Re: [PATCH v2 2/7] btrfs: do not take the uuid_mutex in btrfs_rm_device

[Anand Jain <anand.jain@oracle.com>]

On 28/07/2021 05:01, Josef Bacik wrote:
> 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.


I think uuid_mutex should stay. Here is why.

  While thread A takes %device at line 816 and deref at line 880.
  Thread B can completely remove and free that %device.
  As of now these threads are mutual exclusive using uuid_mutex.

Thread A

btrfs_control_ioctl()
   mutex_lock(&uuid_mutex);
     btrfs_scan_one_device()
       device_list_add()
       {
  815                 mutex_lock(&fs_devices->device_list_mutex);

  816                 device = btrfs_find_device(fs_devices, devid,
  817                                 disk_super->dev_item.uuid, NULL);

  880         } else if (!device->name || strcmp(device->name->str, path)) {

  933                         if (device->bdev->bd_dev != path_dev) {

  982         mutex_unlock(&fs_devices->device_list_mutex);
        }


Thread B

btrfs_rm_device()

2069         mutex_lock(&uuid_mutex);  <-- proposed to remove

2150         mutex_lock(&fs_devices->device_list_mutex);

2172         mutex_unlock(&fs_devices->device_list_mutex);

2180                 btrfs_scratch_superblocks(fs_info, device->bdev,
2181                                           device->name->str);

2183         btrfs_close_bdev(device);
2184         synchronize_rcu();
2185         btrfs_free_device(device);

2194         mutex_unlock(&uuid_mutex);  <-- proposed to remove


Well, I don't have a better option to fix this issue as of now.


> 
> 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:
>