block, writeback: wait for writeback to finish before detaching wb

__blkdev_put() could surprise writeback thread by detaching the
wb object from an inode that hasn't cleared the I_SYNC flag yet.
This causes a NULL pointer dereference as seen below:

  BUG: unable to handle kernel NULL pointer dereference at (null)
  IP: locked_inode_to_wb_and_lock_list+0x38/0x440
  PGD 0
  Oops: 0000 [#1] SMP
  CPU: 0 PID: 34 Comm: kworker/u8:1 Not tainted 4.11.0-rc1+ torvalds#202
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
  Workqueue: writeback wb_workfn (flush-8:16)
  task: ffff88013aa780c0 task.stack: ffffc9000012c000
  RIP: 0010:locked_inode_to_wb_and_lock_list+0x38/0x440
  RSP: 0018:ffffc9000012fb70 EFLAGS: 00010202
  RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000018
  RDX: ffff88013aa780c0 RSI: ffff880139a478f8 RDI: ffff88013aa788b8
  RBP: ffffc9000012fba0 R08: 0000000000000001 R09: 0000000000000000
  R10: 00000000969da8e2 R11: 0000000000000000 R12: ffff880139a47858
  R13: ffff880139a478e0 R14: ffff880139a478f8 R15: ffff8801371f4058
  FS:  0000000000000000(0000) GS:ffff88013ae00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000000 CR3: 0000000001012000 CR4: 00000000000006f0
  Call Trace:
   writeback_sb_inodes+0x3e1/0x7a0
   __writeback_inodes_wb+0x87/0xc0
   wb_writeback+0x2e7/0x5c0
   wb_workfn+0x2d1/0x9c0
   process_one_work+0x1d3/0x620
   worker_thread+0x126/0x4a0
   kthread+0x10a/0x140
   ret_from_fork+0x2e/0x40
  RIP: locked_inode_to_wb_and_lock_list+0x38/0x440 RSP: ffffc9000012fb70
  CR2: 0000000000000000
  ---[ end trace e0ea8a2695f4c86c ]---

Make __blkdev_put() wait for the I_SYNC flag to clear before detaching
wb.

Fixes: 43d1c0e ("block: detach bdev inode from its wb in __blkdev_put()")
Signed-off-by: Tahsin Erdogan <[email protected]>

Author: Tahsin Erdogan

fs/block_dev.c

@@ -1880,7 +1880,10 @@ static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
 		 * Detaching bdev inode from its wb in __destroy_inode()
 		 * is too late: the queue which embeds its bdi (along with
 		 * root wb) can be gone as soon as we put_disk() below.
+		 * Before detaching wb, wait for any writeback activity for
+		 * inode to settle.
 		 */
+		inode_wait_for_writeback(bdev->bd_inode);
 		inode_detach_wb(bdev->bd_inode);
 	}
 	if (bdev->bd_contains == bdev) {