btrfs: fix BUG_ON condition in btrfs_cancel_balance

Pausing and canceling balance can race to interrupt balance lead to BUG_ON
panic in btrfs_cancel_balance. The BUG_ON condition in btrfs_cancel_balance
does not take this race scenario into account.

However, the race condition has no other side effects. We can fix that.

Reproducing it with panic trace like this:

  kernel BUG at fs/btrfs/volumes.c:4618!
  RIP: 0010:btrfs_cancel_balance+0x5cf/0x6a0
  Call Trace:
   <TASK>
   ? do_nanosleep+0x60/0x120
   ? hrtimer_nanosleep+0xb7/0x1a0
   ? sched_core_clone_cookie+0x70/0x70
   btrfs_ioctl_balance_ctl+0x55/0x70
   btrfs_ioctl+0xa46/0xd20
   __x64_sys_ioctl+0x7d/0xa0
   do_syscall_64+0x38/0x80
   entry_SYSCALL_64_after_hwframe+0x63/0xcd

  Race scenario as follows:
  > mutex_unlock(&fs_info->balance_mutex);
  > --------------------
  > .......issue pause and cancel req in another thread
  > --------------------
  > ret = __btrfs_balance(fs_info);
  >
  > mutex_lock(&fs_info->balance_mutex);
  > if (ret == -ECANCELED && atomic_read(&fs_info->balance_pause_req)) {
  >         btrfs_info(fs_info, "balance: paused");
  >         btrfs_exclop_balance(fs_info, BTRFS_EXCLOP_BALANCE_PAUSED);
  > }

CC: stable@vger.kernel.org # 4.19+
Signed-off-by: xiaoshoukui <xiaoshoukui@ruijie.com.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index efa19a528c33eba230088d465a66870e16810894..a3085c7daaf798b43e81d286053c4263c87d7419 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4641,8 +4641,7 @@ int btrfs_cancel_balance(struct btrfs_fs_info *fs_info)
                }
        }
 
-       BUG_ON(fs_info->balance_ctl ||
-               test_bit(BTRFS_FS_BALANCE_RUNNING, &fs_info->flags));
+       ASSERT(!test_bit(BTRFS_FS_BALANCE_RUNNING, &fs_info->flags));
        atomic_dec(&fs_info->balance_cancel_req);
        mutex_unlock(&fs_info->balance_mutex);
        return 0;