[PATCH] mm: backing-dev: use kfree_rcu() instead of synchronize_rcu_expedited()

[Zqiang]

<IRQ>
 __init_work+0x2d/0x50
 synchronize_rcu_expedited+0x3af/0x650
 bdi_remove_from_list [inline]
 bdi_unregister+0x17f/0x5c0
 release_bdi+0xa1/0xc0
 kref_put [inline]
 bdi_put+0x72/0xa0
 bdev_free_inode+0x11e/0x220
 i_callback+0x3f/0x70
 rcu_do_batch [inline]
 rcu_core+0x76d/0x16c0
 __do_softirq+0x1d7/0x93b
 invoke_softirq [inline]
 __irq_exit_rcu [inline]
 irq_exit_rcu+0xf2/0x130
 sysvec_apic_timer_interrupt+0x93/0xc0

The bdi_remove_from_list() is called in RCU softirq, however the
synchronize_rcu_expedited() will produce sleep action, use kfree_rcu()
instead of it.

Reported-by: Hao Sun <sunhao.th@xxxxxxxxx>
Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx>
---
 include/linux/backing-dev-defs.h | 1 +
 mm/backing-dev.c                 | 4 +---
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 33207004cfde..35a093384518 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -202,6 +202,7 @@ struct backing_dev_info {
 #ifdef CONFIG_DEBUG_FS
 	struct dentry *debug_dir;
 #endif
+	struct rcu_head rcu;
 };
 
 enum {
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index c878d995af06..45d866a3a4a2 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -935,8 +935,6 @@ static void bdi_remove_from_list(struct backing_dev_info *bdi)
 	rb_erase(&bdi->rb_node, &bdi_tree);
 	list_del_rcu(&bdi->bdi_list);
 	spin_unlock_bh(&bdi_lock);
-
-	synchronize_rcu_expedited();
 }
 
 void bdi_unregister(struct backing_dev_info *bdi)
@@ -969,7 +967,7 @@ static void release_bdi(struct kref *ref)
 		bdi_unregister(bdi);
 	WARN_ON_ONCE(bdi->dev);
 	wb_exit(&bdi->wb);
-	kfree(bdi);
+	kfree_rcu(bdi, rcu);
 }
 
 void bdi_put(struct backing_dev_info *bdi)
-- 
2.17.1