syzbot is reporting circular locking problem at __loop_clr_fd() [1], for
commit 87579e9b7d8dc36e ("loop: use worker per cgroup instead of kworker")
is calling destroy_workqueue() with disk->open_mutex held.
This circular dependency cannot be broken unless we call __loop_clr_fd()
without holding disk->open_mutex. Therefore, defer __loop_clr_fd() from
lo_release() to a WQ context.
Link: https://syzkaller.appspot.com/bug?extid=643e4ce4b6ad1347d372 [1]
Reported-by: syzbot <syzbot+643e4ce4b6ad1347d372@xxxxxxxxxxxxxxxxxxxxxxxxx>
Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
---
drivers/block/loop.c | 65 ++++++++++++++++++++++++--------------------
drivers/block/loop.h | 1 +
2 files changed, 37 insertions(+), 29 deletions(-)
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index ba76319b5544..7f4ea06534c2 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1082,7 +1082,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
return error;
}
-static void __loop_clr_fd(struct loop_device *lo, bool release)
+static void __loop_clr_fd(struct loop_device *lo)
{
struct file *filp;
gfp_t gfp = lo->old_gfp_mask;
@@ -1144,8 +1144,6 @@ static void __loop_clr_fd(struct loop_device *lo, bool release)
/* let user-space know about this change */
kobject_uevent(&disk_to_dev(lo->lo_disk)->kobj, KOBJ_CHANGE);
mapping_set_gfp_mask(filp->f_mapping, gfp);
- /* This is safe: open() is still holding a reference. */
- module_put(THIS_MODULE);
blk_mq_unfreeze_queue(lo->lo_queue);
disk_force_media_change(lo->lo_disk, DISK_EVENT_MEDIA_CHANGE);
@@ -1153,44 +1151,52 @@ static void __loop_clr_fd(struct loop_device *lo, bool release)
if (lo->lo_flags & LO_FLAGS_PARTSCAN) {
int err;
- /*
- * open_mutex has been held already in release path, so don't
- * acquire it if this function is called in such case.
- *
- * If the reread partition isn't from release path, lo_refcnt
- * must be at least one and it can only become zero when the
- * current holder is released.
- */
- if (!release)
- mutex_lock(&lo->lo_disk->open_mutex);
+ mutex_lock(&lo->lo_disk->open_mutex);
err = bdev_disk_changed(lo->lo_disk, false);
- if (!release)
- mutex_unlock(&lo->lo_disk->open_mutex);
+ mutex_unlock(&lo->lo_disk->open_mutex);
if (err)
pr_warn("%s: partition scan of loop%d failed (rc=%d)\n",
__func__, lo->lo_number, err);
/* Device is gone, no point in returning error */
}
- /*
- * lo->lo_state is set to Lo_unbound here after above partscan has
- * finished. There cannot be anybody else entering __loop_clr_fd() as
- * Lo_rundown state protects us from all the other places trying to
- * change the 'lo' device.
- */
lo->lo_flags = 0;
if (!part_shift)
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);
+}
- /*
- * Need not hold lo_mutex to fput backing file. Calling fput holding
- * lo_mutex triggers a circular lock dependency possibility warning as
- * fput can take open_mutex which is usually taken before lo_mutex.
- */
- fput(filp);
+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)
@@ -1222,7 +1228,8 @@ static int loop_clr_fd(struct loop_device *lo)
lo->lo_state = Lo_rundown;
mutex_unlock(&lo->lo_mutex);
- __loop_clr_fd(lo, false);
+ __loop_clr_fd(lo);
+ loop_rundown_completed(lo);
return 0;
}
@@ -1747,7 +1754,7 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
* In autoclear mode, stop the loop thread
* and remove configuration after last close.
*/
- __loop_clr_fd(lo, true);
+ loop_schedule_rundown(lo);
return;
} else if (lo->lo_state == Lo_bound) {
/*
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index 082d4b6bfc6a..918a7a2dc025 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -56,6 +56,7 @@ struct loop_device {
struct gendisk *lo_disk;
struct mutex lo_mutex;
bool idr_visible;
+ struct work_struct rundown_work;
};
struct loop_cmd {
--
2.18.4
