[PATCH] bcache: fix panic due to cache_set is null

[Yi Li]

bcache_device_detach will release the cache_set after hotunplug cache
disk. update_writeback_rate should check validate of cache_set.

  IP: [<ffffffffa03730c9>] update_writeback_rate+0x59/0x3a0 [bcache]
  PGD 879620067 PUD 8755d3067 PMD 0
  Oops: 0000 [#1] SMP
  CPU: 8 PID: 1005702 Comm: kworker/8:0 Tainted: G 4.4.0+10 #1
  Hardware name: Intel BIOS SE5C610.86B.01.01.0021.032120170601 03/21/2017
  Workqueue: events update_writeback_rate [bcache]
  task: ffff8808786f3800 ti: ffff88077082c000 task.ti: ffff88077082c000
  RIP: e030:[<ffffffffa03730c9>] update_writeback_rate+0x59/0x3a0 [bcache]
  RSP: e02b:ffff88077082fde0  EFLAGS: 00010202
  RAX: 0000000000000018 RBX: ffff8808047f0b08 RCX: 0000000000000000
  RDX: 0000000000000001 RSI: ffff88088170dab8 RDI: ffff88088170dab8
  RBP: ffff88077082fe18 R08: 000000000000000a R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000017bc8 R12: 0000000000000000
  R13: ffff8808047f0000 R14: 0000000000000200 R15: ffff8808047f0b08
  FS:  00007f157b6d6700(0000) GS:ffff880881700000(0000) knlGS:0000000000000000
  CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000368 CR3: 0000000875c05000 CR4: 0000000000040660
  Stack:
   0000000000000001 0000000000007ff0 ffff88085ff600c0 ffff880881714e80
   ffff880881719500 0000000000000200 ffff8808047f0b08 ffff88077082fe60
   ffffffff81088c0c 0000000081714e80 0000000000000000 ffff880881714e80
  Call Trace:
   [<ffffffff81088c0c>] process_one_work+0x1fc/0x3b0
   [<ffffffff81089575>] worker_thread+0x2a5/0x470
   [<ffffffff815a2f58>] ? __schedule+0x648/0x870
   [<ffffffff810892d0>] ? rescuer_thread+0x300/0x300
   [<ffffffff8108e3d5>] kthread+0xd5/0xe0
   [<ffffffff8108e300>] ? kthread_stop+0x110/0x110
   [<ffffffff815a704f>] ret_from_fork+0x3f/0x70
   [<ffffffff8108e300>] ? kthread_stop+0x110/0x110

Reported-by: Guo Chao <guochao@xxxxxxxxxxx>
Signed-off-by: Guo Chao <guochao@xxxxxxxxxxx>
Signed-off-by: Yi Li <yili@xxxxxxxxxxx>
---
 drivers/md/bcache/writeback.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 3c74996978da..186c4c6e1607 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -175,7 +175,15 @@ static void update_writeback_rate(struct work_struct *work)
 	struct cached_dev *dc = container_of(to_delayed_work(work),
 					     struct cached_dev,
 					     writeback_rate_update);
-	struct cache_set *c = dc->disk.c;
+	struct cache_set *c = NULL;
+
+	mutex_lock(&bch_register_lock);
+	c = dc->disk.c;
+
+	if (c == NULL) {
+		mutex_unlock(&bch_register_lock);
+		return;
+	}
 
 	/*
 	 * should check BCACHE_DEV_RATE_DW_RUNNING before calling
@@ -194,6 +202,7 @@ static void update_writeback_rate(struct work_struct *work)
 		clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
 		/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
 		smp_mb__after_atomic();
+		mutex_unlock(&bch_register_lock);
 		return;
 	}
 
@@ -230,6 +239,7 @@ static void update_writeback_rate(struct work_struct *work)
 	clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
 	/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
 	smp_mb__after_atomic();
+	mutex_unlock(&bch_register_lock);
 }
 
 static unsigned int writeback_delay(struct cached_dev *dc,
-- 
2.25.3