Lockup in BTRFS_IOC_CLONE/Kernel 4.2.0-rc5

["Elias Probst" <mail@eliasprobst.eu>]

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I can reproduce a hard btrfs lockup (process issuing the ioctl() is in
D-state, same goes for btrfs-transacti process) on Kernel 4.2.0-rc5.

I had the same issue on 4.1, so it's unlikely a regression introduced in
4.2.

## With the following steps, I can reproduce the problem:

1. Create a new clean btrfs volume for /var/lib/machines
machinectl set-limit 6G

2. Paste this to /tmp/yum.conf
[main]
reposdir=/dev/null
gpgcheck=0
logfile=/var/log/yum.log
installroot=/var/lib/machines/centos7.1-base
assumeyes=1

[base]
name=CentOS 7.1.1503 - x86_64
baseurl=http://mirror.centos.org/centos/7.1.1503/os/x86_64/
enabled=1

3. Bootstrap a CentOS 7.1 base image
/usr/bin/yum -c /tmp/yum.conf groupinstall Base

4. Start an ephemeral systemd-nspawn container based on 'centos7.1-base'
strace -o /tmp/systemd-nspawn.out -s 500 -f systemd-nspawn -xbD
/var/lib/machines/centos7.1-base/


`systemd-nspawn` will now just hang forever.
I couldn't come up yet with a shorter/more low-level way to reproduce this as I lack quite a bit of btrfs experience.

## Results:

- Last 'strace' lines
6095  fchown(16, 0, 0)                  = 0
6095  fchmod(16, 0755)                  = 0
6095  utimensat(16, NULL, {{1402362275, 0}, {1438761285, 819041906}}, 0) = 0
6095  flistxattr(15, "", 100)           = 0
6095  getdents(15, /* 3 entries */, 32768) = 80
6095  newfstatat(15, "coreutils.mo", {st_mode=S_IFREG|0644, st_size=357263, ...}, AT_SYMLINK_NOFOLLOW) = 0
6095  openat(15, "coreutils.mo", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = 17
6095  openat(16, "coreutils.mo", O_WRONLY|O_CREAT|O_EXCL|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC, 0644) = 18
6095  fstat(18, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
6095  ioctl(18, BTRFS_IOC_CLONE

- call trace in Kernel journal:
Aug 05 10:10:03 moria kernel: INFO: task btrfs-transacti:4175 blocked for more than 120 seconds.
Aug 05 10:10:03 moria kernel:       Tainted: G           O    4.2.0-rc5 #2
Aug 05 10:10:03 moria kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Aug 05 10:10:03 moria kernel: btrfs-transacti D ffff8800b13279f8     0  4175      2 0x00080080
Aug 05 10:10:03 moria kernel:  ffff8800b13279f8 ffff88018fd3a380 ffff8800ab4521c0 0000000000000246
Aug 05 10:10:03 moria kernel:  ffff8800b1328000 ffff88018d5c8518 ffff88018debdba0 ffff880232d64990
Aug 05 10:10:03 moria kernel:  0000000000000197 ffff8800b1327a18 ffffffff86999201 0000000000000000
Aug 05 10:10:03 moria kernel: Call Trace:
Aug 05 10:10:03 moria kernel:  [<ffffffff86999201>] schedule+0x74/0x83
Aug 05 10:10:03 moria kernel:  [<ffffffff863ef8f0>] btrfs_tree_lock+0xa7/0x1b7
Aug 05 10:10:03 moria kernel:  [<ffffffff86137ed7>] ? wait_woken+0x74/0x74
Aug 05 10:10:03 moria kernel:  [<ffffffff8639d30f>] push_leaf_right+0x9a/0x19f
Aug 05 10:10:03 moria kernel:  [<ffffffff8639dd9b>] split_leaf+0x100/0x63f
Aug 05 10:10:03 moria kernel:  [<ffffffff86398f09>] ? leaf_space_used+0xbb/0xea
Aug 05 10:10:03 moria kernel:  [<ffffffff863efa61>] ? btrfs_set_lock_blocking_rw+0x52/0x95
Aug 05 10:10:03 moria kernel:  [<ffffffff8639ea46>] btrfs_search_slot+0x76c/0x8b3
Aug 05 10:10:03 moria kernel:  [<ffffffff863a0107>] btrfs_insert_empty_items+0x58/0xa3
Aug 05 10:10:03 moria kernel:  [<ffffffff8640805a>] btrfs_insert_delayed_items+0x7f/0x3bb
Aug 05 10:10:03 moria kernel:  [<ffffffff8640842e>] __btrfs_run_delayed_items+0x98/0x1c0
Aug 05 10:10:03 moria kernel:  [<ffffffff86408739>] btrfs_run_delayed_items+0xc/0xe
Aug 05 10:10:03 moria kernel:  [<ffffffff863bdc50>] btrfs_commit_transaction+0x298/0xb66
Aug 05 10:10:03 moria kernel:  [<ffffffff863be8d0>] ? start_transaction+0x3b2/0x535
Aug 05 10:10:03 moria kernel:  [<ffffffff863b9cd9>] transaction_kthread+0x100/0x1d6
Aug 05 10:10:03 moria kernel:  [<ffffffff863b9bd9>] ? btrfs_cleanup_transaction+0x49f/0x49f
Aug 05 10:10:03 moria kernel:  [<ffffffff8611eca9>] kthread+0xcd/0xd5
Aug 05 10:10:03 moria kernel:  [<ffffffff8611ebdc>] ? kthread_create_on_node+0x17d/0x17d
Aug 05 10:10:03 moria kernel:  [<ffffffff8699d29f>] ret_from_fork+0x3f/0x70
Aug 05 10:10:03 moria kernel:  [<ffffffff8611ebdc>] ? kthread_create_on_node+0x17d/0x17d
Aug 05 10:10:03 moria kernel: INFO: task systemd-nspawn:6095 blocked for more than 120 seconds.
Aug 05 10:10:03 moria kernel:       Tainted: G           O    4.2.0-rc5 #2
Aug 05 10:10:03 moria kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Aug 05 10:10:03 moria kernel: systemd-nspawn  D ffff88019f3e3668     0  6095   6090 0x00080083
Aug 05 10:10:03 moria kernel:  ffff88019f3e3668 ffffffff86e5d480 ffff88018fd3a380 0000000000000246
Aug 05 10:10:03 moria kernel:  ffff88019f3e4000 ffff88018debdc08 ffff88018fd3a380 ffff88019f3e36b8
Aug 05 10:10:03 moria kernel:  ffff88018fd3a380 ffff88019f3e3688 ffffffff86999201 0000000000000000
Aug 05 10:10:03 moria kernel: Call Trace:
Aug 05 10:10:03 moria kernel:  [<ffffffff86999201>] schedule+0x74/0x83
Aug 05 10:10:03 moria kernel:  [<ffffffff863ef64c>] btrfs_tree_read_lock+0xc0/0xea
Aug 05 10:10:03 moria kernel:  [<ffffffff86137ed7>] ? wait_woken+0x74/0x74
Aug 05 10:10:03 moria kernel:  [<ffffffff8639f0a7>] btrfs_search_old_slot+0x51a/0x810
Aug 05 10:10:03 moria kernel:  [<ffffffff863a129b>] btrfs_next_old_leaf+0xdf/0x3ce
Aug 05 10:10:03 moria kernel:  [<ffffffff86413a00>] ? ulist_add_merge+0x1b/0x127
Aug 05 10:10:03 moria kernel:  [<ffffffff86411688>] __resolve_indirect_refs+0x62a/0x667
Aug 05 10:10:03 moria kernel:  [<ffffffff863ef546>] ? btrfs_clear_lock_blocking_rw+0x78/0xbe
Aug 05 10:10:03 moria kernel:  [<ffffffff864122d3>] find_parent_nodes+0xaf3/0xfc6
Aug 05 10:10:03 moria kernel:  [<ffffffff86412838>] __btrfs_find_all_roots+0x92/0xf0
Aug 05 10:10:03 moria kernel:  [<ffffffff864128f2>] btrfs_find_all_roots+0x45/0x65
Aug 05 10:10:03 moria kernel:  [<ffffffff8639a75b>] ? btrfs_get_tree_mod_seq+0x2b/0x88
Aug 05 10:10:03 moria kernel:  [<ffffffff863e852e>] check_ref+0x64/0xc4
Aug 05 10:10:03 moria kernel:  [<ffffffff863e9e01>] btrfs_clone+0x66e/0xb5d
Aug 05 10:10:03 moria kernel:  [<ffffffff863ea77f>] btrfs_ioctl_clone+0x48f/0x5bb
Aug 05 10:10:03 moria kernel:  [<ffffffff86048a68>] ? native_sched_clock+0x28/0x77
Aug 05 10:10:03 moria kernel:  [<ffffffff863ed9b0>] btrfs_ioctl+0xabc/0x25cb
Aug 05 10:10:03 moria kernel:  [<ffffffff86998f25>] ? __schedule+0x590/0x7f8
Aug 05 10:10:03 moria kernel:  [<ffffffff86999201>] ? schedule+0x74/0x83
Aug 05 10:10:03 moria kernel:  [<ffffffff86111b2e>] ? ptrace_stop+0x227/0x236
Aug 05 10:10:03 moria kernel:  [<ffffffff8612ba10>] ? account_system_time+0xf2/0x10f
Aug 05 10:10:03 moria kernel:  [<ffffffff86111bbc>] ? ptrace_do_notify+0x7f/0x8c
Aug 05 10:10:03 moria kernel:  [<ffffffff861fff2e>] do_vfs_ioctl+0x369/0x423
Aug 05 10:10:03 moria kernel:  [<ffffffff8617c29f>] ? __audit_syscall_entry+0xba/0xdc
Aug 05 10:10:03 moria kernel:  [<ffffffff86200021>] SyS_ioctl+0x39/0x61
Aug 05 10:10:03 moria kernel:  [<ffffffff8699cfd5>] tracesys_phase2+0x84/0x89


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