Commit 322c4293 authored by Tetsuo Handa's avatar Tetsuo Handa Committed by Jens Axboe
Browse files

loop: make autoclear operation asynchronous 

syzbot is reporting circular locking problem at __loop_clr_fd() [1], for
commit 87579e9b ("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: default avatarsyzbot <syzbot+643e4ce4b6ad1347d372@syzkaller.appspotmail.com>
Suggested-by: default avatarChristoph Hellwig <hch@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Tested-by: default avatar <syzbot+643e4ce4b6ad1347d372@syzkaller.appspotmail.com>
Signed-off-by: default avatarTetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/1ed7df28-ebd6-71fb-70e5-1c2972e05ddb@i-love.sakura.ne.jp


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent c5eafd79

drivers/block/loop.c

+36 −29
Original line number Diff line number Diff line
@@ -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);
@@ -1153,18 +1151,8 @@ 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);

		err = bdev_disk_changed(lo->lo_disk, false);

		if (!release)

		mutex_unlock(&lo->lo_disk->open_mutex);

		if (err)

			pr_warn("%s: partition scan of loop%d failed (rc=%d)\n",
@@ -1172,25 +1160,43 @@ static void __loop_clr_fd(struct loop_device *lo, bool release) 
		/* 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) {

		/*

drivers/block/loop.h

+1 −0
Original line number Diff line number Diff line
@@ -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 {