[PATCH 2/2] loop: use task_work for autoclear operation

[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.

Try to make the autoclear operation upon close() synchronous, by calling
__loop_clr_fd() from current thread's task work rather than a WQ thread.

Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
---
 drivers/block/loop.c | 25 ++++++++++++++++++++-----
 drivers/block/loop.h |  5 ++++-
 2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index b1b05c45c07c..814605e2cbef 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1175,10 +1175,8 @@ static void loop_rundown_completed(struct loop_device *lo)
 	module_put(THIS_MODULE);
 }
 
-static void loop_rundown_workfn(struct work_struct *work)
+static void loop_rundown_start(struct loop_device *lo)
 {
-	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;
 
@@ -1188,6 +1186,18 @@ static void loop_rundown_workfn(struct work_struct *work)
 	loop_rundown_completed(lo);
 }
 
+static void loop_rundown_callbackfn(struct callback_head *callback)
+{
+	loop_rundown_start(container_of(callback, struct loop_device,
+					rundown.callback));
+}
+
+static void loop_rundown_workfn(struct work_struct *work)
+{
+	loop_rundown_start(container_of(work, struct loop_device,
+					rundown.work));
+}
+
 static void loop_schedule_rundown(struct loop_device *lo)
 {
 	struct block_device *bdev = lo->lo_device;
@@ -1195,8 +1205,13 @@ static void loop_schedule_rundown(struct loop_device *lo)
 
 	__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);
+	if (!(current->flags & PF_KTHREAD)) {
+		init_task_work(&lo->rundown.callback, loop_rundown_callbackfn);
+		if (!task_work_add(current, &lo->rundown.callback, TWA_RESUME))
+			return;
+	}
+	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)
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index 918a7a2dc025..596472f9cde3 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -56,7 +56,10 @@ struct loop_device {
 	struct gendisk		*lo_disk;
 	struct mutex		lo_mutex;
 	bool			idr_visible;
-	struct work_struct      rundown_work;
+	union {
+		struct work_struct   work;
+		struct callback_head callback;
+	} rundown;
 };
 
 struct loop_cmd {
-- 
2.32.0