Hi,
I've seen the following lockdep warning after the 4.18 merges, it's
probably a cross-subsystem locking issue so I waited some time if this
will not go away after merge window.
Slab shrinker calls evict inode, in parallel there's an unmount in
progress and at some point locks get taken in the wrong order.
[ 635.423621] ======================================================
[ 635.429802] WARNING: possible circular locking dependency detected
[ 635.432211] 4.18.0-rc1-default+ #164 Not tainted
[ 635.434006] ------------------------------------------------------
[ 635.436888] kswapd0/77 is trying to acquire lock:
[ 635.438777] 00000000f6fa0d1f (&delayed_node->mutex){+.+.}, at:
__btrfs_release_delayed_node+0x49/0x2b0 [btrfs]
[ 635.440579]
[ 635.440579] but task is already holding lock:
[ 635.441636] 0000000025dee6ec (fs_reclaim){+.+.}, at:
__fs_reclaim_acquire+0x5/0x30
[ 635.442851]
[ 635.442851] which lock already depends on the new lock.
[ 635.442851]
[ 635.444221]
[ 635.444221] the existing dependency chain (in reverse order) is:
[ 635.445398]
[ 635.445398] -> #3 (fs_reclaim){+.+.}:
[ 635.446499] fs_reclaim_acquire.part.111+0x29/0x30
[ 635.447332] kmem_cache_alloc+0x29/0x2b0
[ 635.448066] btrfs_alloc_inode+0x24/0x260 [btrfs]
[ 635.448889] alloc_inode+0x18/0x80
[ 635.449549] new_inode_pseudo+0xc/0x60
[ 635.450304] new_inode+0x12/0x30
[ 635.451601] iget5_locked+0xb1/0xf0
[ 635.452524] btrfs_iget+0x57/0xf0 [btrfs]
[ 635.453276] __lookup_free_space_inode+0xd8/0x150 [btrfs]
[ 635.454205] lookup_free_space_inode+0x63/0xc0 [btrfs]
[ 635.455058] load_free_space_cache+0x6e/0x190 [btrfs]
[ 635.455949] cache_block_group+0x1c6/0x460 [btrfs]
[ 635.456793] find_free_extent+0x889/0x14e0 [btrfs]
[ 635.457640] btrfs_reserve_extent+0x9b/0x180 [btrfs]
[ 635.458463] btrfs_alloc_tree_block+0x1e8/0x440 [btrfs]
[ 635.459204] __btrfs_cow_block+0x109/0x700 [btrfs]
[ 635.460124] btrfs_cow_block+0x129/0x2f0 [btrfs]
[ 635.460938] btrfs_search_slot+0x22f/0xa70 [btrfs]
[ 635.461977] btrfs_lookup_inode+0x3a/0xc0 [btrfs]
[ 635.463158] __btrfs_update_delayed_inode+0x75/0x270 [btrfs]
[ 635.464032] __btrfs_run_delayed_items+0x147/0x1d0 [btrfs]
[ 635.464825] btrfs_commit_transaction+0x18a/0x9e0 [btrfs]
[ 635.465737] sync_filesystem+0x6b/0x90
[ 635.466439] generic_shutdown_super+0x22/0x100
[ 635.467229] kill_anon_super+0xe/0x20
[ 635.467994] btrfs_kill_super+0x12/0xa0 [btrfs]
[ 635.468827] deactivate_locked_super+0x29/0x60
[ 635.469622] cleanup_mnt+0x3b/0x70
[ 635.470363] task_work_run+0x9b/0xd0
[ 635.471479] exit_to_usermode_loop+0xbb/0xc0
[ 635.472519] do_syscall_64+0x16c/0x170
[ 635.473329] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 635.474309]
[ 635.474309] -> #2 (&caching_ctl->mutex){+.+.}:
[ 635.475430] __mutex_lock+0x86/0x9c0
[ 635.476206] cache_block_group+0x1bb/0x460 [btrfs]
[ 635.477152] find_free_extent+0x889/0x14e0 [btrfs]
[ 635.478101] btrfs_reserve_extent+0x9b/0x180 [btrfs]
[ 635.479074] btrfs_alloc_tree_block+0x1e8/0x440 [btrfs]
[ 635.480081] __btrfs_cow_block+0x109/0x700 [btrfs]
[ 635.481028] btrfs_cow_block+0x129/0x2f0 [btrfs]
[ 635.482020] btrfs_search_slot+0x22f/0xa70 [btrfs]
[ 635.482733] btrfs_insert_empty_items+0x67/0xc0 [btrfs]
[ 635.483459] btrfs_uuid_tree_add+0x1d2/0x2e0 [btrfs]
[ 635.484158] btrfs_uuid_scan_kthread+0x159/0x340 [btrfs]
[ 635.484880] kthread+0x11b/0x140
[ 635.485443] ret_from_fork+0x24/0x30
[ 635.486075]
[ 635.486075] -> #1 (&space_info->groups_sem){++++}:
[ 635.486931] down_read+0x3b/0x60
[ 635.487456] find_free_extent+0x992/0x14e0 [btrfs]
[ 635.488137] btrfs_reserve_extent+0x9b/0x180 [btrfs]
[ 635.488833] btrfs_alloc_tree_block+0x1e8/0x440 [btrfs]
[ 635.489639] __btrfs_cow_block+0x109/0x700 [btrfs]
[ 635.490440] btrfs_cow_block+0x129/0x2f0 [btrfs]
[ 635.491102] btrfs_search_slot+0x22f/0xa70 [btrfs]
[ 635.491782] btrfs_insert_empty_items+0x67/0xc0 [btrfs]
[ 635.492508] btrfs_insert_delayed_items+0x86/0x490 [btrfs]
[ 635.493305] __btrfs_run_delayed_items+0x96/0x1d0 [btrfs]
[ 635.494200] btrfs_commit_transaction+0x297/0x9e0 [btrfs]
[ 635.494989] btrfs_mksubvol+0x4e8/0x530 [btrfs]
[ 635.495651] btrfs_ioctl_snap_create_transid+0x170/0x180 [btrfs]
[ 635.496458] btrfs_ioctl_snap_create_v2+0x124/0x180 [btrfs]
[ 635.497224] btrfs_ioctl+0xc35/0x31a0 [btrfs]
[ 635.498003] do_vfs_ioctl+0xa2/0x6b0
[ 635.498680] ksys_ioctl+0x3a/0x70
[ 635.499331] __x64_sys_ioctl+0x16/0x20
[ 635.500032] do_syscall_64+0x5a/0x170
[ 635.500715] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 635.501574]
[ 635.501574] -> #0 (&delayed_node->mutex){+.+.}:
[ 635.502470] lock_acquire+0x9e/0x1d0
[ 635.503018] __mutex_lock+0x86/0x9c0
[ 635.503581] __btrfs_release_delayed_node+0x49/0x2b0 [btrfs]
[ 635.504352] btrfs_evict_inode+0x24e/0x5f0 [btrfs]
[ 635.505022] evict+0xcd/0x190
[ 635.505515] dispose_list+0x51/0x80
[ 635.506056] prune_icache_sb+0x52/0x70
[ 635.506622] super_cache_scan+0x123/0x1a0
[ 635.507213] shrink_slab.part.50+0x1c0/0x580
[ 635.507831] shrink_node+0x2f2/0x340
[ 635.508379] balance_pgdat+0x1dc/0x310
[ 635.508944] kswapd+0x195/0x540
[ 635.509511] kthread+0x11b/0x140
[ 635.510132] ret_from_fork+0x24/0x30
[ 635.510790]
[ 635.510790] other info that might help us debug this:
[ 635.510790]
[ 635.512112] Chain exists of:
[ 635.512112] &delayed_node->mutex --> &caching_ctl->mutex --> fs_reclaim
[ 635.512112]
[ 635.513826] Possible unsafe locking scenario:
[ 635.513826]
[ 635.514799] CPU0 CPU1
[ 635.515501] ---- ----
[ 635.516211] lock(fs_reclaim);
[ 635.516741] lock(&caching_ctl->mutex);
[ 635.517662] lock(fs_reclaim);
[ 635.518498] lock(&delayed_node->mutex);
[ 635.519158]
[ 635.519158] *** DEADLOCK ***
[ 635.519158]
[ 635.520211] 3 locks held by kswapd0/77:
[ 635.520831] #0: 0000000025dee6ec (fs_reclaim){+.+.}, at:
__fs_reclaim_acquire+0x5/0x30
[ 635.522092] #1: 000000009cc3bd11 (shrinker_rwsem){++++}, at:
shrink_slab.part.50+0x40/0x580
[ 635.523308] #2: 00000000e9e0ce72 (&type->s_umount_key#26){++++}, at:
trylock_super+0x16/0x50
[ 635.524387]
[ 635.524387] stack backtrace:
[ 635.525055] CPU: 1 PID: 77 Comm: kswapd0 Not tainted 4.18.0-rc1-default+ #164
[ 635.526078] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.0.0-prebuilt.qemu-project.org 04/01/2014
[ 635.527360] Call Trace:
[ 635.527753] dump_stack+0x85/0xc0
[ 635.528228] print_circular_bug.isra.36.cold.52+0x15c/0x195
[ 635.528925] check_prev_add.constprop.43+0x529/0x780
[ 635.529564] ? unwind_next_frame+0x54/0x430
[ 635.530126] __lock_acquire+0x9f4/0xf60
[ 635.530652] lock_acquire+0x9e/0x1d0
[ 635.531519] ? __btrfs_release_delayed_node+0x49/0x2b0 [btrfs]
[ 635.532416] ? __btrfs_release_delayed_node+0x49/0x2b0 [btrfs]
[ 635.533292] __mutex_lock+0x86/0x9c0
[ 635.533900] ? __btrfs_release_delayed_node+0x49/0x2b0 [btrfs]
[ 635.534770] ? __lock_acquire+0x9f4/0xf60
[ 635.535428] ? __btrfs_release_delayed_node+0x49/0x2b0 [btrfs]
[ 635.536314] __btrfs_release_delayed_node+0x49/0x2b0 [btrfs]
[ 635.537178] btrfs_evict_inode+0x24e/0x5f0 [btrfs]
[ 635.537921] evict+0xcd/0x190
[ 635.538441] dispose_list+0x51/0x80
[ 635.539009] prune_icache_sb+0x52/0x70
[ 635.539613] super_cache_scan+0x123/0x1a0
[ 635.540251] shrink_slab.part.50+0x1c0/0x580
[ 635.540867] shrink_node+0x2f2/0x340
[ 635.541441] balance_pgdat+0x1dc/0x310
[ 635.542040] kswapd+0x195/0x540
[ 635.542530] ? __schedule+0x306/0x940
[ 635.543039] ? finish_wait+0x80/0x80
[ 635.543539] ? balance_pgdat+0x310/0x310
[ 635.544073] kthread+0x11b/0x140
[ 635.544540] ? kthread_create_worker_on_cpu+0x70/0x70
[ 635.545188] ret_from_fork+0x24/0x30
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