syzbot

======================================================
WARNING: possible circular locking dependency detected
6.14.0-syzkaller-05877-g1a9239bb4253 #0 Not tainted
------------------------------------------------------
syz.0.89/6264 is trying to acquire lock:
ffff888144395d98 (&q->elevator_lock){+.+.}-{4:4}, at: blk_mq_elv_switch_none block/blk-mq.c:4942 [inline]
ffff888144395d98 (&q->elevator_lock){+.+.}-{4:4}, at: __blk_mq_update_nr_hw_queues block/blk-mq.c:5022 [inline]
ffff888144395d98 (&q->elevator_lock){+.+.}-{4:4}, at: blk_mq_update_nr_hw_queues+0x3e3/0x1a90 block/blk-mq.c:5074

but task is already holding lock:
ffff888144395868 (&q->q_usage_counter(io)#49){++++}-{0:0}, at: __blk_mq_update_nr_hw_queues block/blk-mq.c:5014 [inline]
ffff888144395868 (&q->q_usage_counter(io)#49){++++}-{0:0}, at: blk_mq_update_nr_hw_queues+0x316/0x1a90 block/blk-mq.c:5074

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&q->q_usage_counter(io)#49){++++}-{0:0}:
       lock_acquire+0x116/0x2f0 kernel/locking/lockdep.c:5866
       blk_alloc_queue+0x542/0x620 block/blk-core.c:461
       blk_mq_alloc_queue block/blk-mq.c:4349 [inline]
       __blk_mq_alloc_disk+0x162/0x380 block/blk-mq.c:4396
       nbd_dev_add+0x504/0xc00 drivers/block/nbd.c:1933
       nbd_init+0x210/0x2c0 drivers/block/nbd.c:2670
       do_one_initcall+0x24c/0x940 init/main.c:1257
       do_initcall_level+0x157/0x210 init/main.c:1319
       do_initcalls+0x71/0xd0 init/main.c:1335
       kernel_init_freeable+0x432/0x5d0 init/main.c:1567
       kernel_init+0x1d/0x2b0 init/main.c:1457
       ret_from_fork+0x4d/0x80 arch/x86/kernel/process.c:153
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #1 (fs_reclaim){+.+.}-{0:0}:
       lock_acquire+0x116/0x2f0 kernel/locking/lockdep.c:5866
       __fs_reclaim_acquire mm/page_alloc.c:3853 [inline]
       fs_reclaim_acquire+0x88/0x130 mm/page_alloc.c:3867
       might_alloc include/linux/sched/mm.h:318 [inline]
       xa_insert include/linux/xarray.h:783 [inline]
       blk_mq_init_hctx block/blk-mq.c:3924 [inline]
       blk_mq_alloc_and_init_hctx+0xa4f/0xd50 block/blk-mq.c:4457
       blk_mq_realloc_hw_ctxs+0x19e/0xb00 block/blk-mq.c:4486
       blk_mq_init_allocated_queue+0x3fb/0x14d0 block/blk-mq.c:4540
       blk_mq_alloc_queue block/blk-mq.c:4353 [inline]
       __blk_mq_alloc_disk+0x1b0/0x380 block/blk-mq.c:4396
       loop_add+0x445/0xaf0 drivers/block/loop.c:2067
       loop_init+0x168/0x220 drivers/block/loop.c:2302
       do_one_initcall+0x24c/0x940 init/main.c:1257
       do_initcall_level+0x157/0x210 init/main.c:1319
       do_initcalls+0x71/0xd0 init/main.c:1335
       kernel_init_freeable+0x432/0x5d0 init/main.c:1567
       kernel_init+0x1d/0x2b0 init/main.c:1457
       ret_from_fork+0x4d/0x80 arch/x86/kernel/process.c:153
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #0 (&q->elevator_lock){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3166 [inline]
       check_prevs_add kernel/locking/lockdep.c:3285 [inline]
       validate_chain+0xa69/0x24e0 kernel/locking/lockdep.c:3909
       __lock_acquire+0xad5/0xd80 kernel/locking/lockdep.c:5235
       lock_acquire+0x116/0x2f0 kernel/locking/lockdep.c:5866
       __mutex_lock_common kernel/locking/mutex.c:587 [inline]
       __mutex_lock+0x1bf/0x1000 kernel/locking/mutex.c:732
       blk_mq_elv_switch_none block/blk-mq.c:4942 [inline]
       __blk_mq_update_nr_hw_queues block/blk-mq.c:5022 [inline]
       blk_mq_update_nr_hw_queues+0x3e3/0x1a90 block/blk-mq.c:5074
       nbd_start_device+0x163/0xab0 drivers/block/nbd.c:1476
       nbd_start_device_ioctl drivers/block/nbd.c:1527 [inline]
       __nbd_ioctl drivers/block/nbd.c:1602 [inline]
       nbd_ioctl+0x628/0xf40 drivers/block/nbd.c:1642
       blkdev_ioctl+0x5e1/0x710 block/ioctl.c:698
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:906 [inline]
       __se_sys_ioctl+0xf3/0x160 fs/ioctl.c:892
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xf3/0x230 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:
  &q->elevator_lock --> fs_reclaim --> &q->q_usage_counter(io)#49

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&q->q_usage_counter(io)#49);
                               lock(fs_reclaim);
                               lock(&q->q_usage_counter(io)#49);
  lock(&q->elevator_lock);

 *** DEADLOCK ***

4 locks held by syz.0.89/6264:
 #0: ffff88802575f198 (&nbd->config_lock){+.+.}-{4:4}, at: nbd_ioctl+0x149/0xf40 drivers/block/nbd.c:1635
 #1: ffff88802575f0d8 (&set->tag_list_lock){+.+.}-{4:4}, at: blk_mq_update_nr_hw_queues+0x9f/0x1a90 block/blk-mq.c:5073
 #2: ffff888144395868 (&q->q_usage_counter(io)#49){++++}-{0:0}, at: __blk_mq_update_nr_hw_queues block/blk-mq.c:5014 [inline]
 #2: ffff888144395868 (&q->q_usage_counter(io)#49){++++}-{0:0}, at: blk_mq_update_nr_hw_queues+0x316/0x1a90 block/blk-mq.c:5074
 #3: ffff8881443958a0 (&q->q_usage_counter(queue)){+.+.}-{0:0}, at: __blk_mq_update_nr_hw_queues block/blk-mq.c:5014 [inline]
 #3: ffff8881443958a0 (&q->q_usage_counter(queue)){+.+.}-{0:0}, at: blk_mq_update_nr_hw_queues+0x316/0x1a90 block/blk-mq.c:5074

stack backtrace:
CPU: 0 UID: 0 PID: 6264 Comm: syz.0.89 Not tainted 6.14.0-syzkaller-05877-g1a9239bb4253 #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
 print_circular_bug+0x2e1/0x300 kernel/locking/lockdep.c:2079
 check_noncircular+0x142/0x160 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+0xa69/0x24e0 kernel/locking/lockdep.c:3909
 __lock_acquire+0xad5/0xd80 kernel/locking/lockdep.c:5235
 lock_acquire+0x116/0x2f0 kernel/locking/lockdep.c:5866
 __mutex_lock_common kernel/locking/mutex.c:587 [inline]
 __mutex_lock+0x1bf/0x1000 kernel/locking/mutex.c:732
 blk_mq_elv_switch_none block/blk-mq.c:4942 [inline]
 __blk_mq_update_nr_hw_queues block/blk-mq.c:5022 [inline]
 blk_mq_update_nr_hw_queues+0x3e3/0x1a90 block/blk-mq.c:5074
 nbd_start_device+0x163/0xab0 drivers/block/nbd.c:1476
 nbd_start_device_ioctl drivers/block/nbd.c:1527 [inline]
 __nbd_ioctl drivers/block/nbd.c:1602 [inline]
 nbd_ioctl+0x628/0xf40 drivers/block/nbd.c:1642
 blkdev_ioctl+0x5e1/0x710 block/ioctl.c:698
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl+0xf3/0x160 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f193d78d169
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f193e638038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f193d9a5fa0 RCX: 00007f193d78d169
RDX: 0000000000000000 RSI: 000000000000ab03 RDI: 000000000000000a
RBP: 00007f193d80e2a0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007f193d9a5fa0 R15: 00007ffc8a0c9c68
 </TASK>
block nbd0: shutting down sockets