[PATCH v4 RESEND] fs/writeback: use rcu_barrier() to wait for inflight wb switches going into workqueue when umount

[Jiufei Xue <jiufei.xue@linux.alibaba.com>]

synchronize_rcu() didn't wait for call_rcu() callbacks, so inode wb
switch may not go to the workqueue after synchronize_rcu(). Thus
previous scheduled switches was not finished even flushing the
workqueue, which will cause a NULL pointer dereferenced followed below.

VFS: Busy inodes after unmount of vdd. Self-destruct in 5 seconds.  Have a nice day...
BUG: unable to handle kernel NULL pointer dereference at 0000000000000278
[<ffffffff8126a303>] evict+0xb3/0x180
[<ffffffff8126a760>] iput+0x1b0/0x230
[<ffffffff8127c690>] inode_switch_wbs_work_fn+0x3c0/0x6a0
[<ffffffff810a5b2e>] worker_thread+0x4e/0x490
[<ffffffff810a5ae0>] ? process_one_work+0x410/0x410
[<ffffffff810ac056>] kthread+0xe6/0x100
[<ffffffff8173c199>] ret_from_fork+0x39/0x50

Replace the synchronize_rcu() call with a rcu_barrier() to wait for all
pending callbacks to finish. And inc isw_nr_in_flight after call_rcu()
in inode_switch_wbs() to make more sense.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: stable@kernel.org
---
 fs/fs-writeback.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 36855c1f8daf..b16645b417d9 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -523,8 +523,6 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 
 	isw->inode = inode;
 
-	atomic_inc(&isw_nr_in_flight);
-
 	/*
 	 * In addition to synchronizing among switchers, I_WB_SWITCH tells
 	 * the RCU protected stat update paths to grab the i_page
@@ -532,6 +530,9 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	 * Let's continue after I_WB_SWITCH is guaranteed to be visible.
 	 */
 	call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
+
+	atomic_inc(&isw_nr_in_flight);
+
 	goto out_unlock;
 
 out_free:
@@ -901,7 +902,11 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 void cgroup_writeback_umount(void)
 {
 	if (atomic_read(&isw_nr_in_flight)) {
-		synchronize_rcu();
+		/*
+		 * Use rcu_barrier() to wait for all pending callbacks to
+		 * ensure that all in-flight wb switches are in the workqueue.
+		 */
+		rcu_barrier();
 		flush_workqueue(isw_wq);
 	}
 }
-- 
2.19.1.856.g8858448bb