Re: [PATCH net-next] rcu: prevent RCU_LOCKDEP_WARN() from swallowing the condition

[nikolay@cumulusnetworks.com]

On 8 September 2020 20:36:24 EEST, Jakub Kicinski <kuba@kernel.org> wrote:
>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@kernel.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;
> }

Ah, you want to solve it for all. :) 
Looks and sounds good to me, 
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>