[PATCH] block: loop: fix deadlock between open and remove

[Ming Lei <ming.lei@redhat.com>]

Commit c76f48eb5c08 ("block: take bd_mutex around delete_partitions in
del_gendisk") adds disk->part0->bd_mutex in del_gendisk(), this way
causes the following AB/BA deadlock between removing loop and opening
loop:

1) loop_control_ioctl(LOOP_CTL_REMOVE)
- mutex_lock(&loop_ctl_mutex)
- mutex_lock(&disk->part0->bd_mutex)	//del_gendisk

2) open look device
- mutex_lock(&disk->part0->bd_mutex)	//blkdev_get_by_dev
- mutex_lock(&loop_ctl_mutex)		//lo_open() <- __blkdev_get

Fixes the issue by not holding loop_ctl_mutex in lo_open(), and cover
the protection on bdev->bd_disk->private_data via disk->part0->bd_mutex.

Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: c76f48eb5c08 ("block: take bd_mutex around delete_partitions in del_gendisk")
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/loop.c | 31 +++++++++++++++----------------
 1 file changed, 15 insertions(+), 16 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index d58d68f3c7cd..b03d8f4c1cdf 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1883,20 +1883,14 @@ static int lo_open(struct block_device *bdev, fmode_t mode)
 	int err;
 
 	/*
-	 * take loop_ctl_mutex to protect lo pointer from race with
-	 * loop_control_ioctl(LOOP_CTL_REMOVE), however, to reduce contention
-	 * release it prior to updating lo->lo_refcnt.
+	 * both ->private_data and ->lo_refcnt are covered by disk's
+	 * open_mutex, so race between open and remove can be avoided
 	 */
-	err = mutex_lock_killable(&loop_ctl_mutex);
-	if (err)
-		return err;
 	lo = bdev->bd_disk->private_data;
-	if (!lo) {
-		mutex_unlock(&loop_ctl_mutex);
+	if (!lo)
 		return -ENXIO;
-	}
+
 	err = mutex_lock_killable(&lo->lo_mutex);
-	mutex_unlock(&loop_ctl_mutex);
 	if (err)
 		return err;
 	atomic_inc(&lo->lo_refcnt);
@@ -2272,21 +2266,26 @@ static long loop_control_ioctl(struct file *file, unsigned int cmd,
 		ret = loop_lookup(&lo, parm);
 		if (ret < 0)
 			break;
-		ret = mutex_lock_killable(&lo->lo_mutex);
+		/* cover removing vs. opening loop device */
+		ret = mutex_lock_killable(&lo->lo_disk->part0->bd_mutex);
 		if (ret)
 			break;
-		if (lo->lo_state != Lo_unbound) {
-			ret = -EBUSY;
-			mutex_unlock(&lo->lo_mutex);
+		ret = mutex_lock_killable(&lo->lo_mutex);
+		if (ret) {
+			mutex_unlock(&lo->lo_disk->part0->bd_mutex);
 			break;
 		}
-		if (atomic_read(&lo->lo_refcnt) > 0) {
+		if (lo->lo_state != Lo_unbound ||
+				atomic_read(&lo->lo_refcnt) > 0) {
 			ret = -EBUSY;
 			mutex_unlock(&lo->lo_mutex);
+			mutex_unlock(&lo->lo_disk->part0->bd_mutex);
 			break;
 		}
-		lo->lo_disk->private_data = NULL;
 		mutex_unlock(&lo->lo_mutex);
+		lo->lo_disk->private_data = NULL;
+		mutex_unlock(&lo->lo_disk->part0->bd_mutex);
+
 		idr_remove(&loop_index_idr, lo->lo_number);
 		loop_remove(lo);
 		break;
-- 
2.29.2