[syzbot] [btrfs?] WARNING: suspicious RCU usage in btrfs_device_init_dev_stats

[syzbot] [btrfs?] WARNING: suspicious RCU usage in btrfs_device_init_dev_stats

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    7d6661873f6b Add linux-next specific files for 20260226
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=105ad1aa580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=98a1e192f758c1c1
dashboard link: https://syzkaller.appspot.com/bug?extid=d4957dbe80e471232035
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/4cd2824adb8a/disk-7d666187.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/bbdd4a130d86/vmlinux-7d666187.xz
kernel image: https://storage.googleapis.com/syzbot-assets/6e3bea3e96f8/bzImage-7d666187.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d4957dbe80e471232035@syzkaller.appspotmail.com

=============================
WARNING: suspicious RCU usage
syzkaller #0 Not tainted
-----------------------------
fs/btrfs/volumes.h:872 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
3 locks held by syz.2.1057/8526:
 #0: ffff88807cc920e0 (&type->s_umount_key#58/1){+.+.}-{4:4}, at: alloc_super+0x28c/0xab0 fs/super.c:345
 #1: ffff888057f2e0d8 (&fs_devs->device_list_mutex){+.+.}-{4:4}, at: btrfs_init_dev_stats+0x5b/0x190 fs/btrfs/volumes.c:8157
 #2: ffff8880772940f8 (btrfs-dev-00){.+.+}-{4:4}, at: btrfs_tree_read_lock_nested+0x33/0x250 fs/btrfs/locking.c:146

stack backtrace:
CPU: 0 UID: 0 PID: 8526 Comm: syz.2.1057 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 lockdep_rcu_suspicious+0x13f/0x1d0 kernel/locking/lockdep.c:6876
 btrfs_dev_name fs/btrfs/volumes.h:872 [inline]
 btrfs_dev_stat_print_on_load fs/btrfs/volumes.c:8302 [inline]
 btrfs_device_init_dev_stats+0x971/0xb90 fs/btrfs/volumes.c:8140
 btrfs_init_dev_stats+0x8e/0x190 fs/btrfs/volumes.c:8159
 open_ctree+0x1be9/0x2e40 fs/btrfs/disk-io.c:3534
 btrfs_fill_super+0x187/0x2e0 fs/btrfs/super.c:981
 btrfs_get_tree_super fs/btrfs/super.c:1944 [inline]
 btrfs_get_tree_subvol fs/btrfs/super.c:2087 [inline]
 btrfs_get_tree+0xe77/0x1370 fs/btrfs/super.c:2121
 vfs_get_tree+0x92/0x2a0 fs/super.c:1754
 fc_mount fs/namespace.c:1193 [inline]
 do_new_mount_fc fs/namespace.c:3763 [inline]
 do_new_mount+0x341/0xd30 fs/namespace.c:3839
 do_mount fs/namespace.c:4172 [inline]
 __do_sys_mount fs/namespace.c:4361 [inline]
 __se_sys_mount+0x31d/0x420 fs/namespace.c:4338
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f197b19da0a
Code: 48 c7 c2 e8 ff ff ff f7 d8 64 89 02 b8 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 49 89 ca b8 a5 00 00 00 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:00007f197bf8be58 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007f197bf8bee0 RCX: 00007f197b19da0a
RDX: 0000200000005100 RSI: 0000200000000040 RDI: 00007f197bf8bea0
RBP: 0000200000005100 R08: 00007f197bf8bee0 R09: 0000000000a08811
R10: 0000000000a08811 R11: 0000000000000246 R12: 0000200000000040
R13: 00007f197bf8bea0 R14: 0000000000005163 R15: 0000200000000100
 </TASK>
BTRFS info (device loop2 state ECS): bdev /dev/loop2 errs: wr 150994944, rd 0, flush 0, corrupt 0, gen 0
BTRFS info (device loop2 state ECS): setting nodatasum
BTRFS info (device loop2 state ECS): disabling tree log
BTRFS info (device loop2 state ECS): enabling free space tree
BTRFS info (device loop2 state ECS): ignoring bad roots
BTRFS info (device loop2 state ECS): ignoring meta csums


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

Re: [syzbot] [btrfs?] WARNING: suspicious RCU usage in btrfs_device_init_dev_stats

[David Sterba]

On Mon, Mar 02, 2026 at 10:45:41AM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    7d6661873f6b Add linux-next specific files for 20260226
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=105ad1aa580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=98a1e192f758c1c1
> dashboard link: https://syzkaller.appspot.com/bug?extid=d4957dbe80e471232035
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/4cd2824adb8a/disk-7d666187.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/bbdd4a130d86/vmlinux-7d666187.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/6e3bea3e96f8/bzImage-7d666187.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+d4957dbe80e471232035@syzkaller.appspotmail.com
> 
> =============================
> WARNING: suspicious RCU usage
> syzkaller #0 Not tainted
> -----------------------------
> fs/btrfs/volumes.h:872 suspicious rcu_dereference_check() usage!

#syz invalid

Marking as invalid so it does not appear in the list of issues but
otherwise it's a valid report caused by patch that has been removed.

https://lore.kernel.org/linux-btrfs/20260205114546.210418-1-dsterba@suse.com/