[PATCH 2/2] loop: make autoclear operation synchronous again

[Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>]

The kernel test robot is reporting that xfstest can fail at

  umount ext2 on xfs
  umount xfs

sequence, for commit 322c4293ecc58110 ("loop: make autoclear operation
asynchronous") broke what commit ("loop: Make explicit loop device
destruction lazy") wanted to achieve.

Although we cannot guarantee that nobody is holding a reference when
"umount xfs" is called, we should try to close a race window opened
by asynchronous autoclear operation.

Make the autoclear operation upon close() synchronous, by performing
__loop_clr_fd() from post_release() block device callback than from
release() block device callback.

Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Fixes: 322c4293ecc58110 ("loop: make autoclear operation asynchronous")
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
---
 drivers/block/loop.c | 42 ++++++++++++------------------------------
 drivers/block/loop.h |  2 +-
 2 files changed, 13 insertions(+), 31 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index b1b05c45c07c..faa3a3097b22 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1165,40 +1165,12 @@ static void __loop_clr_fd(struct loop_device *lo)
 		lo->lo_disk->flags |= GENHD_FL_NO_PART;
 
 	fput(filp);
-}
-
-static void loop_rundown_completed(struct loop_device *lo)
-{
 	mutex_lock(&lo->lo_mutex);
 	lo->lo_state = Lo_unbound;
 	mutex_unlock(&lo->lo_mutex);
 	module_put(THIS_MODULE);
 }
 
-static void loop_rundown_workfn(struct work_struct *work)
-{
-	struct loop_device *lo = container_of(work, struct loop_device,
-					      rundown_work);
-	struct block_device *bdev = lo->lo_device;
-	struct gendisk *disk = lo->lo_disk;
-
-	__loop_clr_fd(lo);
-	kobject_put(&bdev->bd_device.kobj);
-	module_put(disk->fops->owner);
-	loop_rundown_completed(lo);
-}
-
-static void loop_schedule_rundown(struct loop_device *lo)
-{
-	struct block_device *bdev = lo->lo_device;
-	struct gendisk *disk = lo->lo_disk;
-
-	__module_get(disk->fops->owner);
-	kobject_get(&bdev->bd_device.kobj);
-	INIT_WORK(&lo->rundown_work, loop_rundown_workfn);
-	queue_work(system_long_wq, &lo->rundown_work);
-}
-
 static int loop_clr_fd(struct loop_device *lo)
 {
 	int err;
@@ -1229,7 +1201,6 @@ static int loop_clr_fd(struct loop_device *lo)
 	mutex_unlock(&lo->lo_mutex);
 
 	__loop_clr_fd(lo);
-	loop_rundown_completed(lo);
 	return 0;
 }
 
@@ -1754,7 +1725,7 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
 		 * In autoclear mode, stop the loop thread
 		 * and remove configuration after last close.
 		 */
-		loop_schedule_rundown(lo);
+		lo->rundown_owner = current;
 		return;
 	} else if (lo->lo_state == Lo_bound) {
 		/*
@@ -1769,10 +1740,21 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
 	mutex_unlock(&lo->lo_mutex);
 }
 
+static void lo_post_release(struct gendisk *disk)
+{
+	struct loop_device *lo = disk->private_data;
+
+	if (lo->rundown_owner != current)
+		return;
+	lo->rundown_owner = NULL;
+	__loop_clr_fd(lo);
+}
+
 static const struct block_device_operations lo_fops = {
 	.owner =	THIS_MODULE,
 	.open =		lo_open,
 	.release =	lo_release,
+	.post_release = lo_post_release,
 	.ioctl =	lo_ioctl,
 #ifdef CONFIG_COMPAT
 	.compat_ioctl =	lo_compat_ioctl,
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index 918a7a2dc025..f2e9f38694dc 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -56,7 +56,7 @@ struct loop_device {
 	struct gendisk		*lo_disk;
 	struct mutex		lo_mutex;
 	bool			idr_visible;
-	struct work_struct      rundown_work;
+	struct task_struct	*rundown_owner; /* current or NULL */
 };
 
 struct loop_cmd {
-- 
2.32.0