We got the following lockdep splat while running xfstests (specifically
btrfs/003 and btrfs/020 in a row) with the new rc.  This was uncovered
by 87579e9b7d8d ("loop: use worker per cgroup instead of kworker") which
converted loop to using workqueues, which comes with lockdep
annotations that don't exist with kworkers.  The lockdep splat is as
follows

======================================================
WARNING: possible circular locking dependency detected
5.14.0-rc2-custom+ #34 Not tainted
------------------------------------------------------
losetup/156417 is trying to acquire lock:
ffff9c7645b02d38 ((wq_completion)loop0){+.+.}-{0:0}, at: flush_workqueue+0x84/0x600

but task is already holding lock:
ffff9c7647395468 (&lo->lo_mutex){+.+.}-{3:3}, at: __loop_clr_fd+0x41/0x650 [loop]

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #5 (&lo->lo_mutex){+.+.}-{3:3}:
       __mutex_lock+0xba/0x7c0
       lo_open+0x28/0x60 [loop]
       blkdev_get_whole+0x28/0xf0
       blkdev_get_by_dev.part.0+0x168/0x3c0
       blkdev_open+0xd2/0xe0
       do_dentry_open+0x163/0x3a0
       path_openat+0x74d/0xa40
       do_filp_open+0x9c/0x140
       do_sys_openat2+0xb1/0x170
       __x64_sys_openat+0x54/0x90
       do_syscall_64+0x3b/0x90
       entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #4 (&disk->open_mutex){+.+.}-{3:3}:
       __mutex_lock+0xba/0x7c0
       blkdev_get_by_dev.part.0+0xd1/0x3c0
       blkdev_get_by_path+0xc0/0xd0
       btrfs_scan_one_device+0x52/0x1f0 [btrfs]
       btrfs_control_ioctl+0xac/0x170 [btrfs]
       __x64_sys_ioctl+0x83/0xb0
       do_syscall_64+0x3b/0x90
       entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #3 (uuid_mutex){+.+.}-{3:3}:
       __mutex_lock+0xba/0x7c0
       btrfs_rm_device+0x48/0x6a0 [btrfs]
       btrfs_ioctl+0x2d1c/0x3110 [btrfs]
       __x64_sys_ioctl+0x83/0xb0
       do_syscall_64+0x3b/0x90
       entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #2 (sb_writers#11){.+.+}-{0:0}:
       lo_write_bvec+0x112/0x290 [loop]
       loop_process_work+0x25f/0xcb0 [loop]
       process_one_work+0x28f/0x5d0
       worker_thread+0x55/0x3c0
       kthread+0x140/0x170
       ret_from_fork+0x22/0x30

-> #1 ((work_completion)(&lo->rootcg_work)){+.+.}-{0:0}:
       process_one_work+0x266/0x5d0
       worker_thread+0x55/0x3c0
       kthread+0x140/0x170
       ret_from_fork+0x22/0x30

-> #0 ((wq_completion)loop0){+.+.}-{0:0}:
       __lock_acquire+0x1130/0x1dc0
       lock_acquire+0xf5/0x320
       flush_workqueue+0xae/0x600
       drain_workqueue+0xa0/0x110
       destroy_workqueue+0x36/0x250
       __loop_clr_fd+0x9a/0x650 [loop]
       lo_ioctl+0x29d/0x780 [loop]
       block_ioctl+0x3f/0x50
       __x64_sys_ioctl+0x83/0xb0
       do_syscall_64+0x3b/0x90
       entry_SYSCALL_64_after_hwframe+0x44/0xae

other info that might help us debug this:
Chain exists of:
  (wq_completion)loop0 --> &disk->open_mutex --> &lo->lo_mutex
 Possible unsafe locking scenario:
       CPU0                    CPU1
       ----                    ----
  lock(&lo->lo_mutex);
                               lock(&disk->open_mutex);
                               lock(&lo->lo_mutex);
  lock((wq_completion)loop0);

 *** DEADLOCK ***
1 lock held by losetup/156417:
 #0: ffff9c7647395468 (&lo->lo_mutex){+.+.}-{3:3}, at: __loop_clr_fd+0x41/0x650 [loop]

stack backtrace:
CPU: 8 PID: 156417 Comm: losetup Not tainted 5.14.0-rc2-custom+ #34
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
Call Trace:
 dump_stack_lvl+0x57/0x72
 check_noncircular+0x10a/0x120
 __lock_acquire+0x1130/0x1dc0
 lock_acquire+0xf5/0x320
 ? flush_workqueue+0x84/0x600
 flush_workqueue+0xae/0x600
 ? flush_workqueue+0x84/0x600
 drain_workqueue+0xa0/0x110
 destroy_workqueue+0x36/0x250
 __loop_clr_fd+0x9a/0x650 [loop]
 lo_ioctl+0x29d/0x780 [loop]
 ? __lock_acquire+0x3a0/0x1dc0
 ? update_dl_rq_load_avg+0x152/0x360
 ? lock_is_held_type+0xa5/0x120
 ? find_held_lock.constprop.0+0x2b/0x80
 block_ioctl+0x3f/0x50
 __x64_sys_ioctl+0x83/0xb0
 do_syscall_64+0x3b/0x90
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f645884de6b

Usually the uuid_mutex exists to protect the fs_devices that map
together all of the devices that match a specific uuid.  In rm_device
we're messing with the uuid of a device, so it makes sense to protect
that here.

However in doing that it pulls in a whole host of lockdep dependencies,
as we call mnt_may_write() on the sb before we grab the uuid_mutex, thus
we end up with the dependency chain under the uuid_mutex being added
under the normal sb write dependency chain, which causes problems with
loop devices.

We don't need the uuid mutex here however.  If we call
btrfs_scan_one_device() before we scratch the super block we will find
the fs_devices and not find the device itself and return EBUSY because
the fs_devices is open.  If we call it after the scratch happens it will
not appear to be a valid btrfs file system.

We do not need to worry about other fs_devices modifying operations here
because we're protected by the exclusive operations locking.

So drop the uuid_mutex here in order to fix the lockdep splat.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/volumes.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 5217b93172b4..0e7372f637eb 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c

@@ -2082,8 +2082,6 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
        u64 num_devices;
        int ret = 0;

-       mutex_lock(&uuid_mutex);
-
        num_devices = btrfs_num_devices(fs_info);

        ret = btrfs_check_raid_min_devices(fs_info, num_devices - 1);

@@ -2127,11 +2125,9 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
                mutex_unlock(&fs_info->chunk_mutex);
        }

-       mutex_unlock(&uuid_mutex);
        ret = btrfs_shrink_device(device, 0);
        if (!ret)
                btrfs_reada_remove_dev(device);
-       mutex_lock(&uuid_mutex);

        if (ret)
                goto error_undo;

@@ -2215,7 +2211,6 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
        }

 out:
-       mutex_unlock(&uuid_mutex);
        return ret;

 error_undo:
--

2.26.3