linux-kernel - [PATCH tip/core/rcu 7/7] rcu: Prevent RCU_LOCKDEP

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Message-Id: <20201105232454.19919-7-paulmck@kernel.org>
Date:   Thu,  5 Nov 2020 15:24:54 -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,
        Jakub Kicinski <kuba@...nel.org>, paulmck@...nel.org
Subject: [PATCH tip/core/rcu 7/7] rcu: Prevent RCU_LOCKDEP_WARN() from swallowing the condition

From: Jakub Kicinski <kuba@...nel.org>

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.

This commit therefore prevents this 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
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 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 f9533bb..de08264 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -328,7 +328,7 @@ 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 */
-- 
2.9.5