[PATCH v2 22/35] percpu-rwsem: enable percpu_sem destruction in atomic context

[Michel Lespinasse <michel@lespinasse.org>]

From: Suren Baghdasaryan <surenb@google.com>

Calling percpu_free_rwsem in atomic context results in "scheduling while
atomic" bug being triggered:

BUG: scheduling while atomic: klogd/158/0x00000002
...
  __schedule_bug+0x191/0x290
  schedule_debug+0x97/0x180
  __schedule+0xdc/0xba0
  schedule+0xda/0x250
  schedule_timeout+0x92/0x2d0
  __wait_for_common+0x25b/0x430
  wait_for_completion+0x1f/0x30
  rcu_barrier+0x440/0x4f0
  rcu_sync_dtor+0xaa/0x190
  percpu_free_rwsem+0x41/0x80

Introduce percpu_rwsem_destroy function to perform semaphore destruction
in a worker thread.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
---
 include/linux/percpu-rwsem.h  | 13 ++++++++++++-
 kernel/locking/percpu-rwsem.c | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index 5fda40f97fe9..bf1668fc9c5e 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -13,7 +13,14 @@ struct percpu_rw_semaphore {
 	struct rcu_sync		rss;
 	unsigned int __percpu	*read_count;
 	struct rcuwait		writer;
-	wait_queue_head_t	waiters;
+	/*
+	 * destroy_list_entry is used during object destruction when waiters
+	 * can't be used, therefore reusing the same space.
+	 */
+	union {
+		wait_queue_head_t	waiters;
+		struct list_head	destroy_list_entry;
+	};
 	atomic_t		block;
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	dep_map;
@@ -127,8 +134,12 @@ extern void percpu_up_write(struct percpu_rw_semaphore *);
 extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
 				const char *, struct lock_class_key *);
 
+/* Can't be called in atomic context. */
 extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
 
+/* Invokes percpu_free_rwsem and frees the semaphore from a worker thread. */
+extern void percpu_rwsem_async_destroy(struct percpu_rw_semaphore *sem);
+
 #define percpu_init_rwsem(sem)					\
 ({								\
 	static struct lock_class_key rwsem_key;			\
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 70a32a576f3f..a3d37bf83c60 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -7,6 +7,7 @@
 #include <linux/rcupdate.h>
 #include <linux/sched.h>
 #include <linux/sched/task.h>
+#include <linux/slab.h>
 #include <linux/errno.h>
 
 int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
@@ -268,3 +269,34 @@ void percpu_up_write(struct percpu_rw_semaphore *sem)
 	rcu_sync_exit(&sem->rss);
 }
 EXPORT_SYMBOL_GPL(percpu_up_write);
+
+static LIST_HEAD(destroy_list);
+static DEFINE_SPINLOCK(destroy_list_lock);
+
+static void destroy_list_workfn(struct work_struct *work)
+{
+	struct percpu_rw_semaphore *sem, *sem2;
+	LIST_HEAD(to_destroy);
+
+	spin_lock(&destroy_list_lock);
+	list_splice_init(&destroy_list, &to_destroy);
+	spin_unlock(&destroy_list_lock);
+
+	if (list_empty(&to_destroy))
+		return;
+
+	list_for_each_entry_safe(sem, sem2, &to_destroy, destroy_list_entry) {
+		percpu_free_rwsem(sem);
+		kfree(sem);
+	}
+}
+
+static DECLARE_WORK(destroy_list_work, destroy_list_workfn);
+
+void percpu_rwsem_async_destroy(struct percpu_rw_semaphore *sem)
+{
+	spin_lock(&destroy_list_lock);
+	list_add_tail(&sem->destroy_list_entry, &destroy_list);
+	spin_unlock(&destroy_list_lock);
+	schedule_work(&destroy_list_work);
+}
-- 
2.20.1