btrfs: fix potential dead lock in size class loading logic

btrfs: fix potential dead lock in size class loading logic

As reported by Filipe, there's a potential deadlock caused by
using btrfs_search_forward on commit_root. The locking there is
unconditional, even if ->skip_locking and ->search_commit_root is set.
It's not meant to be used for commit roots, so it always needs to do
locking.

So if another task is COWing a child node of the same root node and
then needs to wait for block group caching to complete when trying to
allocate a metadata extent, it deadlocks.

For example:

[539604.239315] sysrq: Show Blocked State
[539604.240133] task:kworker/u16:6   state:D stack:0     pid:2119594 ppid:2      flags:0x00004000
[539604.241613] Workqueue: btrfs-cache btrfs_work_helper [btrfs]
[539604.242673] Call Trace:
[539604.243129]  <TASK>
[539604.243925]  __schedule+0x41d/0xee0
[539604.244797]  ? rcu_read_lock_sched_held+0x12/0x70
[539604.245399]  ? rwsem_down_read_slowpath+0x185/0x490
[539604.246111]  schedule+0x5d/0xf0
[539604.246593]  rwsem_down_read_slowpath+0x2da/0x490
[539604.247290]  ? rcu_barrier_tasks_trace+0x10/0x20
[539604.248090]  __down_read_common+0x3d/0x150
[539604.248702]  down_read_nested+0xc3/0x140
[539604.249280]  __btrfs_tree_read_lock+0x24/0x100 [btrfs]
[539604.250097]  btrfs_read_lock_root_node+0x48/0x60 [btrfs]
[539604.250915]  btrfs_search_forward+0x59/0x460 [btrfs]
[539604.251781]  ? btrfs_global_root+0x50/0x70 [btrfs]
[539604.252476]  caching_thread+0x1be/0x920 [btrfs]
[539604.253167]  btrfs_work_helper+0xf6/0x400 [btrfs]
[539604.253848]  process_one_work+0x24f/0x5a0
[539604.254476]  worker_thread+0x52/0x3b0
[539604.255166]  ? __pfx_worker_thread+0x10/0x10
[539604.256047]  kthread+0xf0/0x120
[539604.256591]  ? __pfx_kthread+0x10/0x10
[539604.257212]  ret_from_fork+0x29/0x50
[539604.257822]  </TASK>
[539604.258233] task:btrfs-transacti state:D stack:0     pid:2236474 ppid:2      flags:0x00004000
[539604.259802] Call Trace:
[539604.260243]  <TASK>
[539604.260615]  __schedule+0x41d/0xee0
[539604.261205]  ? rcu_read_lock_sched_held+0x12/0x70
[539604.262000]  ? rwsem_down_read_slowpath+0x185/0x490
[539604.262822]  schedule+0x5d/0xf0
[539604.263374]  rwsem_down_read_slowpath+0x2da/0x490
[539604.266228]  ? lock_acquire+0x160/0x310
[539604.266917]  ? rcu_read_lock_sched_held+0x12/0x70
[539604.267996]  ? lock_contended+0x19e/0x500
[539604.268720]  __down_read_common+0x3d/0x150
[539604.269400]  down_read_nested+0xc3/0x140
[539604.270057]  __btrfs_tree_read_lock+0x24/0x100 [btrfs]
[539604.271129]  btrfs_read_lock_root_node+0x48/0x60 [btrfs]
[539604.272372]  btrfs_search_slot+0x143/0xf70 [btrfs]
[539604.273295]  update_block_group_item+0x9e/0x190 [btrfs]
[539604.274282]  btrfs_start_dirty_block_groups+0x1c4/0x4f0 [btrfs]
[539604.275381]  ? __mutex_unlock_slowpath+0x45/0x280
[539604.276390]  btrfs_commit_transaction+0xee/0xed0 [btrfs]
[539604.277391]  ? lock_acquire+0x1a4/0x310
[539604.278080]  ? start_transaction+0xcb/0x6c0 [btrfs]
[539604.279099]  transaction_kthread+0x142/0x1c0 [btrfs]
[539604.279996]  ? __pfx_transaction_kthread+0x10/0x10 [btrfs]
[539604.280673]  kthread+0xf0/0x120
[539604.281050]  ? __pfx_kthread+0x10/0x10
[539604.281496]  ret_from_fork+0x29/0x50
[539604.281966]  </TASK>
[539604.282255] task:fsstress        state:D stack:0     pid:2236483 ppid:1      flags:0x00004006
[539604.283897] Call Trace:
[539604.284700]  <TASK>
[539604.285088]  __schedule+0x41d/0xee0
[539604.285660]  schedule+0x5d/0xf0
[539604.286175]  btrfs_wait_block_group_cache_progress+0xf2/0x170 [btrfs]
[539604.287342]  ? __pfx_autoremove_wake_function+0x10/0x10
[539604.288450]  find_free_extent+0xd93/0x1750 [btrfs]
[539604.289256]  ? _raw_spin_unlock+0x29/0x50
[539604.289911]  ? btrfs_get_alloc_profile+0x127/0x2a0 [btrfs]
[539604.290843]  btrfs_reserve_extent+0x147/0x290 [btrfs]
[539604.291943]  btrfs_alloc_tree_block+0xcb/0x3e0 [btrfs]
[539604.292903]  __btrfs_cow_block+0x138/0x580 [btrfs]
[539604.293773]  btrfs_cow_block+0x10e/0x240 [btrfs]
[539604.294595]  btrfs_search_slot+0x7f3/0xf70 [btrfs]
[539604.295585]  btrfs_update_device+0x71/0x1b0 [btrfs]
[539604.296459]  btrfs_chunk_alloc_add_chunk_item+0xe0/0x340 [btrfs]
[539604.297489]  btrfs_chunk_alloc+0x1bf/0x490 [btrfs]
[539604.298335]  find_free_extent+0x6fa/0x1750 [btrfs]
[539604.299174]  ? _raw_spin_unlock+0x29/0x50
[539604.299950]  ? btrfs_get_alloc_profile+0x127/0x2a0 [btrfs]
[539604.300918]  btrfs_reserve_extent+0x147/0x290 [btrfs]
[539604.301797]  btrfs_alloc_tree_block+0xcb/0x3e0 [btrfs]
[539604.303017]  ? lock_release+0x224/0x4a0
[539604.303855]  __btrfs_cow_block+0x138/0x580 [btrfs]
[539604.304789]  btrfs_cow_block+0x10e/0x240 [btrfs]
[539604.305611]  btrfs_search_slot+0x7f3/0xf70 [btrfs]
[539604.306682]  ? btrfs_global_root+0x50/0x70 [btrfs]
[539604.308198]  lookup_inline_extent_backref+0x17b/0x7a0 [btrfs]
[539604.309254]  lookup_extent_backref+0x43/0xd0 [btrfs]
[539604.310122]  __btrfs_free_extent+0xf8/0x810 [btrfs]
[539604.310874]  ? lock_release+0x224/0x4a0
[539604.311724]  ? btrfs_merge_delayed_refs+0x17b/0x1d0 [btrfs]
[539604.313023]  __btrfs_run_delayed_refs+0x2ba/0x1260 [btrfs]
[539604.314271]  btrfs_run_delayed_refs+0x8f/0x1c0 [btrfs]
[539604.315445]  ? rcu_read_lock_sched_held+0x12/0x70
[539604.316706]  btrfs_commit_transaction+0xa2/0xed0 [btrfs]
[539604.317855]  ? do_raw_spin_unlock+0x4b/0xa0
[539604.318544]  ? _raw_spin_unlock+0x29/0x50
[539604.319240]  create_subvol+0x53d/0x6e0 [btrfs]
[539604.320283]  btrfs_mksubvol+0x4f5/0x590 [btrfs]
[539604.321220]  __btrfs_ioctl_snap_create+0x11b/0x180 [btrfs]
[539604.322307]  btrfs_ioctl_snap_create_v2+0xc6/0x150 [btrfs]
[539604.323295]  btrfs_ioctl+0x9f7/0x33e0 [btrfs]
[539604.324331]  ? rcu_read_lock_sched_held+0x12/0x70
[539604.325137]  ? lock_release+0x224/0x4a0
[539604.325808]  ? __x64_sys_ioctl+0x87/0xc0
[539604.326467]  __x64_sys_ioctl+0x87/0xc0
[539604.327109]  do_syscall_64+0x38/0x90
[539604.327875]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
[539604.328792] RIP: 0033:0x7f05a7babaeb

This needs to use regular btrfs_search_slot() with some skip and stop
logic.

Since we only consider five samples (five search slots), don't bother
with the complexity of looking for commit_root_sem contention. If
necessary, it can be added to the load function in between samples.

Reported-by: Filipe Manana <fdmanana@kernel.org>
Link: https://lore.kernel.org/linux-btrfs/CAL3q7H7eKMD44Z1+=Kb-1RFMMeZpAm2fwyO59yeBwCcSOU80Pg@mail.gmail.com/
Fixes: c7eec3d9aa95 ("btrfs: load block group size class when caching")
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>