possible deadlock in __alloc_workqueue (3)

Status: upstream: reported on 2026/07/20 22:51
Subsystems: crypto
Labels: prio:normal
[Documentation on labels]
Reported-by: syzbot+5f117b3024eab6a14c35@syzkaller.appspotmail.com
First crash: 30d, last: 11d
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ID Workflow Result Correct Bug Created Started Finished Revision
23eece1b assessment-security DenialOfService: ✅ Exploitable: ❌ FilesystemTrigger: ❌ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ❌ UserNamespace: ✅ VMGuestTrigger: ❌ VMHostTrigger: ❌ ❓ possible deadlock in __alloc_workqueue (3) 2026/07/18 00:01 2026/07/18 00:01 2026/07/18 00:08 8aaff637
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Title Replies (including bot) Last reply
[syzbot] [crypto?] possible deadlock in __alloc_workqueue (3) 0 (1) 2026/07/20 22:51
▶ ▼ Similar bugs (1)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
upstream possible deadlock in __alloc_workqueue kernel 4 6 313d 335d 29/29 fixed on 2025/10/29 21:02

Sample crash report:
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.4.673/8249 is trying to acquire lock:
ffffffff8ec848e0 (fs_reclaim){+.+.}-{0:0}, at: might_alloc include/linux/sched/mm.h:317 [inline]
ffffffff8ec848e0 (fs_reclaim){+.+.}-{0:0}, at: slab_pre_alloc_hook mm/slub.c:4537 [inline]
ffffffff8ec848e0 (fs_reclaim){+.+.}-{0:0}, at: slab_alloc_node mm/slub.c:4897 [inline]
ffffffff8ec848e0 (fs_reclaim){+.+.}-{0:0}, at: __do_kmalloc_node mm/slub.c:5333 [inline]
ffffffff8ec848e0 (fs_reclaim){+.+.}-{0:0}, at: __kmalloc_noprof+0xbc/0x720 mm/slub.c:5359

but task is already holding lock:
ffffffff8e9f6d00 (wq_pool_mutex){+.+.}-{4:4}, at: __alloc_workqueue+0xa9c/0x2060 kernel/workqueue.c:5895

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #8 (wq_pool_mutex){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
       __alloc_workqueue+0xa9c/0x2060 kernel/workqueue.c:5895
       alloc_workqueue_va kernel/workqueue.c:5946 [inline]
       alloc_workqueue_noprof+0xe3/0x210 kernel/workqueue.c:5962
       padata_alloc+0xbe/0x360 kernel/padata.c:964
       pcrypt_init_padata+0x27/0x100 crypto/pcrypt.c:335
       pcrypt_init+0x60/0xc0 crypto/pcrypt.c:360
       do_one_initcall+0x250/0x870 init/main.c:1347
       do_initcall_level+0x10a/0x1a0 init/main.c:1409
       do_initcalls+0x59/0xa0 init/main.c:1425
       kernel_init_freeable+0x29d/0x3e0 init/main.c:1658
       kernel_init+0x1d/0x1d0 init/main.c:1548
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #7 (cpu_hotplug_lock){++++}-{0:0}:
       percpu_down_read_internal include/linux/percpu-rwsem.h:53 [inline]
       percpu_down_read include/linux/percpu-rwsem.h:77 [inline]
       cpus_read_lock+0x42/0x160 kernel/cpu.c:490
       static_key_slow_inc+0x12/0x30 kernel/jump_label.c:190
       udp_tunnel_encap_enable include/net/udp_tunnel.h:232 [inline]
       setup_udp_tunnel_sock+0x2df/0x4f0 net/ipv4/udp_tunnel_core.c:90
       l2tp_tunnel_register+0xe77/0x1570 net/l2tp/l2tp_core.c:1687
       pppol2tp_tunnel_get net/l2tp/l2tp_ppp.c:663 [inline]
       pppol2tp_connect+0x8e3/0x18b0 net/l2tp/l2tp_ppp.c:711
       __sys_connect_file net/socket.c:2135 [inline]
       __sys_connect+0x323/0x460 net/socket.c:2154
       __do_sys_connect net/socket.c:2160 [inline]
       __se_sys_connect net/socket.c:2157 [inline]
       __x64_sys_connect+0x7a/0x90 net/socket.c:2157
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #6 (sk_lock-AF_INET6){+.+.}-{0:0}:
       lock_sock_nested+0x41/0x100 net/core/sock.c:3828
       lock_sock include/net/sock.h:1713 [inline]
       inet_shutdown+0x6a/0x390 net/ipv4/af_inet.c:913
       nbd_mark_nsock_dead+0x2cb/0x550 drivers/block/nbd.c:318
       recv_work+0x1cee/0x1e10 drivers/block/nbd.c:1021
       process_one_work kernel/workqueue.c:3322 [inline]
       process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
       worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
       kthread+0x388/0x470 kernel/kthread.c:436
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #5 (&nsock->tx_lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
       nbd_handle_cmd drivers/block/nbd.c:1143 [inline]
       nbd_queue_rq+0x373/0x1150 drivers/block/nbd.c:1207
       blk_mq_dispatch_rq_list+0x499/0x1990 block/blk-mq.c:2117
       __blk_mq_do_dispatch_sched block/blk-mq-sched.c:168 [inline]
       blk_mq_do_dispatch_sched block/blk-mq-sched.c:182 [inline]
       __blk_mq_sched_dispatch_requests+0xd36/0x1580 block/blk-mq-sched.c:307
       blk_mq_sched_dispatch_requests+0xd7/0x190 block/blk-mq-sched.c:329
       blk_mq_run_work_fn+0x16c/0x300 block/blk-mq.c:2532
       process_one_work kernel/workqueue.c:3322 [inline]
       process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
       worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
       kthread+0x388/0x470 kernel/kthread.c:436
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #4 (&cmd->lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
       nbd_queue_rq+0xc1/0x1150 drivers/block/nbd.c:1199
       blk_mq_dispatch_rq_list+0x499/0x1990 block/blk-mq.c:2117
       __blk_mq_do_dispatch_sched block/blk-mq-sched.c:168 [inline]
       blk_mq_do_dispatch_sched block/blk-mq-sched.c:182 [inline]
       __blk_mq_sched_dispatch_requests+0xd36/0x1580 block/blk-mq-sched.c:307
       blk_mq_sched_dispatch_requests+0xd7/0x190 block/blk-mq-sched.c:329
       blk_mq_run_work_fn+0x16c/0x300 block/blk-mq.c:2532
       process_one_work kernel/workqueue.c:3322 [inline]
       process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
       worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
       kthread+0x388/0x470 kernel/kthread.c:436
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #3 (set->srcu){.+.+}-{0:0}:
       srcu_lock_sync include/linux/srcu.h:199 [inline]
       __synchronize_srcu+0xc9/0x2f0 kernel/rcu/srcutree.c:1481
       elevator_switch+0x1e8/0x7b0 block/elevator.c:576
       elevator_change+0x2fa/0x480 block/elevator.c:681
       elevator_set_default+0x375/0x440 block/elevator.c:754
       blk_register_queue+0x3f3/0x4e0 block/blk-sysfs.c:992
       __add_disk+0x6cb/0xe30 block/genhd.c:524
       add_disk_fwnode+0x100/0x3a0 block/genhd.c:593
       add_disk include/linux/blkdev.h:800 [inline]
       nbd_dev_add+0x733/0xb60 drivers/block/nbd.c:2021
       nbd_init+0x15f/0x1e0 drivers/block/nbd.c:2729
       do_one_initcall+0x250/0x870 init/main.c:1347
       do_initcall_level+0x10a/0x1a0 init/main.c:1409
       do_initcalls+0x59/0xa0 init/main.c:1425
       kernel_init_freeable+0x29d/0x3e0 init/main.c:1658
       kernel_init+0x1d/0x1d0 init/main.c:1548
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #2 (&q->elevator_lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
       elevator_change+0x1af/0x480 block/elevator.c:679
       elevator_set_none+0xb5/0x140 block/elevator.c:769
       blk_mq_elv_switch_none block/blk-mq.c:5101 [inline]
       __blk_mq_update_nr_hw_queues block/blk-mq.c:5146 [inline]
       blk_mq_update_nr_hw_queues+0x5ef/0x19f0 block/blk-mq.c:5211
       nbd_start_device+0x189/0xb30 drivers/block/nbd.c:1526
       nbd_genl_connect+0x1597/0x1c10 drivers/block/nbd.c:2276
       genl_family_rcv_msg_doit+0x233/0x340 net/netlink/genetlink.c:1114
       genl_family_rcv_msg net/netlink/genetlink.c:1194 [inline]
       genl_rcv_msg+0x614/0x7a0 net/netlink/genetlink.c:1209
       netlink_rcv_skb+0x226/0x4a0 net/netlink/af_netlink.c:2556
       genl_rcv+0x28/0x40 net/netlink/genetlink.c:1218
       netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
       netlink_unicast+0x7bb/0x940 net/netlink/af_netlink.c:1345
       netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1900
       sock_sendmsg_nosec+0x13a/0x180 net/socket.c:775
       __sock_sendmsg net/socket.c:790 [inline]
       ____sys_sendmsg+0x54e/0x850 net/socket.c:2684
       ___sys_sendmsg+0x2a5/0x360 net/socket.c:2738
       __sys_sendmsg net/socket.c:2770 [inline]
       __do_sys_sendmsg net/socket.c:2775 [inline]
       __se_sys_sendmsg net/socket.c:2773 [inline]
       __x64_sys_sendmsg+0x1b1/0x290 net/socket.c:2773
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&q->q_usage_counter(io)#50){++++}-{0:0}:
       blk_alloc_queue+0x544/0x690 block/blk-core.c:504
       blk_mq_alloc_queue block/blk-mq.c:4420 [inline]
       __blk_mq_alloc_disk+0x194/0x390 block/blk-mq.c:4467
       nbd_dev_add+0x494/0xb60 drivers/block/nbd.c:1991
       nbd_init+0x15f/0x1e0 drivers/block/nbd.c:2729
       do_one_initcall+0x250/0x870 init/main.c:1347
       do_initcall_level+0x10a/0x1a0 init/main.c:1409
       do_initcalls+0x59/0xa0 init/main.c:1425
       kernel_init_freeable+0x29d/0x3e0 init/main.c:1658
       kernel_init+0x1d/0x1d0 init/main.c:1548
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #0 (fs_reclaim){+.+.}-{0:0}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain kernel/locking/lockdep.c:3908 [inline]
       __lock_acquire+0x1520/0x2cf0 kernel/locking/lockdep.c:5237
       lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
       __fs_reclaim_acquire mm/page_alloc.c:4329 [inline]
       fs_reclaim_acquire+0x71/0x100 mm/page_alloc.c:4343
       might_alloc include/linux/sched/mm.h:317 [inline]
       slab_pre_alloc_hook mm/slub.c:4537 [inline]
       slab_alloc_node mm/slub.c:4897 [inline]
       __do_kmalloc_node mm/slub.c:5333 [inline]
       __kmalloc_noprof+0xbc/0x720 mm/slub.c:5359
       _kmalloc_noprof include/linux/slab.h:992 [inline]
       _kzalloc_noprof include/linux/slab.h:1309 [inline]
       apply_wqattrs_prepare+0xee/0xdc0 kernel/workqueue.c:5410
       apply_workqueue_attrs_locked kernel/workqueue.c:5495 [inline]
       alloc_and_link_pwqs kernel/workqueue.c:5645 [inline]
       __alloc_workqueue+0x117c/0x2060 kernel/workqueue.c:5897
       alloc_workqueue_va kernel/workqueue.c:5946 [inline]
       alloc_workqueue_noprof+0xe3/0x210 kernel/workqueue.c:5962
       nci_register_device+0x22c/0xa00 net/nfc/nci/core.c:1261
       virtual_ncidev_open+0x129/0x1a0 drivers/nfc/virtual_ncidev.c:153
       misc_open+0x2d5/0x350 drivers/char/misc.c:163
       chrdev_open+0x4d9/0x600 fs/char_dev.c:411
       do_dentry_open+0x816/0x1380 fs/open.c:947
       vfs_open+0x3b/0x340 fs/open.c:1052
       do_open fs/namei.c:4700 [inline]
       path_openat+0x2e44/0x3830 fs/namei.c:4863
       do_file_open+0x23e/0x4a0 fs/namei.c:4892
       do_sys_openat2+0x115/0x200 fs/open.c:1368
       do_sys_open fs/open.c:1374 [inline]
       __do_sys_openat fs/open.c:1390 [inline]
       __se_sys_openat fs/open.c:1385 [inline]
       __x64_sys_openat+0x138/0x170 fs/open.c:1385
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x174/0x580 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:
  fs_reclaim --> cpu_hotplug_lock --> wq_pool_mutex

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(wq_pool_mutex);
                               lock(cpu_hotplug_lock);
                               lock(wq_pool_mutex);
  lock(fs_reclaim);

 *** DEADLOCK ***

2 locks held by syz.4.673/8249:
 #0: ffffffff8f47d3a0 (misc_mtx){+.+.}-{4:4}, at: misc_open+0x51/0x350 drivers/char/misc.c:125
 #1: ffffffff8e9f6d00 (wq_pool_mutex){+.+.}-{4:4}, at: __alloc_workqueue+0xa9c/0x2060 kernel/workqueue.c:5895

stack backtrace:
CPU: 0 UID: 0 PID: 8249 Comm: syz.4.673 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e1/0x300 kernel/locking/lockdep.c:2043
 check_noncircular+0x12e/0x150 kernel/locking/lockdep.c:2175
 check_prev_add kernel/locking/lockdep.c:3165 [inline]
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain kernel/locking/lockdep.c:3908 [inline]
 __lock_acquire+0x1520/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
 __fs_reclaim_acquire mm/page_alloc.c:4329 [inline]
 fs_reclaim_acquire+0x71/0x100 mm/page_alloc.c:4343
 might_alloc include/linux/sched/mm.h:317 [inline]
 slab_pre_alloc_hook mm/slub.c:4537 [inline]
 slab_alloc_node mm/slub.c:4897 [inline]
 __do_kmalloc_node mm/slub.c:5333 [inline]
 __kmalloc_noprof+0xbc/0x720 mm/slub.c:5359
 _kmalloc_noprof include/linux/slab.h:992 [inline]
 _kzalloc_noprof include/linux/slab.h:1309 [inline]
 apply_wqattrs_prepare+0xee/0xdc0 kernel/workqueue.c:5410
 apply_workqueue_attrs_locked kernel/workqueue.c:5495 [inline]
 alloc_and_link_pwqs kernel/workqueue.c:5645 [inline]
 __alloc_workqueue+0x117c/0x2060 kernel/workqueue.c:5897
 alloc_workqueue_va kernel/workqueue.c:5946 [inline]
 alloc_workqueue_noprof+0xe3/0x210 kernel/workqueue.c:5962
 nci_register_device+0x22c/0xa00 net/nfc/nci/core.c:1261
 virtual_ncidev_open+0x129/0x1a0 drivers/nfc/virtual_ncidev.c:153
 misc_open+0x2d5/0x350 drivers/char/misc.c:163
 chrdev_open+0x4d9/0x600 fs/char_dev.c:411
 do_dentry_open+0x816/0x1380 fs/open.c:947
 vfs_open+0x3b/0x340 fs/open.c:1052
 do_open fs/namei.c:4700 [inline]
 path_openat+0x2e44/0x3830 fs/namei.c:4863
 do_file_open+0x23e/0x4a0 fs/namei.c:4892
 do_sys_openat2+0x115/0x200 fs/open.c:1368
 do_sys_open fs/open.c:1374 [inline]
 __do_sys_openat fs/open.c:1390 [inline]
 __se_sys_openat fs/open.c:1385 [inline]
 __x64_sys_openat+0x138/0x170 fs/open.c:1385
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ff6f639e019
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:00007ff6f45f6028 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007ff6f6625fa0 RCX: 00007ff6f639e019
RDX: 0000000000000002 RSI: 0000200000000080 RDI: ffffffffffffff9c
RBP: 00007ff6f643500c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ff6f6626038 R14: 00007ff6f6625fa0 R15: 00007ffeda2b6ca8
 </TASK>

Crashes (2):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/08/04 17:44 net cf6f8b29befb d4abbeac .config console log report info [disk image] [vmlinux] [kernel image] ci-upstream-net-this-kasan-gce possible deadlock in __alloc_workqueue
2026/07/16 22:49 net-next f6f3b36c15ed 8f30cfd7 .config console log report info [disk image] [vmlinux] [kernel image] ci-upstream-net-kasan-gce possible deadlock in __alloc_workqueue
* Struck through repros no longer work on HEAD.