block: loop: fix deadlock between open and remove

Commit 4a92cb3acc41 ("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)
     -> del_gendisk
         -> mutex_lock(&disk->part0->bd_mutex)

 2) blkdev_get_by_dev
     -> mutex_lock(&disk->part0->bd_mutex)
     -> lo_open
         -> mutex_lock(&loop_ctl_mutex)

Add a new Lo_deleting state to remove the need for clearing
->private_data and thus holding loop_ctl_mutex in the ioctl
LOOP_CTL_REMOVE path.

Based on an analysis and earlier patch from
Ming Lei <ming.lei@redhat.com>.

Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: 4a92cb3acc41 ("block: take bd_mutex around delete_partitions in del_gendisk")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210605140950.5800-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index a370cde3ddd49ad7e8c8d005279f904fa793bb57..f280a96d4de2d8723049ee3c6e8c79b12c44d653 100644 (file)
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1878,29 +1878,18 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode,
 
 static int lo_open(struct block_device *bdev, fmode_t mode)
 {
-       struct loop_device *lo;
+       struct loop_device *lo = bdev->bd_disk->private_data;
        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.
-        */
-       err = mutex_lock_killable(&loop_ctl_mutex);
-       if (err)
-               return err;
-       lo = bdev->bd_disk->private_data;
-       if (!lo) {
-               mutex_unlock(&loop_ctl_mutex);
-               return -ENXIO;
-       }
        err = mutex_lock_killable(&lo->lo_mutex);
-       mutex_unlock(&loop_ctl_mutex);
        if (err)
                return err;
-       atomic_inc(&lo->lo_refcnt);
+       if (lo->lo_state == Lo_deleting)
+               err = -ENXIO;
+       else
+               atomic_inc(&lo->lo_refcnt);
        mutex_unlock(&lo->lo_mutex);
-       return 0;
+       return err;
 }
 
 static void lo_release(struct gendisk *disk, fmode_t mode)
@@ -2284,7 +2273,7 @@ static long loop_control_ioctl(struct file *file, unsigned int cmd,
                        mutex_unlock(&lo->lo_mutex);
                        break;
                }
-               lo->lo_disk->private_data = NULL;
+               lo->lo_state = Lo_deleting;
                mutex_unlock(&lo->lo_mutex);
                idr_remove(&loop_index_idr, lo->lo_number);
                loop_remove(lo);

diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index a3c04f310672e8e3968fbb30a1c596368002d340..5beb959b94d36b79bf38a40faa0c502146d8c26f 100644 (file)
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -22,6 +22,7 @@ enum {
        Lo_unbound,
        Lo_bound,
        Lo_rundown,
+       Lo_deleting,
 };
 
 struct loop_func_table;