btrfs: fix sleep while in non-sleep context during qgroup removal

While removing a qgroup's sysfs entry we end up taking the kernfs_mutex,
through kobject_del(), while holding the fs_info->qgroup_lock spinlock,
producing the following trace:

  [821.843637] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:281
  [821.843641] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 28214, name: podman
  [821.843644] CPU: 3 PID: 28214 Comm: podman Tainted: G        W         5.11.6 #15
  [821.843646] Hardware name: Dell Inc. PowerEdge R330/084XW4, BIOS 2.11.0 12/08/2020
  [821.843647] Call Trace:
  [821.843650]  dump_stack+0xa1/0xfb
  [821.843656]  ___might_sleep+0x144/0x160
  [821.843659]  mutex_lock+0x17/0x40
  [821.843662]  kernfs_remove_by_name_ns+0x1f/0x80
  [821.843666]  sysfs_remove_group+0x7d/0xe0
  [821.843668]  sysfs_remove_groups+0x28/0x40
  [821.843670]  kobject_del+0x2a/0x80
  [821.843672]  btrfs_sysfs_del_one_qgroup+0x2b/0x40 [btrfs]
  [821.843685]  __del_qgroup_rb+0x12/0x150 [btrfs]
  [821.843696]  btrfs_remove_qgroup+0x288/0x2a0 [btrfs]
  [821.843707]  btrfs_ioctl+0x3129/0x36a0 [btrfs]
  [821.843717]  ? __mod_lruvec_page_state+0x5e/0xb0
  [821.843719]  ? page_add_new_anon_rmap+0xbc/0x150
  [821.843723]  ? kfree+0x1b4/0x300
  [821.843725]  ? mntput_no_expire+0x55/0x330
  [821.843728]  __x64_sys_ioctl+0x5a/0xa0
  [821.843731]  do_syscall_64+0x33/0x70
  [821.843733]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
  [821.843736] RIP: 0033:0x4cd3fb
  [821.843741] RSP: 002b:000000c000906b20 EFLAGS: 00000206 ORIG_RAX: 0000000000000010
  [821.843744] RAX: ffffffffffffffda RBX: 000000c000050000 RCX: 00000000004cd3fb
  [821.843745] RDX: 000000c000906b98 RSI: 000000004010942a RDI: 000000000000000f
  [821.843747] RBP: 000000c000907cd0 R08: 000000c000622901 R09: 0000000000000000
  [821.843748] R10: 000000c000d992c0 R11: 0000000000000206 R12: 000000000000012d
  [821.843749] R13: 000000000000012c R14: 0000000000000200 R15: 0000000000000049

Fix this by removing the qgroup sysfs entry while not holding the spinlock,
since the spinlock is only meant for protection of the qgroup rbtree.

Reported-by: Stuart Shelton <srcshelton@gmail.com>
Link: https://lore.kernel.org/linux-btrfs/7A5485BB-0628-419D-A4D3-27B1AF47E25A@gmail.com/
Fixes: 49e5fb4 ("btrfs: qgroup: export qgroups in sysfs")
CC: stable@vger.kernel.org # 5.10+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/qgroup.c

@@ -226,7 +226,6 @@ static void __del_qgroup_rb(struct btrfs_fs_info *fs_info,
 {
 	struct btrfs_qgroup_list *list;
 
-	btrfs_sysfs_del_one_qgroup(fs_info, qgroup);
 	list_del(&qgroup->dirty);
 	while (!list_empty(&qgroup->groups)) {
 		list = list_first_entry(&qgroup->groups,
@@ -243,7 +242,6 @@ static void __del_qgroup_rb(struct btrfs_fs_info *fs_info,
 		list_del(&list->next_member);
 		kfree(list);
 	}
-	kfree(qgroup);
 }
 
 /* must be called with qgroup_lock held */
@@ -569,6 +567,8 @@ void btrfs_free_qgroup_config(struct btrfs_fs_info *fs_info)
 		qgroup = rb_entry(n, struct btrfs_qgroup, node);
 		rb_erase(n, &fs_info->qgroup_tree);
 		__del_qgroup_rb(fs_info, qgroup);
+		btrfs_sysfs_del_one_qgroup(fs_info, qgroup);
+		kfree(qgroup);
 	}
 	/*
 	 * We call btrfs_free_qgroup_config() when unmounting
@@ -1578,6 +1578,14 @@ int btrfs_remove_qgroup(struct btrfs_trans_handle *trans, u64 qgroupid)
 	spin_lock(&fs_info->qgroup_lock);
 	del_qgroup_rb(fs_info, qgroupid);
 	spin_unlock(&fs_info->qgroup_lock);
+
+	/*
+	 * Remove the qgroup from sysfs now without holding the qgroup_lock
+	 * spinlock, since the sysfs_remove_group() function needs to take
+	 * the mutex kernfs_mutex through kernfs_remove_by_name_ns().
+	 */
+	btrfs_sysfs_del_one_qgroup(fs_info, qgroup);
+	kfree(qgroup);
 out:
 	mutex_unlock(&fs_info->qgroup_ioctl_lock);
 	return ret;