linux-kernel - [tip: core/rcu] rcu: Prevent RCU_LOCKDEP_WARN() from swallowing the condition

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Date:   Sun, 13 Dec 2020 19:01:13 -0000
From:   "tip-bot2 for Jakub Kicinski" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: core/rcu] rcu: Prevent RCU_LOCKDEP_WARN() from swallowing the condition

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     65e9eb1ccfe56b41a0d8bfec651ea014968413cb
Gitweb:        https://git.kernel.org/tip/65e9eb1ccfe56b41a0d8bfec651ea014968413cb
Author:        Jakub Kicinski <kuba@...nel.org>
AuthorDate:    Wed, 16 Sep 2020 11:45:28 -07:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Mon, 02 Nov 2020 17:10:01 -08:00

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.

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 */