fs/writeback.c: use rcu_barrier() to wait for inflight wb switches going into workqueue when umount

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
    evict+0xb3/0x180
    iput+0x1b0/0x230
    inode_switch_wbs_work_fn+0x3c0/0x6a0
    worker_thread+0x4e/0x490
    ? process_one_work+0x410/0x410
    kthread+0xe6/0x100
    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.

Link: http://lkml.kernel.org/r/20190429024108.54150-1-jiufei.xue@linux.alibaba.com
Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
Acked-by: Tejun Heo <tj@kernel.org>
Suggested-by: Tejun Heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files 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 @@ restart:
 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);
         }
 }