Re: [PATCH] btrfs: qgroup: Don't hold qgroup_ioctl_lock in btrfs_qgroup_inherit()

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2019/6/12 下午9:53, David Sterba wrote:
> On Wed, Jun 12, 2019 at 03:57:45PM +0800, Qu Wenruo wrote:
>> [BUG]
>> Lockdep will report the following circular locking dependency:
>>
>>   WARNING: possible circular locking dependency detected
>>   5.2.0-rc2-custom #24 Tainted: G           O
>>   ------------------------------------------------------
>>   btrfs/8631 is trying to acquire lock:
>>   000000002536438c (&fs_info->qgroup_ioctl_lock#2){+.+.}, at: btrfs_qgroup_inherit+0x40/0x620 [btrfs]
>>
>>   but task is already holding lock:
>>   000000003d52cc23 (&fs_info->tree_log_mutex){+.+.}, at: create_pending_snapshot+0x8b6/0xe60 [btrfs]
>>
>>   which lock already depends on the new lock.
>>
>>   the existing dependency chain (in reverse order) is:
>>
>>   -> #2 (&fs_info->tree_log_mutex){+.+.}:
>>          __mutex_lock+0x76/0x940
>>          mutex_lock_nested+0x1b/0x20
>>          btrfs_commit_transaction+0x475/0xa00 [btrfs]
>>          btrfs_commit_super+0x71/0x80 [btrfs]
>>          close_ctree+0x2bd/0x320 [btrfs]
>>          btrfs_put_super+0x15/0x20 [btrfs]
>>          generic_shutdown_super+0x72/0x110
>>          kill_anon_super+0x18/0x30
>>          btrfs_kill_super+0x16/0xa0 [btrfs]
>>          deactivate_locked_super+0x3a/0x80
>>          deactivate_super+0x51/0x60
>>          cleanup_mnt+0x3f/0x80
>>          __cleanup_mnt+0x12/0x20
>>          task_work_run+0x94/0xb0
>>          exit_to_usermode_loop+0xd8/0xe0
>>          do_syscall_64+0x210/0x240
>>          entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>>   -> #1 (&fs_info->reloc_mutex){+.+.}:
>>          __mutex_lock+0x76/0x940
>>          mutex_lock_nested+0x1b/0x20
>>          btrfs_commit_transaction+0x40d/0xa00 [btrfs]
>>          btrfs_quota_enable+0x2da/0x730 [btrfs]
>>          btrfs_ioctl+0x2691/0x2b40 [btrfs]
>>          do_vfs_ioctl+0xa9/0x6d0
>>          ksys_ioctl+0x67/0x90
>>          __x64_sys_ioctl+0x1a/0x20
>>          do_syscall_64+0x65/0x240
>>          entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>>   -> #0 (&fs_info->qgroup_ioctl_lock#2){+.+.}:
>>          lock_acquire+0xa7/0x190
>>          __mutex_lock+0x76/0x940
>>          mutex_lock_nested+0x1b/0x20
>>          btrfs_qgroup_inherit+0x40/0x620 [btrfs]
>>          create_pending_snapshot+0x9d7/0xe60 [btrfs]
>>          create_pending_snapshots+0x94/0xb0 [btrfs]
>>          btrfs_commit_transaction+0x415/0xa00 [btrfs]
>>          btrfs_mksubvol+0x496/0x4e0 [btrfs]
>>          btrfs_ioctl_snap_create_transid+0x174/0x180 [btrfs]
>>          btrfs_ioctl_snap_create_v2+0x11c/0x180 [btrfs]
>>          btrfs_ioctl+0xa90/0x2b40 [btrfs]
>>          do_vfs_ioctl+0xa9/0x6d0
>>          ksys_ioctl+0x67/0x90
>>          __x64_sys_ioctl+0x1a/0x20
>>          do_syscall_64+0x65/0x240
>>          entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>>   other info that might help us debug this:
>>
>>   Chain exists of:
>>     &fs_info->qgroup_ioctl_lock#2 --> &fs_info->reloc_mutex --> &fs_info->tree_log_mutex
>>
>>    Possible unsafe locking scenario:
>>
>>          CPU0                    CPU1
>>          ----                    ----
>>     lock(&fs_info->tree_log_mutex);
>>                                  lock(&fs_info->reloc_mutex);
>>                                  lock(&fs_info->tree_log_mutex);
>>     lock(&fs_info->qgroup_ioctl_lock#2);
>>
>>    *** DEADLOCK ***
>>
>>   6 locks held by btrfs/8631:
>>    #0: 00000000ed8f23f6 (sb_writers#12){.+.+}, at: mnt_want_write_file+0x28/0x60
>>    #1: 000000009fb1597a (&type->i_mutex_dir_key#10/1){+.+.}, at: btrfs_mksubvol+0x70/0x4e0 [btrfs]
>>    #2: 0000000088c5ad88 (&fs_info->subvol_sem){++++}, at: btrfs_mksubvol+0x128/0x4e0 [btrfs]
>>    #3: 000000009606fc3e (sb_internal#2){.+.+}, at: start_transaction+0x37a/0x520 [btrfs]
>>    #4: 00000000f82bbdf5 (&fs_info->reloc_mutex){+.+.}, at: btrfs_commit_transaction+0x40d/0xa00 [btrfs]
>>    #5: 000000003d52cc23 (&fs_info->tree_log_mutex){+.+.}, at: create_pending_snapshot+0x8b6/0xe60 [btrfs]
>>
>> [CAUSE]
>> Due to the delayed subvolume creation, we need to call
>> btrfs_qgroup_inherit() inside commit transaction code, with a lot of
>> other mutex hold.
>> This hell of lock chain can lead to above problem.
>>
>> [FIX]
>> On the other hand, we don't really need to hold qgroup_ioctl_lock if
>> we're in the context of create_pending_snapshot().
>> As in that context, we're the only one being able to modify qgroup.
>>
>> All other qgroup functions which needs qgroup_ioctl_lock are either
>> holding a transaction handle, or will start a new transaction:
>>   Functions will start a new transaction():
>>   * btrfs_quota_enable()
>>   * btrfs_quota_disable()
>>   Functions hold a transaction handler:
>>   * btrfs_add_qgroup_relation()
>>   * btrfs_del_qgroup_relation()
>>   * btrfs_create_qgroup()
>>   * btrfs_remove_qgroup()
>>   * btrfs_limit_qgroup()
>>   * btrfs_qgroup_inherit() call inside create_subvol()
>>
>> So we have a higher level protection provided by transaction, thus we
>> don't need to always hold qgroup_ioctl_lock in btrfs_qgroup_inherit().
>>
>> Only the btrfs_qgroup_inherit() call in create_subvol() needs to hold
>> qgroup_ioctl_lock, while the btrfs_qgroup_inherit() call in
>> create_pending_snapshot() is already protected by transaction.
>>
>> So the fix is to manually hold qgroup_ioctl_lock inside create_subvol()
>> while skip the lock inside create_pending_snapshot.
>
> Would it be possible to add that as a run-time assertion? Eg. check the
> state of the transaction if it's inside commit, and if not then check
> the locks?
>

Oh, that's a much better solution!

Thank you very much for the hint,
Qu