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    Btrfs: fix race between cleaner kthread and space cache writeout · 036a9348
    Filipe Manana 提交于
    When a block group becomes unused and the cleaner kthread is currently
    running, we can end up getting the current transaction aborted with error
    -ENOENT when we try to commit the transaction, leading to the following
    trace:
    
      [59779.258768] WARNING: CPU: 3 PID: 5990 at fs/btrfs/extent-tree.c:3740 btrfs_write_dirty_block_groups+0x17c/0x214 [btrfs]()
      [59779.272594] BTRFS: Transaction aborted (error -2)
      (...)
      [59779.291137] Call Trace:
      [59779.291621]  [<ffffffff812566f4>] dump_stack+0x4e/0x79
      [59779.292543]  [<ffffffff8104d0a6>] warn_slowpath_common+0x9f/0xb8
      [59779.293435]  [<ffffffffa04cb81f>] ? btrfs_write_dirty_block_groups+0x17c/0x214 [btrfs]
      [59779.295000]  [<ffffffff8104d107>] warn_slowpath_fmt+0x48/0x50
      [59779.296138]  [<ffffffffa04c2721>] ? write_one_cache_group.isra.32+0x77/0x82 [btrfs]
      [59779.297663]  [<ffffffffa04cb81f>] btrfs_write_dirty_block_groups+0x17c/0x214 [btrfs]
      [59779.299141]  [<ffffffffa0549b0d>] commit_cowonly_roots+0x1de/0x261 [btrfs]
      [59779.300359]  [<ffffffffa04dd5b6>] btrfs_commit_transaction+0x4c4/0x99c [btrfs]
      [59779.301805]  [<ffffffffa04b5df4>] btrfs_sync_fs+0x145/0x1ad [btrfs]
      [59779.302893]  [<ffffffff81196634>] sync_filesystem+0x7f/0x93
      (...)
      [59779.318186] ---[ end trace 577e2daff90da33a ]---
    
    The following diagram illustrates a sequence of steps leading to this
    problem:
    
           CPU 1                                             CPU 2
    
                               <at transaction N>
    
                                                            adds bg A to list
                                                            fs_info->unused_bgs
    
                                                            adds bg B to list
                                                            fs_info->unused_bgs
    
                               <transaction kthread
                                commits transaction N
                                and wakes up the
                                cleaner kthread>
    
      cleaner kthread
        delete_unused_bgs()
    
          sees bg A in list
          fs_info->unused_bgs
    
          btrfs_start_transaction()
    
                               <transaction N + 1 starts>
    
          deletes bg A
    
                                                            update_block_group(bg C)
    
                                                              --> adds bg C to list
                                                                  fs_info->unused_bgs
    
          deletes bg B
    
          sees bg C in the list
          fs_info->unused_bgs
    
          btrfs_remove_chunk(bg C)
            btrfs_remove_block_group(bg C)
    
              --> checks if the block group
                  is in a dirty list, and
                  because it isn't now, it
                  does nothing
    
              --> the block group item
                  is deleted from the
                  extent tree
    
                                                              --> adds bg C to list
                                                                  transaction->dirty_bgs
    
                                                             some task calls
                                                             btrfs_commit_transaction(t N + 1)
                                                               commit_cowonly_roots()
                                                                 btrfs_write_dirty_block_groups()
                                                                   --> sees bg C in cur_trans->dirty_bgs
                                                                   --> calls write_one_cache_group()
                                                                       which returns -ENOENT because
                                                                       it did not find the block group
                                                                       item in the extent tree
                                                                   --> transaction aborte with -ENOENT
                                                                       because write_one_cache_group()
                                                                       returned that error
    
    So fix this by adding a block group to the list of dirty block groups
    before adding it to the list of unused block groups.
    
    This happened on a stress test using fsstress plus concurrent calls to
    fallocate 20G and truncate (releasing part of the space allocated with
    fallocate).
    Signed-off-by: NFilipe Manana <fdmanana@suse.com>
    Signed-off-by: NChris Mason <clm@fb.com>
    036a9348
extent-tree.c 288.4 KB