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

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Date:   Tue,  8 Sep 2020 10:36:24 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     davem@...emloft.net
Cc:     netdev@...r.kernel.org, paulmck@...nel.org, joel@...lfernandes.org,
        josh@...htriplett.org, peterz@...radead.org,
        christian.brauner@...ntu.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, nikolay@...ulusnetworks.com,
        sfr@...b.auug.org.au, roopa@...dia.com,
        Jakub Kicinski <kuba@...nel.org>
Subject: [PATCH net-next] 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.
We need to un-hide the declaration of lockdep_tasklist_lock_is_held()
and fix a bug the crept into a net/sched header.

Signed-off-by: Jakub Kicinski <kuba@...nel.org>
---
 include/linux/rcupdate.h   | 2 +-
 include/linux/sched/task.h | 2 --
 include/net/sch_generic.h  | 2 +-
 3 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index d15d46db61f7..cf3d3ba3f3e4 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -320,7 +320,7 @@ 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 */
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index a98965007eef..9f943c391df9 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -47,9 +47,7 @@ extern spinlock_t mmlist_lock;
 extern union thread_union init_thread_union;
 extern struct task_struct init_task;
 
-#ifdef CONFIG_PROVE_RCU
 extern int lockdep_tasklist_lock_is_held(void);
-#endif /* #ifdef CONFIG_PROVE_RCU */
 
 extern asmlinkage void schedule_tail(struct task_struct *prev);
 extern void init_idle(struct task_struct *idle, int cpu);
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index d60e7c39d60c..eb68cc6e4e79 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -443,7 +443,7 @@ static inline bool lockdep_tcf_proto_is_locked(struct tcf_proto *tp)
 	return lockdep_is_held(&tp->lock);
 }
 #else
-static inline bool lockdep_tcf_chain_is_locked(struct tcf_block *chain)
+static inline bool lockdep_tcf_chain_is_locked(struct tcf_chain *chain)
 {
 	return true;
 }
-- 
2.24.1