On Wed, Apr 29, 2020 at 11:50:34AM +0200, Greg KH wrote:

On Wed, Apr 29, 2020 at 07:46:27AM +0000, Luis Chamberlain wrote:

quoted

Be pedantic on removal as well and hold the mutex.
This should prevent uses of addition while we exit.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 drivers/block/loop.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index da693e6a834e..6dccba22c9b5 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c

@@ -2333,6 +2333,8 @@ static void __exit loop_exit(void)
 
 	range = max_loop ? max_loop << part_shift : 1UL << MINORBITS;
 
+	mutex_lock(&loop_ctl_mutex);
+
 	idr_for_each(&loop_index_idr, &loop_exit_cb, NULL);
 	idr_destroy(&loop_index_idr);
 

@@ -2340,6 +2342,8 @@ static void __exit loop_exit(void)
 	unregister_blkdev(LOOP_MAJOR, "loop");
 
 	misc_deregister(&loop_misc);
+
+	mutex_unlock(&loop_ctl_mutex);
 }
 
 module_init(loop_init);

What type of issue is this helping with?  Can it be triggered today?  if
so, shouldn't it be backported to stable kernels?

Just code inspection. I can't trigger a userspace test script to crash
the kernel yet, but suspect a race still does exist.

  Luis