ONCE() when updating the 'prev' pointer for the 'next' node.
This is reasonable because concurrent RCU readers only access the 'next'
pointers.

Although it's obvious after reading the code, add a comment.

Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Will Deacon <will@...nel.org>
---
 include/linux/list.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/list.h b/include/linux/list.h
index 4d9f5f9ed1a8..ec1f780d1449 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -109,6 +109,7 @@ static inline void list_add_tail(struct list_head *new, struct list_head *head)
  */
 static inline void __list_del(struct list_head * prev, struct list_head * next)
 {
+	/* RCU readers don't read the 'prev' pointer */
 	next->prev = prev;
 	WRITE_ONCE(prev->next, next);
 }
-- 
2.20.1