[PATCH] writeback: Judge bdi->dev when set worker desc in bdi_writeback_workfn.

[majianpeng <majianpeng@gmail.com>]

Met a oops when remove a busy-writing disk.The kernel messages are:
[  253.105528] PGD 1366b6067 PUD 136690067 PMD 0
[  253.105531] Oops: 0000 [#1] SMP
[  253.105541] CPU: 3 PID: 748 Comm: kworker/u8:2 Tainted: G        W 3.11.0+ #183
[  253.105542] Hardware name: To Be Filled By O.E.M. To Be Filled By
O.E.M./To be filled by O.E.M., BIOS 080015  01/06/2011[  253.105543]
Workqueue: writeback bdi_writeback_workfn
[  253.105545] task: ffff880136d98000 ti: ffff880136dfa000 task.ti:ffff880136dfa000
[  253.105547] RIP: 0010:[<ffffffff811952ca>]  [<ffffffff811952ca>] bdi_writeback_workfn+0x3a/0x3c0
[  253.105548] RSP: 0018:ffff880136dfbcc8  EFLAGS: 00010292
[  253.105549] RAX: 0000000000000000 RBX: ffff880136cf4580 RCX: 0000000000000000
[  253.105549] RDX: ffff880136dc1a70 RSI: 0000000000584000 RDI: ffff880136dc1d68
[  253.105550] RBP: ffff880136dfbd68 R08: 0000000000000000 R09: ffff880136d98750
[  253.105551] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88013a812000
[  253.105552] R13: ffff880139d0b900 R14: ffff880136dc1d68 R15: 0000000000000100
[  253.105553] FS:  0000000000000000(0000) GS:ffff88013b400000(0000) knlGS:0000000000000000
[  253.105554] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  253.105555] CR2: 0000000000000050 CR3: 00000001366b5000 CR4: 00000000000407e0
[  253.105555] Stack:
[  253.105561]  ffff880136dc1d68 ffffffff8106546e ffff880136dc1a70 ffff880136cf4580
[  253.105563]  ffff880136dc1d50 0000000000000000 0000000000000000 0000000000000000
[  253.105565]  ffff880136dfbd68 0000000000000246 ffffffff81065463 0000000000000000
[  253.105566] Call Trace:
[  253.105575]  [<ffffffff8106546e>] ? process_one_work+0x18e/0x500
[  253.105577]  [<ffffffff81065463>] ? process_one_work+0x183/0x500
[  253.105579]  [<ffffffff810654cc>] process_one_work+0x1ec/0x500
[  253.105580]  [<ffffffff81065463>] ? process_one_work+0x183/0x500
[  253.105582]  [<ffffffff81065c52>] worker_thread+0x122/0x380
[  253.105584]  [<ffffffff81065b30>] ? rescuer_thread+0x310/0x310
[  253.105586]  [<ffffffff8106d67b>] kthread+0xdb/0xe0
[  253.105589]  [<ffffffff8106d5a0>] ? flush_kthread_work+0x1c0/0x1c0
[  253.105591]  [<ffffffff816f6c1c>] ret_from_fork+0x7c/0xb0
[  253.105593]  [<ffffffff8106d5a0>] ? flush_kthread_work+0x1c0/0x1c0
[  253.105610] Code: e5 41 57 41 56 41 55 41 54 53 48 83 ec 78 48 8b 57
e8 48 89 45 80 48 89 bd 60 ff ff ff 48 8b 82 a8 04 00 00 48 89 95 70 ff
ff ff <48> 8b 70 50 48 85 f6 0f 84 68 02 00 00 31 c0 48 c7 c7 6e e2 a2
[  253.105611] RIP  [<ffffffff811952ca>] bdi_writeback_workfn+0x3a/0x3c0
[  253.105612]  RSP <ffff880136dfbcc8>
[  253.105613] CR2: 0000000000000050
[  253.105615] ---[ end trace 60404b6c9a2e6b32 ]---

This bug introduced by commit ef3b101925f2170c.
I think the situation is like:
remove disk			     flush_thread_work
bdi_destroy()
  bdi_unregister()
    bdi->dev = NULL
				       bdi_writeback_workfn()

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
---
 fs/fs-writeback.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 68851ff..9beb967 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1007,7 +1007,8 @@ void bdi_writeback_workfn(struct work_struct *work)
 	struct backing_dev_info *bdi = wb->bdi;
 	long pages_written;
 
-	set_worker_desc("flush-%s", dev_name(bdi->dev));
+	set_worker_desc("flush-%s", bdi->dev ?
+			dev_name(bdi->dev) : bdi->name);
 	current->flags |= PF_SWAPWRITE;
 
 	if (likely(!current_is_workqueue_rescuer() ||
-- 
1.8.1.2