• Filipe Manana's avatar
    Btrfs: fix deadlock between fiemap and transaction commits · a6d155d2
    Filipe Manana authored
    The fiemap handler locks a file range that can have unflushed delalloc,
    and after locking the range, it tries to attach to a running transaction.
    If the running transaction started its commit, that is, it is in state
    TRANS_STATE_COMMIT_START, and either the filesystem was mounted with the
    flushoncommit option or the transaction is creating a snapshot for the
    subvolume that contains the file that fiemap is operating on, we end up
    deadlocking. This happens because fiemap is blocked on the transaction,
    waiting for it to complete, and the transaction is waiting for the flushed
    dealloc to complete, which requires locking the file range that the fiemap
    task already locked. The following stack traces serve as an example of
    when this deadlock happens:
    
      (...)
      [404571.515510] Workqueue: btrfs-endio-write btrfs_endio_write_helper [btrfs]
      [404571.515956] Call Trace:
      [404571.516360]  ? __schedule+0x3ae/0x7b0
      [404571.516730]  schedule+0x3a/0xb0
      [404571.517104]  lock_extent_bits+0x1ec/0x2a0 [btrfs]
      [404571.517465]  ? remove_wait_queue+0x60/0x60
      [404571.517832]  btrfs_finish_ordered_io+0x292/0x800 [btrfs]
      [404571.518202]  normal_work_helper+0xea/0x530 [btrfs]
      [404571.518566]  process_one_work+0x21e/0x5c0
      [404571.518990]  worker_thread+0x4f/0x3b0
      [404571.519413]  ? process_one_work+0x5c0/0x5c0
      [404571.519829]  kthread+0x103/0x140
      [404571.520191]  ? kthread_create_worker_on_cpu+0x70/0x70
      [404571.520565]  ret_from_fork+0x3a/0x50
      [404571.520915] kworker/u8:6    D    0 31651      2 0x80004000
      [404571.521290] Workqueue: btrfs-flush_delalloc btrfs_flush_delalloc_helper [btrfs]
      (...)
      [404571.537000] fsstress        D    0 13117  13115 0x00004000
      [404571.537263] Call Trace:
      [404571.537524]  ? __schedule+0x3ae/0x7b0
      [404571.537788]  schedule+0x3a/0xb0
      [404571.538066]  wait_current_trans+0xc8/0x100 [btrfs]
      [404571.538349]  ? remove_wait_queue+0x60/0x60
      [404571.538680]  start_transaction+0x33c/0x500 [btrfs]
      [404571.539076]  btrfs_check_shared+0xa3/0x1f0 [btrfs]
      [404571.539513]  ? extent_fiemap+0x2ce/0x650 [btrfs]
      [404571.539866]  extent_fiemap+0x2ce/0x650 [btrfs]
      [404571.540170]  do_vfs_ioctl+0x526/0x6f0
      [404571.540436]  ksys_ioctl+0x70/0x80
      [404571.540734]  __x64_sys_ioctl+0x16/0x20
      [404571.540997]  do_syscall_64+0x60/0x1d0
      [404571.541279]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
      (...)
      [404571.543729] btrfs           D    0 14210  14208 0x00004000
      [404571.544023] Call Trace:
      [404571.544275]  ? __schedule+0x3ae/0x7b0
      [404571.544526]  ? wait_for_completion+0x112/0x1a0
      [404571.544795]  schedule+0x3a/0xb0
      [404571.545064]  schedule_timeout+0x1ff/0x390
      [404571.545351]  ? lock_acquire+0xa6/0x190
      [404571.545638]  ? wait_for_completion+0x49/0x1a0
      [404571.545890]  ? wait_for_completion+0x112/0x1a0
      [404571.546228]  wait_for_completion+0x131/0x1a0
      [404571.546503]  ? wake_up_q+0x70/0x70
      [404571.546775]  btrfs_wait_ordered_extents+0x27c/0x400 [btrfs]
      [404571.547159]  btrfs_commit_transaction+0x3b0/0xae0 [btrfs]
      [404571.547449]  ? btrfs_mksubvol+0x4a4/0x640 [btrfs]
      [404571.547703]  ? remove_wait_queue+0x60/0x60
      [404571.547969]  btrfs_mksubvol+0x605/0x640 [btrfs]
      [404571.548226]  ? __sb_start_write+0xd4/0x1c0
      [404571.548512]  ? mnt_want_write_file+0x24/0x50
      [404571.548789]  btrfs_ioctl_snap_create_transid+0x169/0x1a0 [btrfs]
      [404571.549048]  btrfs_ioctl_snap_create_v2+0x11d/0x170 [btrfs]
      [404571.549307]  btrfs_ioctl+0x133f/0x3150 [btrfs]
      [404571.549549]  ? mem_cgroup_charge_statistics+0x4c/0xd0
      [404571.549792]  ? mem_cgroup_commit_charge+0x84/0x4b0
      [404571.550064]  ? __handle_mm_fault+0xe3e/0x11f0
      [404571.550306]  ? do_raw_spin_unlock+0x49/0xc0
      [404571.550608]  ? _raw_spin_unlock+0x24/0x30
      [404571.550976]  ? __handle_mm_fault+0xedf/0x11f0
      [404571.551319]  ? do_vfs_ioctl+0xa2/0x6f0
      [404571.551659]  ? btrfs_ioctl_get_supported_features+0x30/0x30 [btrfs]
      [404571.552087]  do_vfs_ioctl+0xa2/0x6f0
      [404571.552355]  ksys_ioctl+0x70/0x80
      [404571.552621]  __x64_sys_ioctl+0x16/0x20
      [404571.552864]  do_syscall_64+0x60/0x1d0
      [404571.553104]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
      (...)
    
    If we were joining the transaction instead of attaching to it, we would
    not risk a deadlock because a join only blocks if the transaction is in a
    state greater then or equals to TRANS_STATE_COMMIT_DOING, and the delalloc
    flush performed by a transaction is done before it reaches that state,
    when it is in the state TRANS_STATE_COMMIT_START. However a transaction
    join is intended for use cases where we do modify the filesystem, and
    fiemap only needs to peek at delayed references from the current
    transaction in order to determine if extents are shared, and, besides
    that, when there is no current transaction or when it blocks to wait for
    a current committing transaction to complete, it creates a new transaction
    without reserving any space. Such unnecessary transactions, besides doing
    unnecessary IO, can cause transaction aborts (-ENOSPC) and unnecessary
    rotation of the precious backup roots.
    
    So fix this by adding a new transaction join variant, named join_nostart,
    which behaves like the regular join, but it does not create a transaction
    when none currently exists or after waiting for a committing transaction
    to complete.
    
    Fixes: 03628cdb ("Btrfs: do not start a transaction during fiemap")
    Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
    a6d155d2
transaction.h 7.37 KB