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linux-btrfs@vger.kernel.org On 28/07/2021 05:01, Josef Bacik wrote: > I got the following lockdep splat while testing seed devices > > ====================================================== > WARNING: possible circular locking dependency detected > 5.14.0-rc2+ #409 Not tainted > ------------------------------------------------------ > mount/34004 is trying to acquire lock: > ffff9eaac48188e0 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: clone_fs_devices+0x4d/0x170 > > but task is already holding lock: > ffff9eaac766d438 (btrfs-chunk-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x24/0x100 > > which lock already depends on the new lock. > > the existing dependency chain (in reverse order) is: > > -> #2 (btrfs-chunk-00){++++}-{3:3}: > down_read_nested+0x46/0x60 > __btrfs_tree_read_lock+0x24/0x100 > btrfs_read_lock_root_node+0x31/0x40 > btrfs_search_slot+0x480/0x930 > btrfs_update_device+0x63/0x180 > btrfs_chunk_alloc_add_chunk_item+0xdc/0x3a0 > btrfs_chunk_alloc+0x281/0x540 > find_free_extent+0x10ca/0x1790 > btrfs_reserve_extent+0xbf/0x1d0 > btrfs_alloc_tree_block+0xb1/0x320 > __btrfs_cow_block+0x136/0x5f0 > btrfs_cow_block+0x107/0x210 > btrfs_search_slot+0x56a/0x930 > btrfs_truncate_inode_items+0x187/0xef0 > btrfs_truncate_free_space_cache+0x11c/0x210 > delete_block_group_cache+0x6f/0xb0 > btrfs_relocate_block_group+0xf8/0x350 > btrfs_relocate_chunk+0x38/0x120 > btrfs_balance+0x79b/0xf00 > btrfs_ioctl_balance+0x327/0x400 > __x64_sys_ioctl+0x80/0xb0 > do_syscall_64+0x38/0x90 > entry_SYSCALL_64_after_hwframe+0x44/0xae > > -> #1 (&fs_info->chunk_mutex){+.+.}-{3:3}: > __mutex_lock+0x7d/0x750 > btrfs_init_new_device+0x6d6/0x1540 > btrfs_ioctl+0x1b12/0x2d30 > __x64_sys_ioctl+0x80/0xb0 > do_syscall_64+0x38/0x90 > entry_SYSCALL_64_after_hwframe+0x44/0xae > > -> #0 (&fs_devs->device_list_mutex){+.+.}-{3:3}: > __lock_acquire+0x10ea/0x1d90 > lock_acquire+0xb5/0x2b0 > __mutex_lock+0x7d/0x750 > clone_fs_devices+0x4d/0x170 > btrfs_read_chunk_tree+0x32f/0x800 > open_ctree+0xae3/0x16f0 > btrfs_mount_root.cold+0x12/0xea > legacy_get_tree+0x2d/0x50 > vfs_get_tree+0x25/0xc0 > vfs_kern_mount.part.0+0x71/0xb0 > btrfs_mount+0x10d/0x380 > legacy_get_tree+0x2d/0x50 > vfs_get_tree+0x25/0xc0 > path_mount+0x433/0xb60 > __x64_sys_mount+0xe3/0x120 > do_syscall_64+0x38/0x90 > entry_SYSCALL_64_after_hwframe+0x44/0xae > > other info that might help us debug this: > > Chain exists of: > &fs_devs->device_list_mutex --> &fs_info->chunk_mutex --> btrfs-chunk-00 > > Possible unsafe locking scenario: > > CPU0 CPU1 > ---- ---- > lock(btrfs-chunk-00); > lock(&fs_info->chunk_mutex); > lock(btrfs-chunk-00); > lock(&fs_devs->device_list_mutex); > > *** DEADLOCK *** > > 3 locks held by mount/34004: > #0: ffff9eaad75c00e0 (&type->s_umount_key#47/1){+.+.}-{3:3}, at: alloc_super+0xd5/0x3b0 > #1: ffffffffbd2dcf08 (uuid_mutex){+.+.}-{3:3}, at: btrfs_read_chunk_tree+0x59/0x800 > #2: ffff9eaac766d438 (btrfs-chunk-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x24/0x100 > > stack backtrace: > CPU: 0 PID: 34004 Comm: mount Not tainted 5.14.0-rc2+ #409 > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014 > Call Trace: > dump_stack_lvl+0x57/0x72 > check_noncircular+0xcf/0xf0 > __lock_acquire+0x10ea/0x1d90 > lock_acquire+0xb5/0x2b0 > ? clone_fs_devices+0x4d/0x170 > ? lock_is_held_type+0xa5/0x120 > __mutex_lock+0x7d/0x750 > ? clone_fs_devices+0x4d/0x170 > ? clone_fs_devices+0x4d/0x170 > ? lockdep_init_map_type+0x47/0x220 > ? debug_mutex_init+0x33/0x40 > clone_fs_devices+0x4d/0x170 > ? lock_is_held_type+0xa5/0x120 > btrfs_read_chunk_tree+0x32f/0x800 > ? find_held_lock+0x2b/0x80 > open_ctree+0xae3/0x16f0 > btrfs_mount_root.cold+0x12/0xea > ? rcu_read_lock_sched_held+0x3f/0x80 > ? kfree+0x1f6/0x410 > legacy_get_tree+0x2d/0x50 > vfs_get_tree+0x25/0xc0 > vfs_kern_mount.part.0+0x71/0xb0 > btrfs_mount+0x10d/0x380 > ? kfree+0x1f6/0x410 > legacy_get_tree+0x2d/0x50 > vfs_get_tree+0x25/0xc0 > path_mount+0x433/0xb60 > __x64_sys_mount+0xe3/0x120 > do_syscall_64+0x38/0x90 > entry_SYSCALL_64_after_hwframe+0x44/0xae > RIP: 0033:0x7f6cbcd9788e > > It is because we take the ->device_list_mutex in this path while holding > onto the tree locks in the chunk root. However we do not need the lock > here, because we're already holding onto the uuid_mutex, and in fact > have removed all other uses of the ->device_list_mutex in this path > because of this. Remove the ->device_list_mutex locking here, add an > assert for the uuid_mutex and the problem is fixed. > > Signed-off-by: Josef Bacik > --- > fs/btrfs/volumes.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c > index f622e93a6ff1..bdfcc35335c3 100644 > --- a/fs/btrfs/volumes.c > +++ b/fs/btrfs/volumes.c > @@ -1000,11 +1000,12 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig) > struct btrfs_device *orig_dev; > int ret = 0; > > + lockdep_assert_held(&uuid_mutex); > + > fs_devices = alloc_fs_devices(orig->fsid, NULL); > if (IS_ERR(fs_devices)) > return fs_devices; > > - mutex_lock(&orig->device_list_mutex); > fs_devices->total_devices = orig->total_devices; > > list_for_each_entry(orig_dev, &orig->devices, dev_list) { > @@ -1036,10 +1037,8 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig) > device->fs_devices = fs_devices; > fs_devices->num_devices++; > } > - mutex_unlock(&orig->device_list_mutex); > return fs_devices; > error: > - mutex_unlock(&orig->device_list_mutex); > free_fs_devices(fs_devices); > return ERR_PTR(ret); > } > This fix is same as in [1] [1] https://patchwork.kernel.org/project/linux-btrfs/patch/23a8830f3be500995e74b45f18862e67c0634c3d.1614793362.git.anand.jain@oracle.com/