[BUG] btrfs: circular locking dependency in zoned mode (mmap_lock <-> kernfs_rwsem)
From: wheatfox17
Date: Wed Mar 04 2026 - 02:06:23 EST
There’s a circular locking dependency detected by lockdep when running Syzkaller
on a Btrfs filesystem with zoned mode. The deadlock involves a four-stage dependency
chain starting from mmap_lock and ending back at it via kernfs_rwsem.
Upstream commit: 7.0.0-rc2, 0031c06807cfa8aa51a759ff8aa09e1aa48149af
Kernel Architecture: x86_64
Kernel Config: https://pastebin.com/CAZu430t
Hardware: FEMU b3272c0130faa5fd04826303541a831731a8dfb2 ZNS SSD mode
Reported-by: Yulong Han <wheatfox17@xxxxxxxxxx>
======================================================
SYZKALLER BUG REPORT AND REPRODUCER
======================================================
Syzkaller hit 'possible deadlock in lock_mm_and_find_vma' bug.
syz-executor (436) used greatest stack depth: 24176 bytes left
======================================================
WARNING: possible circular locking dependency detected
7.0.0-rc2-g0031c06807cf #7 Not tainted
------------------------------------------------------
syz.3.82/1572 is trying to acquire lock:
ffff88811270e140 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock_killable include/linux/mmap_lock.h:601 [inline]
ffff88811270e140 (&mm->mmap_lock){++++}-{4:4}, at: get_mmap_lock_carefully mm/mmap_lock.c:450 [inline]
ffff88811270e140 (&mm->mmap_lock){++++}-{4:4}, at: lock_mm_and_find_vma+0x485/0x1200 mm/mmap_lock.c:501
but task is already holding lock:
ffff88810021e988 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_fop_readdir+0x15b/0x990 fs/kernfs/dir.c:1898
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #3 (&root->kernfs_rwsem){++++}-{4:4}:
lock_acquire kernel/locking/lockdep.c:5868 [inline]
lock_acquire+0x194/0x300 kernel/locking/lockdep.c:5825
down_write+0x8b/0x1e0 kernel/locking/rwsem.c:1590
kernfs_add_one+0x38/0x850 fs/kernfs/dir.c:796
kernfs_create_dir_ns+0xfc/0x1a0 fs/kernfs/dir.c:1098
sysfs_create_dir_ns+0x13a/0x2b0 fs/sysfs/dir.c:59
create_dir lib/kobject.c:73 [inline]
kobject_add_internal+0x247/0x8f0 lib/kobject.c:240
kobject_add_varg lib/kobject.c:374 [inline]
kobject_add+0x16a/0x1e0 lib/kobject.c:426
btrfs_sysfs_add_block_group_type+0x246/0x500 fs/btrfs/sysfs.c:1866
btrfs_create_pending_block_groups+0xfb0/0x1820 fs/btrfs/block-group.c:2876
__btrfs_end_transaction+0xf5/0x8e0 fs/btrfs/transaction.c:1080
btrfs_zoned_reserve_data_reloc_bg+0x62c/0x930 fs/btrfs/zoned.c:2836
open_ctree+0x423d/0x5f52 fs/btrfs/disk-io.c:3597
btrfs_fill_super fs/btrfs/super.c:981 [inline]
btrfs_get_tree_super fs/btrfs/super.c:1944 [inline]
btrfs_get_tree_subvol fs/btrfs/super.c:2087 [inline]
btrfs_get_tree.cold+0xe1/0x2b7 fs/btrfs/super.c:2121
vfs_get_tree+0x92/0x320 fs/super.c:1754
vfs_cmd_create+0xd7/0x2a0 fs/fsopen.c:231
vfs_fsconfig_locked fs/fsopen.c:295 [inline]
__do_sys_fsconfig+0x55a/0xcb0 fs/fsopen.c:463
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfc/0x670 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
-> #2 (btrfs_trans_num_extwriters){.+.+}-{0:0}:
lock_acquire kernel/locking/lockdep.c:5868 [inline]
lock_acquire+0x194/0x300 kernel/locking/lockdep.c:5825
join_transaction+0x149/0xf40 fs/btrfs/transaction.c:323
start_transaction+0x3c4/0x1fe0 fs/btrfs/transaction.c:708
btrfs_zoned_reserve_data_reloc_bg+0x5bc/0x930 fs/btrfs/zoned.c:2827
open_ctree+0x423d/0x5f52 fs/btrfs/disk-io.c:3597
btrfs_fill_super fs/btrfs/super.c:981 [inline]
btrfs_get_tree_super fs/btrfs/super.c:1944 [inline]
btrfs_get_tree_subvol fs/btrfs/super.c:2087 [inline]
btrfs_get_tree.cold+0xe1/0x2b7 fs/btrfs/super.c:2121
vfs_get_tree+0x92/0x320 fs/super.c:1754
vfs_cmd_create+0xd7/0x2a0 fs/fsopen.c:231
vfs_fsconfig_locked fs/fsopen.c:295 [inline]
__do_sys_fsconfig+0x55a/0xcb0 fs/fsopen.c:463
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfc/0x670 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
-> #1 (btrfs_trans_num_writers){.+.+}-{0:0}:
reacquire_held_locks+0xce/0x1e0 kernel/locking/lockdep.c:5385
__lock_release kernel/locking/lockdep.c:5574 [inline]
lock_release kernel/locking/lockdep.c:5889 [inline]
lock_release+0x11f/0x2c0 kernel/locking/lockdep.c:5875
percpu_up_read include/linux/percpu-rwsem.h:112 [inline]
__sb_end_write include/linux/fs/super.h:14 [inline]
sb_end_intwrite include/linux/fs/super.h:101 [inline]
__btrfs_end_transaction+0x5c7/0x8e0 fs/btrfs/transaction.c:1085
btrfs_dirty_inode+0x131/0x200 fs/btrfs/inode.c:6443
btrfs_update_time fs/btrfs/inode.c:6468 [inline]
btrfs_update_time+0xc4/0x100 fs/btrfs/inode.c:6454
touch_atime+0x255/0x780 fs/inode.c:2271
file_accessed include/linux/fs.h:2263 [inline]
btrfs_file_mmap_prepare+0x206/0x280 fs/btrfs/file.c:2051
vfs_mmap_prepare include/linux/fs.h:2075 [inline]
call_mmap_prepare mm/vma.c:2644 [inline]
__mmap_region+0xe0d/0x2980 mm/vma.c:2743
mmap_region+0x2e0/0x3b0 mm/vma.c:2837
do_mmap+0xc53/0x12d0 mm/mmap.c:559
vm_mmap_pgoff+0x201/0x390 mm/util.c:581
ksys_mmap_pgoff+0x466/0x5b0 mm/mmap.c:605
__do_sys_mmap arch/x86/kernel/sys_x86_64.c:89 [inline]
__se_sys_mmap arch/x86/kernel/sys_x86_64.c:82 [inline]
__x64_sys_mmap+0x125/0x190 arch/x86/kernel/sys_x86_64.c:82
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfc/0x670 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
-> #0 (&mm->mmap_lock){++++}-{4:4}:
check_prev_add+0xeb/0xce0 kernel/locking/lockdep.c:3165
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x128f/0x1a50 kernel/locking/lockdep.c:5237
lock_acquire kernel/locking/lockdep.c:5868 [inline]
lock_acquire+0x194/0x300 kernel/locking/lockdep.c:5825
down_read_killable+0x9c/0x490 kernel/locking/rwsem.c:1560
mmap_read_lock_killable include/linux/mmap_lock.h:601 [inline]
get_mmap_lock_carefully mm/mmap_lock.c:450 [inline]
lock_mm_and_find_vma+0x485/0x1200 mm/mmap_lock.c:501
do_user_addr_fault+0x333/0xe90 arch/x86/mm/fault.c:1357
handle_page_fault arch/x86/mm/fault.c:1474 [inline]
exc_page_fault+0x66/0xc0 arch/x86/mm/fault.c:1527
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
filldir+0x1ec/0x650 fs/readdir.c:296
dir_emit include/linux/fs.h:3566 [inline]
kernfs_fop_readdir+0x40c/0x990 fs/kernfs/dir.c:1915
iterate_dir+0x1e0/0x5e0 fs/readdir.c:108
__do_sys_getdents fs/readdir.c:327 [inline]
__se_sys_getdents fs/readdir.c:312 [inline]
__x64_sys_getdents+0x13b/0x2b0 fs/readdir.c:312
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfc/0x670 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
other info that might help us debug this:
Chain exists of:
&mm->mmap_lock --> btrfs_trans_num_extwriters --> &root->kernfs_rwsem
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
rlock(&root->kernfs_rwsem);
lock(btrfs_trans_num_extwriters);
lock(&root->kernfs_rwsem);
rlock(&mm->mmap_lock);
*** DEADLOCK ***
3 locks held by syz.3.82/1572:
#0: ffff8881035db7b8 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x2aa/0x380 fs/file.c:1261
#1: ffff888106bb99f8 (&type->i_mutex_dir_key#6){++++}-{4:4}, at: iterate_dir+0xdc/0x5e0 fs/readdir.c:101
#2: ffff88810021e988 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_fop_readdir+0x15b/0x990 fs/kernfs/dir.c:1898
stack backtrace:
CPU: 3 UID: 0 PID: 1572 Comm: syz.3.82 Not tainted 7.0.0-rc2-g0031c06807cf #7 PREEMPT(full)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0xba/0x110 lib/dump_stack.c:120
print_circular_bug.cold+0x178/0x1be kernel/locking/lockdep.c:2043
check_noncircular+0x146/0x160 kernel/locking/lockdep.c:2175
check_prev_add+0xeb/0xce0 kernel/locking/lockdep.c:3165
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x128f/0x1a50 kernel/locking/lockdep.c:5237
lock_acquire kernel/locking/lockdep.c:5868 [inline]
lock_acquire+0x194/0x300 kernel/locking/lockdep.c:5825
down_read_killable+0x9c/0x490 kernel/locking/rwsem.c:1560
mmap_read_lock_killable include/linux/mmap_lock.h:601 [inline]
get_mmap_lock_carefully mm/mmap_lock.c:450 [inline]
lock_mm_and_find_vma+0x485/0x1200 mm/mmap_lock.c:501
do_user_addr_fault+0x333/0xe90 arch/x86/mm/fault.c:1357
handle_page_fault arch/x86/mm/fault.c:1474 [inline]
exc_page_fault+0x66/0xc0 arch/x86/mm/fault.c:1527
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
RIP: 0010:filldir+0x1ec/0x650 fs/readdir.c:296
Code: 0f 01 cb 0f ae e8 48 8b 44 24 08 49 89 46 08 e8 6a 2b b6 ff 4c 8b 74 24 30 48 8b 44 24 10 49 89 06 e8 58 2b b6 ff 44 8b 24 24 <66> 45 89 66 10 e8 4a 2b b6 ff 8b 44 24 04 49 63 cc 48 89 4c 24 08
RSP: 0018:ffff88810ebbfce0 EFLAGS: 00050293
RAX: 0000000000000000 RBX: ffff88810ebbfe70 RCX: ffffffff8ed596f8
RDX: ffff888111c0c480 RSI: 0000200000001fd0 RDI: 0000000000000006
RBP: ffff888107f8ad00 R08: 0000000000000000 R09: ffff888111c0c480
R10: 0000200000002010 R11: 0000000000000003 R12: 0000000000000020
R13: ffff88810ebbfe94 R14: 0000200000001ff0 R15: 0000000000000008
dir_emit include/linux/fs.h:3566 [inline]
kernfs_fop_readdir+0x40c/0x990 fs/kernfs/dir.c:1915
iterate_dir+0x1e0/0x5e0 fs/readdir.c:108
__do_sys_getdents fs/readdir.c:327 [inline]
__se_sys_getdents fs/readdir.c:312 [inline]
__x64_sys_getdents+0x13b/0x2b0 fs/readdir.c:312
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfc/0x670 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f1010d9acf9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f1010bff028 EFLAGS: 00000246 ORIG_RAX: 000000000000004e
RAX: ffffffffffffffda RBX: 00007f1011005fa0 RCX: 00007f1010d9acf9
RDX: 0000000000001000 RSI: 0000200000001ac0 RDI: 0000000000000003
RBP: 00007f1010e08bf7 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f1011006038 R14: 00007f1011005fa0 R15: 00007ffd376f51d8
</TASK>
----------------
Code disassembly (best guess):
0: 0f 01 cb stac
3: 0f ae e8 lfence
6: 48 8b 44 24 08 mov 0x8(%rsp),%rax
b: 49 89 46 08 mov %rax,0x8(%r14)
f: e8 6a 2b b6 ff call 0xffb62b7e
14: 4c 8b 74 24 30 mov 0x30(%rsp),%r14
19: 48 8b 44 24 10 mov 0x10(%rsp),%rax
1e: 49 89 06 mov %rax,(%r14)
21: e8 58 2b b6 ff call 0xffb62b7e
26: 44 8b 24 24 mov (%rsp),%r12d
* 2a: 66 45 89 66 10 mov %r12w,0x10(%r14) <-- trapping instruction
2f: e8 4a 2b b6 ff call 0xffb62b7e
34: 8b 44 24 04 mov 0x4(%rsp),%eax
38: 49 63 cc movslq %r12d,%rcx
3b: 48 89 4c 24 08 mov %rcx,0x8(%rsp)
Syzkaller reproducer:
# {Threaded:true Repeat:true RepeatTimes:0 Procs:1 Slowdown:1 Sandbox:none SandboxArg:0 Leak:false NetInjection:false NetDevices:true NetReset:true Cgroups:true BinfmtMisc:true CloseFDs:true KCSAN:false DevlinkPCI:false NicVF:false USB:false VhciInjection:false Wifi:false IEEE802154:false Sysctl:true Swap:false UseTmpDir:true HandleSegv:true Trace:false CallComments:true LegacyOptions:{Collide:false Fault:false FaultCall:0 FaultNth:0}}
r0 = openat(0xffffffffffffff9c, &(0x7f0000000280)='./cgroup\x00', 0x0, 0x0)
getdents(r0, &(0x7f0000001ac0)=""/4096, 0x1000) (async)
fstat(0xffffffffffffffff, 0x0)
newfstatat(0xffffffffffffff9c, 0x0, 0x0, 0x0)