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Date:   Wed, 16 Dec 2020 00:49:27 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 17/19] cpu/hotplug: Add lockdep_is_cpus_held()

On Fri, Nov 13, 2020 at 01:13:32PM +0100, Frederic Weisbecker wrote:
> RCU needs to check if the cpu hotplug lock is held, in the middle of
> other conditions to check the sanity of RCU-nocb. Provide a helper for
> that.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Boqun Feng <boqun.feng@...il.com>
> ---
>  include/linux/cpu.h | 1 +
>  kernel/cpu.c        | 7 +++++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index d6428aaf67e7..27eb3875e45a 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -118,6 +118,7 @@ extern void cpus_read_lock(void);
>  extern void cpus_read_unlock(void);
>  extern int  cpus_read_trylock(void);
>  extern void lockdep_assert_cpus_held(void);
> +extern int lockdep_is_cpus_held(void);
>  extern void cpu_hotplug_disable(void);
>  extern void cpu_hotplug_enable(void);
>  void clear_tasks_mm_cpumask(int cpu);
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 6ff2578ecf17..3cca5b27de25 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -330,6 +330,13 @@ void lockdep_assert_cpus_held(void)
>  	percpu_rwsem_assert_held(&cpu_hotplug_lock);
>  }
>  
> +#ifdef CONFIG_LOCKDEP
> +int lockdep_is_cpus_held(void)
> +{
> +	return percpu_rwsem_is_held(&cpu_hotplug_lock);
> +}
> +#endif
> +
>  static void lockdep_acquire_cpus_lock(void)
>  {
>  	rwsem_acquire(&cpu_hotplug_lock.dep_map, 0, 0, _THIS_IP_);

This patch may trigger a build error with !CONFIG_HOTPLUG.

Here is a fixed version:

---
>From 80609608b29758e43e8d8d6beaca486b8c42bb43 Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <frederic@...nel.org>
Date: Wed, 11 Nov 2020 23:53:13 +0100
Subject: [PATCH] cpu/hotplug: Add lockdep_is_cpus_held()

RCU needs to check if the cpu hotplug lock is held, in the middle of
other conditions to check the sanity of RCU-nocb. Provide a helper for
that.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Boqun Feng <boqun.feng@...il.com>
---
 include/linux/cpu.h | 2 ++
 kernel/cpu.c        | 7 +++++++
 2 files changed, 9 insertions(+)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index d6428aaf67e7..3aaa0687e8df 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -111,6 +111,8 @@ static inline void cpu_maps_update_done(void)
 #endif /* CONFIG_SMP */
 extern struct bus_type cpu_subsys;
 
+extern int lockdep_is_cpus_held(void);
+
 #ifdef CONFIG_HOTPLUG_CPU
 extern void cpus_write_lock(void);
 extern void cpus_write_unlock(void);
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6ff2578ecf17..3cca5b27de25 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -330,6 +330,13 @@ void lockdep_assert_cpus_held(void)
 	percpu_rwsem_assert_held(&cpu_hotplug_lock);
 }
 
+#ifdef CONFIG_LOCKDEP
+int lockdep_is_cpus_held(void)
+{
+	return percpu_rwsem_is_held(&cpu_hotplug_lock);
+}
+#endif
+
 static void lockdep_acquire_cpus_lock(void)
 {
 	rwsem_acquire(&cpu_hotplug_lock.dep_map, 0, 0, _THIS_IP_);
-- 
2.25.1

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