possible circular locking dependency detected (sb_internal/fs_reclaim)

["Zdenek Sojka" <zsojka@seznam.cz>]

Hello,

this is my fourth attempt to post this message to the mailing list; this time, without any attached kernel config (because it has over 100KiB). I also tried contacting the kernel btrfs maintainers directly by email, but they probably also didn't receive the message...

I am running kernel with lock debugging enabled since I am quite often encountering various lockups and hung tasks. Several of the problems have been fixed recently, but not all; I don't know if the following backtrace is related to the hangups, or if it is just a false positive.

$ uname -a
Linux zso 5.2.11-gentoo #2 SMP Fri Aug 30 07:18:03 CEST 2019 x86_64 Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz GenuineIntel GNU/Linux

The kernel has a distro patchset applied (which should not affect this, but you can never say that for sure) and I am compiling at -O3 -fipa-pta -march=native instead of default -O2 (gcc-8.3.0).

Please let me know if I can provide any more information.

Best regards,
Zdenek Sojka

The dmesg warning I recently triggered:

[30560.303721] ======================================================
[30560.303722] WARNING: possible circular locking dependency detected
[30560.303723] 5.2.11-gentoo #2 Not tainted
[30560.303724] ------------------------------------------------------
[30560.303725] kswapd0/711 is trying to acquire lock:
[30560.303726] 000000007777a663 (sb_internal){.+.+}, at: start_transaction+0x3a8/0x500
[30560.303731]
but task is already holding lock:
[30560.303732] 000000000ba86300 (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x0/0x30
[30560.303735]
which lock already depends on the new lock.

[30560.303736]
the existing dependency chain (in reverse order) is:
[30560.303737]
-> #1 (fs_reclaim){+.+.}:
[30560.303740] kmem_cache_alloc+0x1f/0x1c0
[30560.303742] btrfs_alloc_inode+0x1f/0x260
[30560.303744] alloc_inode+0x16/0xa0
[30560.303745] new_inode+0xe/0xb0
[30560.303747] btrfs_new_inode+0x70/0x610
[30560.303749] btrfs_symlink+0xd0/0x420
[30560.303750] vfs_symlink+0x9c/0x100
[30560.303751] do_symlinkat+0x66/0xe0
[30560.303753] do_syscall_64+0x55/0x1c0
[30560.303756] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[30560.303756]
-> #0 (sb_internal){.+.+}:
[30560.303758] __sb_start_write+0xf6/0x150
[30560.303759] start_transaction+0x3a8/0x500
[30560.303760] btrfs_commit_inode_delayed_inode+0x59/0x110
[30560.303761] btrfs_evict_inode+0x19e/0x4c0
[30560.303763] evict+0xbc/0x1f0
[30560.303763] inode_lru_isolate+0x113/0x190
[30560.303765] __list_lru_walk_one.isra.4+0x5c/0x100
[30560.303766] list_lru_walk_one+0x32/0x50
[30560.303766] prune_icache_sb+0x36/0x80
[30560.303768] super_cache_scan+0x14a/0x1d0
[30560.303769] do_shrink_slab+0x131/0x320
[30560.303770] shrink_node+0xf7/0x380
[30560.303772] balance_pgdat+0x2d5/0x640
[30560.303773] kswapd+0x2ba/0x5e0
[30560.303774] kthread+0x147/0x160
[30560.303775] ret_from_fork+0x24/0x30
[30560.303776]
other info that might help us debug this:

[30560.303776] Possible unsafe locking scenario:

[30560.303777] CPU0 CPU1
[30560.303777] ---- ----
[30560.303778] lock(fs_reclaim);
[30560.303778] lock(sb_internal);
[30560.303779] lock(fs_reclaim);
[30560.303780] lock(sb_internal);
[30560.303780]
*** DEADLOCK ***

[30560.303781] 3 locks held by kswapd0/711:
[30560.303782] #0: 000000000ba86300 (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x0/0x30
[30560.303784] #1: 000000004a5100f8 (shrinker_rwsem){++++}, at: shrink_node+0x9a/0x380
[30560.303786] #2: 00000000f956fa46 (&type->s_umount_key#30){++++}, at: super_cache_scan+0x35/0x1d0
[30560.303788]
stack backtrace:
[30560.303789] CPU: 7 PID: 711 Comm: kswapd0 Not tainted 5.2.11-gentoo #2
[30560.303790] Hardware name: Dell Inc. Precision Tower 3620/0MWYPT, BIOS 2.4.2 09/29/2017
[30560.303791] Call Trace:
[30560.303793] dump_stack+0x85/0xc7
[30560.303795] print_circular_bug.cold.40+0x1d9/0x235
[30560.303796] __lock_acquire+0x18b1/0x1f00
[30560.303797] lock_acquire+0xa6/0x170
[30560.303798] ? start_transaction+0x3a8/0x500
[30560.303799] __sb_start_write+0xf6/0x150
[30560.303800] ? start_transaction+0x3a8/0x500
[30560.303801] start_transaction+0x3a8/0x500
[30560.303802] btrfs_commit_inode_delayed_inode+0x59/0x110
[30560.303804] btrfs_evict_inode+0x19e/0x4c0
[30560.303805] ? var_wake_function+0x20/0x20
[30560.303806] evict+0xbc/0x1f0
[30560.303807] inode_lru_isolate+0x113/0x190
[30560.303808] ? discard_new_inode+0xc0/0xc0
[30560.303809] __list_lru_walk_one.isra.4+0x5c/0x100
[30560.303810] ? discard_new_inode+0xc0/0xc0
[30560.303811] list_lru_walk_one+0x32/0x50
[30560.303812] prune_icache_sb+0x36/0x80
[30560.303813] super_cache_scan+0x14a/0x1d0
[30560.303815] do_shrink_slab+0x131/0x320
[30560.303816] shrink_node+0xf7/0x380
[30560.303817] balance_pgdat+0x2d5/0x640
[30560.303819] kswapd+0x2ba/0x5e0
[30560.303820] ? __wake_up_common_lock+0x90/0x90
[30560.303822] kthread+0x147/0x160
[30560.303823] ? balance_pgdat+0x640/0x640
[30560.303824] ? __kthread_create_on_node+0x160/0x160
[30560.303826] ret_from_fork+0x24/0x30