linux-kernel - [RFC PATCH 06/21] list: Remove superfluous WRITE_ONCE() from hlist

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Date:   Tue, 24 Mar 2020 15:36:28 +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 06/21] list: Remove superfluous WRITE_ONCE() from hlist_nulls implementation

Commit 860c8802ace1 ("rcu: Use WRITE_ONCE() for assignments to ->pprev
for hlist_nulls") added WRITE_ONCE() invocations to hlist_nulls_add_head()
and hlist_nulls_del().

Since these functions should not ordinarily run concurrently with other
list accessors, restore the plain C assignments so that KCSAN can yell
if a data race occurs.

Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Will Deacon <will@...nel.org>
---
 include/linux/list_nulls.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
index fa51a801bf32..fd154ceb5b0d 100644
--- a/include/linux/list_nulls.h
+++ b/include/linux/list_nulls.h
@@ -80,10 +80,10 @@ static inline void hlist_nulls_add_head(struct hlist_nulls_node *n,
 	struct hlist_nulls_node *first = h->first;
 
 	n->next = first;
-	WRITE_ONCE(n->pprev, &h->first);
+	n->pprev = &h->first;
 	h->first = n;
 	if (!is_a_nulls(first))
-		WRITE_ONCE(first->pprev, &n->next);
+		first->pprev = &n->next;
 }
 
 static inline void __hlist_nulls_del(struct hlist_nulls_node *n)
@@ -99,7 +99,7 @@ static inline void __hlist_nulls_del(struct hlist_nulls_node *n)
 static inline void hlist_nulls_del(struct hlist_nulls_node *n)
 {
 	__hlist_nulls_del(n);
-	WRITE_ONCE(n->pprev, LIST_POISON2);
+	n->pprev = LIST_POISON2;
 }
 
 /**
-- 
2.20.1