linux-kernel - Re: [syzbot] [nilfs?] possible deadlock in nilfs_segctor

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Message-ID: <CAKFNMokMnXoRuz=GvNRoyaqKLDEJEymQEK7MdvD3BzA03=qZcQ@mail.gmail.com>
Date: Sat, 10 May 2025 22:31:18 +0900
From: Ryusuke Konishi <konishi.ryusuke@...il.com>
To: syzbot <syzbot+81394db39b0e2ed2db06@...kaller.appspotmail.com>
Cc: syzkaller-bugs@...glegroups.com, linux-kernel@...r.kernel.org, 
	eadavis@...com
Subject: Re: [syzbot] [nilfs?] possible deadlock in nilfs_segctor_construct

On Sat, May 10, 2025 at 9:46 PM syzbot
<syzbot+81394db39b0e2ed2db06@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> possible deadlock in nilfs_mdt_get_block
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 6.15.0-rc5-syzkaller-00136-g9c69f8884904-dirty #0 Not tainted
> ------------------------------------------------------
> segctord/5927 is trying to acquire lock:
> ffff88803ffcc610 (sb_internal#2){.+.+}-{0:0}, at: nilfs_mdt_create_block fs/nilfs2/mdt.c:80 [inline]
> ffff88803ffcc610 (sb_internal#2){.+.+}-{0:0}, at: nilfs_mdt_get_block+0x193/0xab0 fs/nilfs2/mdt.c:258
>
> but task is already holding lock:
> ffff8880422660a0 (&nilfs_bmap_mdt_lock_key){++++}-{4:4}, at: nilfs_bmap_propagate+0x2f/0x130 fs/nilfs2/bmap.c:328
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #2 (&nilfs_bmap_mdt_lock_key){++++}-{4:4}:
>        lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5866
>        down_read+0x46/0x2e0 kernel/locking/rwsem.c:1524
>        nilfs_bmap_lookup_at_level+0x92/0x3d0 fs/nilfs2/bmap.c:65
>        nilfs_bmap_lookup fs/nilfs2/bmap.h:182 [inline]
>        nilfs_mdt_submit_block+0x3a3/0x830 fs/nilfs2/mdt.c:144
>        nilfs_mdt_read_block+0x46/0x320 fs/nilfs2/mdt.c:178
>        nilfs_mdt_get_block+0x11f/0xab0 fs/nilfs2/mdt.c:254
>        nilfs_palloc_get_block+0x18d/0x2b0 fs/nilfs2/alloc.c:239
>        nilfs_palloc_get_desc_block fs/nilfs2/alloc.c:296 [inline]
>        nilfs_palloc_prepare_alloc_entry+0x3a3/0xf40 fs/nilfs2/alloc.c:607
>        nilfs_ifile_create_inode+0x111/0x2e0 fs/nilfs2/ifile.c:61
>        nilfs_new_inode+0x1ed/0x750 fs/nilfs2/inode.c:321
>        nilfs_create+0x100/0x2f0 fs/nilfs2/namei.c:98
>        lookup_open fs/namei.c:3701 [inline]
>        open_last_lookups fs/namei.c:3800 [inline]
>        path_openat+0x14f1/0x3830 fs/namei.c:4036
>        do_filp_open+0x1fa/0x410 fs/namei.c:4066
>        do_sys_openat2+0x121/0x1c0 fs/open.c:1429
>        do_sys_open fs/open.c:1444 [inline]
>        __do_sys_openat fs/open.c:1460 [inline]
>        __se_sys_openat fs/open.c:1455 [inline]
>        __x64_sys_openat+0x138/0x170 fs/open.c:1455
>        do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>        do_syscall_64+0xf6/0x210 arch/x86/entry/syscall_64.c:94
>        entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> -> #1 (&nilfs->ns_segctor_sem){++++}-{4:4}:
>        lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5866
>        down_read+0x46/0x2e0 kernel/locking/rwsem.c:1524
>        nilfs_transaction_begin+0x365/0x710 fs/nilfs2/segment.c:221
>        nilfs_create+0xc9/0x2f0 fs/nilfs2/namei.c:95
>        lookup_open fs/namei.c:3701 [inline]
>        open_last_lookups fs/namei.c:3800 [inline]
>        path_openat+0x14f1/0x3830 fs/namei.c:4036
>        do_filp_open+0x1fa/0x410 fs/namei.c:4066
>        do_sys_openat2+0x121/0x1c0 fs/open.c:1429
>        do_sys_open fs/open.c:1444 [inline]
>        __do_sys_openat fs/open.c:1460 [inline]
>        __se_sys_openat fs/open.c:1455 [inline]
>        __x64_sys_openat+0x138/0x170 fs/open.c:1455
>        do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>        do_syscall_64+0xf6/0x210 arch/x86/entry/syscall_64.c:94
>        entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> -> #0 (sb_internal#2){.+.+}-{0:0}:
>        check_prev_add kernel/locking/lockdep.c:3166 [inline]
>        check_prevs_add kernel/locking/lockdep.c:3285 [inline]
>        validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3909
>        __lock_acquire+0xaac/0xd20 kernel/locking/lockdep.c:5235
>        lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5866
>        percpu_down_read include/linux/percpu-rwsem.h:52 [inline]
>        __sb_start_write include/linux/fs.h:1783 [inline]
>        sb_start_intwrite include/linux/fs.h:1966 [inline]
>        nilfs_transaction_begin+0x268/0x710 fs/nilfs2/segment.c:218
>        nilfs_mdt_create_block fs/nilfs2/mdt.c:80 [inline]
>        nilfs_mdt_get_block+0x193/0xab0 fs/nilfs2/mdt.c:258
>        nilfs_palloc_get_block+0x18d/0x2b0 fs/nilfs2/alloc.c:239
>        nilfs_palloc_get_entry_block+0x8c/0xa0 fs/nilfs2/alloc.c:355
>        nilfs_dat_prepare_entry fs/nilfs2/dat.c:45 [inline]
>        nilfs_dat_prepare_alloc fs/nilfs2/dat.c:82 [inline]
>        nilfs_dat_prepare_update+0xb1/0x300 fs/nilfs2/dat.c:250
>        nilfs_direct_propagate+0x1ac/0x370 fs/nilfs2/direct.c:279
>        nilfs_bmap_propagate+0x77/0x130 fs/nilfs2/bmap.c:329
>        nilfs_collect_file_data+0x4f/0xd0 fs/nilfs2/segment.c:589
>        nilfs_segctor_apply_buffers+0x161/0x330 fs/nilfs2/segment.c:1010
>        nilfs_segctor_scan_file+0x68e/0x8e0 fs/nilfs2/segment.c:1059
>        nilfs_segctor_collect_blocks fs/nilfs2/segment.c:1267 [inline]
>        nilfs_segctor_collect fs/nilfs2/segment.c:1547 [inline]
>        nilfs_segctor_do_construct+0x1eac/0x6970 fs/nilfs2/segment.c:2122
>        nilfs_segctor_construct+0x17b/0x690 fs/nilfs2/segment.c:2478
>        nilfs_segctor_thread_construct fs/nilfs2/segment.c:2585 [inline]
>        nilfs_segctor_thread+0x6cd/0xe00 fs/nilfs2/segment.c:2700
>        kthread+0x70e/0x8a0 kernel/kthread.c:464
>        ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:153
>        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>
> other info that might help us debug this:
>
> Chain exists of:
>   sb_internal#2 --> &nilfs->ns_segctor_sem --> &nilfs_bmap_mdt_lock_key
>
>  Possible unsafe locking scenario:
>
>        CPU0                    CPU1
>        ----                    ----
>   lock(&nilfs_bmap_mdt_lock_key);
>                                lock(&nilfs->ns_segctor_sem);
>                                lock(&nilfs_bmap_mdt_lock_key);
>   rlock(sb_internal#2);
>
>  *** DEADLOCK ***
>
> 1 lock held by segctord/5927:
>  #0: ffff8880422660a0 (&nilfs_bmap_mdt_lock_key){++++}-{4:4}, at: nilfs_bmap_propagate+0x2f/0x130 fs/nilfs2/bmap.c:328
>
> stack backtrace:
> CPU: 0 UID: 0 PID: 5927 Comm: segctord Not tainted 6.15.0-rc5-syzkaller-00136-g9c69f8884904-dirty #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
>  print_circular_bug+0x2ee/0x310 kernel/locking/lockdep.c:2079
>  check_noncircular+0x134/0x160 kernel/locking/lockdep.c:2211
>  check_prev_add kernel/locking/lockdep.c:3166 [inline]
>  check_prevs_add kernel/locking/lockdep.c:3285 [inline]
>  validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3909
>  __lock_acquire+0xaac/0xd20 kernel/locking/lockdep.c:5235
>  lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5866
>  percpu_down_read include/linux/percpu-rwsem.h:52 [inline]
>  __sb_start_write include/linux/fs.h:1783 [inline]
>  sb_start_intwrite include/linux/fs.h:1966 [inline]
>  nilfs_transaction_begin+0x268/0x710 fs/nilfs2/segment.c:218
>  nilfs_mdt_create_block fs/nilfs2/mdt.c:80 [inline]
>  nilfs_mdt_get_block+0x193/0xab0 fs/nilfs2/mdt.c:258
>  nilfs_palloc_get_block+0x18d/0x2b0 fs/nilfs2/alloc.c:239
>  nilfs_palloc_get_entry_block+0x8c/0xa0 fs/nilfs2/alloc.c:355
>  nilfs_dat_prepare_entry fs/nilfs2/dat.c:45 [inline]
>  nilfs_dat_prepare_alloc fs/nilfs2/dat.c:82 [inline]
>  nilfs_dat_prepare_update+0xb1/0x300 fs/nilfs2/dat.c:250
>  nilfs_direct_propagate+0x1ac/0x370 fs/nilfs2/direct.c:279
>  nilfs_bmap_propagate+0x77/0x130 fs/nilfs2/bmap.c:329
>  nilfs_collect_file_data+0x4f/0xd0 fs/nilfs2/segment.c:589
>  nilfs_segctor_apply_buffers+0x161/0x330 fs/nilfs2/segment.c:1010
>  nilfs_segctor_scan_file+0x68e/0x8e0 fs/nilfs2/segment.c:1059
>  nilfs_segctor_collect_blocks fs/nilfs2/segment.c:1267 [inline]
>  nilfs_segctor_collect fs/nilfs2/segment.c:1547 [inline]
>  nilfs_segctor_do_construct+0x1eac/0x6970 fs/nilfs2/segment.c:2122
>  nilfs_segctor_construct+0x17b/0x690 fs/nilfs2/segment.c:2478
>  nilfs_segctor_thread_construct fs/nilfs2/segment.c:2585 [inline]
>  nilfs_segctor_thread+0x6cd/0xe00 fs/nilfs2/segment.c:2700
>  kthread+0x70e/0x8a0 kernel/kthread.c:464
>  ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:153
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
>
>
> Tested on:
>
> commit:         9c69f888 Merge tag 'bcachefs-2025-05-08' of git://evil..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11a694d4580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=91c351a0f6229e67
> dashboard link: https://syzkaller.appspot.com/bug?extid=81394db39b0e2ed2db06
> compiler:       Debian clang version 20.1.2 (++20250402124445+58df0ef89dd6-1~exp1~20250402004600.97), Debian LLD 20.1.2
> patch:          https://syzkaller.appspot.com/x/patch.diff?x=10c74768580000
>
>

This reports a different circular lock dependency than all other
reports in this issue, so it appears to be a separate issue. I'd like
to dig deeper.

Ryusuke Konishi