linux-kernel - [PATCH net-next 7/7] rcu: prevent RCU_LOCKDEP

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Date:   Wed, 16 Sep 2020 11:45:28 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     davem@...emloft.net, paulmck@...nel.org, joel@...lfernandes.org
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        rcu@...r.kernel.org, josh@...htriplett.org, peterz@...radead.org,
        christian.brauner@...ntu.com, Jakub Kicinski <kuba@...nel.org>,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com
Subject: [PATCH net-next 7/7] rcu: prevent RCU_LOCKDEP_WARN() from swallowing the condition

We run into a unused variable warning in bridge code when
variable is only used inside the condition of
rcu_dereference_protected().

 #define mlock_dereference(X, br) \
	rcu_dereference_protected(X, lockdep_is_held(&br->multicast_lock))

Since on builds with CONFIG_PROVE_RCU=n rcu_dereference_protected()
compiles to nothing the compiler doesn't see the variable use.

Prevent the warning by adding the condition as dead code.

Signed-off-by: Jakub Kicinski <kuba@...nel.org>
--
CC: paulmck@...nel.org
CC: josh@...htriplett.org
CC: rostedt@...dmis.org
CC: mathieu.desnoyers@...icios.com
CC: joel@...lfernandes.org
CC: jiangshanlai@...il.com
---
 include/linux/rcupdate.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 53f9648cb982..50d45781fa99 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -321,7 +321,7 @@ static inline void rcu_preempt_sleep_check(void) { }
 
 #else /* #ifdef CONFIG_PROVE_RCU */
 
-#define RCU_LOCKDEP_WARN(c, s) do { } while (0)
+#define RCU_LOCKDEP_WARN(c, s) do { } while (0 && (c))
 #define rcu_sleep_check() do { } while (0)
 
 #endif /* #else #ifdef CONFIG_PROVE_RCU */
-- 
2.26.2