Kernel bug: possible deadlock in pcpu_alloc_noprof
Subsystem: block
Status: fixed

Crash report:
======================================================
WARNING: possible circular locking dependency detected
6.15.0-syzkaller-13743-g8630c59e9936 #0 Not tainted
------------------------------------------------------
syz-executor117/5912 is trying to acquire lock:
ffffffff8e52f648 (pcpu_alloc_mutex){+.+.}-{4:4}, at: pcpu_alloc_noprof+0xb4c/0x1470 mm/percpu.c:1782

but task is already holding lock:
ffff888025f9a7c8 (&q->q_usage_counter(io)#18){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:205

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&q->q_usage_counter(io)#18){++++}-{0:0}:
       blk_alloc_queue+0x619/0x760 block/blk-core.c:461
       blk_mq_alloc_queue+0x175/0x290 block/blk-mq.c:4396
       __blk_mq_alloc_disk+0x29/0x120 block/blk-mq.c:4443
       loop_add+0x49e/0xb70 drivers/block/loop.c:2014
       loop_init+0x164/0x270 drivers/block/loop.c:2249
       do_one_initcall+0x120/0x6e0 init/main.c:1273
       do_initcall_level init/main.c:1335 [inline]
       do_initcalls init/main.c:1351 [inline]
       do_basic_setup init/main.c:1370 [inline]
       kernel_init_freeable+0x5c2/0x900 init/main.c:1583
       kernel_init+0x1c/0x2b0 init/main.c:1473
       ret_from_fork+0x5d7/0x6f0 arch/x86/kernel/process.c:148
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #1 (fs_reclaim){+.+.}-{0:0}:
       __fs_reclaim_acquire mm/page_alloc.c:4045 [inline]
       fs_reclaim_acquire+0x102/0x150 mm/page_alloc.c:4059
       might_alloc include/linux/sched/mm.h:318 [inline]
       prepare_alloc_pages+0x162/0x610 mm/page_alloc.c:4727
       __alloc_frozen_pages_noprof+0x18b/0x23f0 mm/page_alloc.c:4948
       __alloc_pages_noprof+0xb/0x1b0 mm/page_alloc.c:4993
       __alloc_pages_node_noprof include/linux/gfp.h:284 [inline]
       alloc_pages_node_noprof include/linux/gfp.h:311 [inline]
       pcpu_alloc_pages mm/percpu-vm.c:95 [inline]
       pcpu_populate_chunk+0x110/0xb00 mm/percpu-vm.c:285
       pcpu_alloc_noprof+0x86a/0x1470 mm/percpu.c:1870
       xt_percpu_counter_alloc+0x13e/0x1b0 net/netfilter/x_tables.c:1931
       find_check_entry.constprop.0+0xbc/0x9b0 net/ipv4/netfilter/ip_tables.c:526
       translate_table+0xc98/0x1720 net/ipv4/netfilter/ip_tables.c:716
       ipt_register_table+0x102/0x430 net/ipv4/netfilter/ip_tables.c:1742
       iptable_security_table_init+0x40/0x60 net/ipv4/netfilter/iptable_security.c:46
       xt_find_table_lock+0x2e4/0x520 net/netfilter/x_tables.c:1260
       xt_request_find_table_lock+0x28/0xf0 net/netfilter/x_tables.c:1285
       get_info+0x190/0x610 net/ipv4/netfilter/ip_tables.c:963
       do_ipt_get_ctl+0x169/0xa10 net/ipv4/netfilter/ip_tables.c:1659
       nf_getsockopt+0x79/0xe0 net/netfilter/nf_sockopt.c:116
       ip_getsockopt+0x18c/0x1e0 net/ipv4/ip_sockglue.c:1777
       tcp_getsockopt+0xa1/0x100 net/ipv4/tcp.c:4760
       do_sock_getsockopt+0x3fc/0x800 net/socket.c:2357
       __sys_getsockopt+0x123/0x1b0 net/socket.c:2386
       __do_sys_getsockopt net/socket.c:2393 [inline]
       __se_sys_getsockopt net/socket.c:2390 [inline]
       __x64_sys_getsockopt+0xbd/0x160 net/socket.c:2390
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xcd/0x490 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (pcpu_alloc_mutex){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3168 [inline]
       check_prevs_add kernel/locking/lockdep.c:3287 [inline]
       validate_chain kernel/locking/lockdep.c:3911 [inline]
       __lock_acquire+0x126f/0x1c90 kernel/locking/lockdep.c:5240
       lock_acquire kernel/locking/lockdep.c:5871 [inline]
       lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5828
       __mutex_lock_common kernel/locking/mutex.c:602 [inline]
       __mutex_lock+0x199/0xb90 kernel/locking/mutex.c:747
       pcpu_alloc_noprof+0xb4c/0x1470 mm/percpu.c:1782
       blk_stat_alloc_callback+0xc8/0x280 block/blk-stat.c:116
       wbt_init+0xac/0x540 block/blk-wbt.c:903
       queue_wb_lat_store+0x354/0x3d0 block/blk-sysfs.c:603
       queue_attr_store+0x279/0x320 block/blk-sysfs.c:805
       sysfs_kf_write+0xef/0x150 fs/sysfs/file.c:145
       kernfs_fop_write_iter+0x351/0x510 fs/kernfs/file.c:334
       new_sync_write fs/read_write.c:593 [inline]
       vfs_write+0x6c7/0x1150 fs/read_write.c:686
       ksys_write+0x12a/0x250 fs/read_write.c:738
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xcd/0x490 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:
  pcpu_alloc_mutex --> fs_reclaim --> &q->q_usage_counter(io)#18

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&q->q_usage_counter(io)#18);
                               lock(fs_reclaim);
                               lock(&q->q_usage_counter(io)#18);
  lock(pcpu_alloc_mutex);

 *** DEADLOCK ***

5 locks held by syz-executor117/5912:
 #0: ffff88803128a428 (sb_writers#7){.+.+}-{0:0}, at: ksys_write+0x12a/0x250 fs/read_write.c:738
 #1: ffff888069ff1888 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x28f/0x510 fs/kernfs/file.c:325
 #2: ffff888025fca3c8 (kn->active#59){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2b2/0x510 fs/kernfs/file.c:326
 #3: ffff888025f9a7c8 (&q->q_usage_counter(io)#18){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:205
 #4: ffff888025f9a800 (&q->q_usage_counter(queue)#19){+.+.}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:205

stack backtrace:
CPU: 1 UID: 0 PID: 5912 Comm: syz-executor117 Not tainted 6.15.0-syzkaller-13743-g8630c59e9936 #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 print_circular_bug+0x275/0x350 kernel/locking/lockdep.c:2046
 check_noncircular+0x14c/0x170 kernel/locking/lockdep.c:2178
 check_prev_add kernel/locking/lockdep.c:3168 [inline]
 check_prevs_add kernel/locking/lockdep.c:3287 [inline]
 validate_chain kernel/locking/lockdep.c:3911 [inline]
 __lock_acquire+0x126f/0x1c90 kernel/locking/lockdep.c:5240
 lock_acquire kernel/locking/lockdep.c:5871 [inline]
 lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5828
 __mutex_lock_common kernel/locking/mutex.c:602 [inline]
 __mutex_lock+0x199/0xb90 kernel/locking/mutex.c:747
 pcpu_alloc_noprof+0xb4c/0x1470 mm/percpu.c:1782
 blk_stat_alloc_callback+0xc8/0x280 block/blk-stat.c:116
 wbt_init+0xac/0x540 block/blk-wbt.c:903
 queue_wb_lat_store+0x354/0x3d0 block/blk-sysfs.c:603
 queue_attr_store+0x279/0x320 block/blk-sysfs.c:805
 sysfs_kf_write+0xef/0x150 fs/sysfs/file.c:145
 kernfs_fop_write_iter+0x351/0x510 fs/kernfs/file.c:334
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x6c7/0x1150 fs/read_write.c:686
 ksys_write+0x12a/0x250 fs/read_write.c:738
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0x490 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f97e87124a9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 c1 1f 00 00 90 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 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc4ca0cea8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f97e87124a9
RDX: 0000000000000009 RSI: 00002000000001c0 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000003 R09: 0000000000000003
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003
R13: 00007ffc4ca0cef0 R14: 00007ffc4ca0cf30 R15: 0000000000000000
 </TASK>