[RFC][PATCH 2/5] cpu-hotplug: Allow get_online_cpus() to nest

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>

Allow get_online_cpus() to be recursive. If a lock is taken while under
"get_online_cpus()", it can call get_online_cpus() as well, just as long as
it is never held without being under get_online_cpus(), but then calling it.

   GOC() -> Lock(X) -> GOC()

is OK, as long as

   Lock(X) -> GOC()

does not exist.

Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 include/linux/sched.h | 1 +
 kernel/cpu.c          | 9 +++++++++
 kernel/fork.c         | 1 +
 3 files changed, 11 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 993e7e25a3a5..8f272ab57685 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -813,6 +813,7 @@ struct task_struct {
 	unsigned int			lockdep_recursion;
 	struct held_lock		held_locks[MAX_LOCK_DEPTH];
 	gfp_t				lockdep_reclaim_gfp;
+	int				goc_depth;
 #endif
 
 #ifdef CONFIG_UBSAN
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 983163ef36ee..0dbdf1e69715 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -209,12 +209,21 @@ DEFINE_STATIC_PERCPU_RWSEM(cpu_hotplug_lock);
 
 void get_online_cpus(void)
 {
+#ifdef CONFIG_LOCKDEP
+	if (current->goc_depth++)
+		return;
+#endif
 	percpu_down_read(&cpu_hotplug_lock);
 }
 EXPORT_SYMBOL_GPL(get_online_cpus);
 
 void put_online_cpus(void)
 {
+#ifdef CONFIG_LOCKDEP
+	WARN_ON_ONCE(current->goc_depth < 1);
+	if (--current->goc_depth)
+		return;
+#endif
 	percpu_up_read(&cpu_hotplug_lock);
 }
 EXPORT_SYMBOL_GPL(put_online_cpus);
diff --git a/kernel/fork.c b/kernel/fork.c
index dd5a371c392a..8aedcd011ccc 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1658,6 +1658,7 @@ static __latent_entropy struct task_struct *copy_process(
 	p->lockdep_depth = 0; /* no locks held yet */
 	p->curr_chain_key = 0;
 	p->lockdep_recursion = 0;
+	p->goc_depth = 0;
 #endif
 
 #ifdef CONFIG_DEBUG_MUTEXES
-- 
2.10.2