• Filipe Manana's avatar
    Btrfs: fix race between transaction commit and empty block group removal · d4b450cd
    Filipe Manana authored
    Committing a transaction can race with automatic removal of empty block
    groups (cleaner kthread), leading to a BUG_ON() in the transaction
    commit code while running btrfs_finish_extent_commit(). The following
    sequence diagram shows how it can happen:
    
               CPU 1                                       CPU 2
    
    btrfs_commit_transaction()
      fs_info->running_transaction = NULL
      btrfs_finish_extent_commit()
        find_first_extent_bit()
          -> found range for block group X
             in fs_info->freed_extents[]
    
                                                   btrfs_delete_unused_bgs()
                                                     -> found block group X
    
                                                     Removed block group X's range
                                                     from fs_info->freed_extents[]
    
                                                     btrfs_remove_chunk()
                                                        btrfs_remove_block_group(bg X)
    
        unpin_extent_range(bg X range)
           btrfs_lookup_block_group(bg X)
              -> returns NULL
                -> BUG_ON()
    
    The trace that results from the BUG_ON() is:
    
    [48665.187808] ------------[ cut here ]------------
    [48665.188032] kernel BUG at fs/btrfs/extent-tree.c:5675!
    [48665.188032] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC
    [48665.188032] Modules linked in: dm_flakey dm_mod crc32c_generic btrfs xor raid6_pq nfsd auth_rpcgss oid_registry nfs_acl nfs lockd grace fscache sunrpc loop parport_pc evdev microcode
    [48665.197388] CPU: 2 PID: 31211 Comm: kworker/u32:16 Tainted: G        W      3.19.0-rc5-btrfs-next-4+ #1
    [48665.197388] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
    [48665.197388] Workqueue: events_unbound btrfs_async_reclaim_metadata_space [btrfs]
    [48665.197388] task: ffff880222011810 ti: ffff8801b56a4000 task.ti: ffff8801b56a4000
    [48665.197388] RIP: 0010:[<ffffffffa0350d05>]  [<ffffffffa0350d05>] unpin_extent_range+0x6a/0x1ba [btrfs]
    [48665.197388] RSP: 0018:ffff8801b56a7b88  EFLAGS: 00010246
    [48665.197388] RAX: 0000000000000000 RBX: ffff8802143a6000 RCX: ffff8802220120c8
    [48665.197388] RDX: 0000000000000001 RSI: 0000000000000001 RDI: ffff8800a3c140b0
    [48665.197388] RBP: ffff8801b56a7bd8 R08: 0000000000000003 R09: 0000000000000000
    [48665.197388] R10: 0000000000000000 R11: 000000000000bbac R12: 0000000012e8e000
    [48665.197388] R13: ffff8800a3c14000 R14: 0000000000000000 R15: 0000000000000000
    [48665.197388] FS:  0000000000000000(0000) GS:ffff88023ec40000(0000) knlGS:0000000000000000
    [48665.197388] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
    [48665.197388] CR2: 00007f065e42f270 CR3: 0000000206f70000 CR4: 00000000000006e0
    [48665.197388] Stack:
    [48665.197388]  ffff8801b56a7bd8 0000000012ea0000 01ff8800a3c14138 0000000012e9ffff
    [48665.197388]  ffff880141df3dd8 ffff8802143a6000 ffff8800a3c14138 ffff880141df3df0
    [48665.197388]  ffff880141df3dd8 0000000000000000 ffff8801b56a7c08 ffffffffa0354227
    [48665.197388] Call Trace:
    [48665.197388]  [<ffffffffa0354227>] btrfs_finish_extent_commit+0xb0/0xd9 [btrfs]
    [48665.197388]  [<ffffffffa0366b4b>] btrfs_commit_transaction+0x791/0x92c [btrfs]
    [48665.197388]  [<ffffffffa0352432>] flush_space+0x43d/0x452 [btrfs]
    [48665.197388]  [<ffffffff814295c3>] ? _raw_spin_unlock+0x28/0x33
    [48665.197388]  [<ffffffffa035255f>] btrfs_async_reclaim_metadata_space+0x118/0x164 [btrfs]
    [48665.197388]  [<ffffffff81059917>] ? process_one_work+0x14b/0x3ab
    [48665.197388]  [<ffffffff810599ac>] process_one_work+0x1e0/0x3ab
    [48665.197388]  [<ffffffff81079fa9>] ? trace_hardirqs_off+0xd/0xf
    [48665.197388]  [<ffffffff8105a55b>] worker_thread+0x210/0x2d0
    [48665.197388]  [<ffffffff8105a34b>] ? rescuer_thread+0x2c3/0x2c3
    [48665.197388]  [<ffffffff8105e5c0>] kthread+0xef/0xf7
    [48665.197388]  [<ffffffff81429682>] ? _raw_spin_unlock_irq+0x2d/0x39
    [48665.197388]  [<ffffffff8105e4d1>] ? __kthread_parkme+0xad/0xad
    [48665.197388]  [<ffffffff81429dec>] ret_from_fork+0x7c/0xb0
    [48665.197388]  [<ffffffff8105e4d1>] ? __kthread_parkme+0xad/0xad
    [48665.197388] Code: 85 f6 74 14 49 8b 06 49 03 46 09 49 39 c4 72 1d 4c 89 f7 e8 83 ec ff ff 4c 89 e6 4c 89 ef e8 1e f1 ff ff 48 85 c0 49 89 c6 75 02 <0f> 0b 49 8b 1e 49 03 5e 09 48 8b
    [48665.197388] RIP  [<ffffffffa0350d05>] unpin_extent_range+0x6a/0x1ba [btrfs]
    [48665.197388]  RSP <ffff8801b56a7b88>
    [48665.272246] ---[ end trace b9c6ab9957521376 ]---
    
    Fix this by ensuring that unpining the block group's range in
    btrfs_finish_extent_commit() is done in a synchronized fashion
    with removing the block group's range from freed_extents[]
    in btrfs_delete_unused_bgs()
    
    This race got introduced with the change:
    
        Btrfs: remove empty block groups automatically
        commit 47ab2a6cSigned-off-by: default avatarFilipe Manana <fdmanana@suse.com>
    Signed-off-by: default avatarChris Mason <clm@fb.com>
    d4b450cd
disk-io.c 117 KB