Message-ID: <67eee338.050a0220.9040b.023b.GAE@google.com>
Date: Thu, 03 Apr 2025 12:36:24 -0700
From: syzbot <syzbot+6279b273d888c2017726@...kaller.appspotmail.com>
To: axboe@...nel.dk, linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [block?] possible deadlock in blk_mq_update_nr_hw_queues

syzbot has found a reproducer for the following issue on:

HEAD commit:    a2cc6ff5ec8f Merge tag 'firewire-updates-6.15' of git://gi..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1007f7b0580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=adffebefc9feb9d6
dashboard link: https://syzkaller.appspot.com/bug?extid=6279b273d888c2017726
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12202fb0580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1417e94c580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-a2cc6ff5.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ebd4f463c5cb/vmlinux-a2cc6ff5.xz
kernel image: https://storage.googleapis.com/syzbot-assets/91fd93bd91b3/bzImage-a2cc6ff5.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+6279b273d888c2017726@...kaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
6.14.0-syzkaller-12966-ga2cc6ff5ec8f #0 Not tainted
------------------------------------------------------
syz-executor876/5939 is trying to acquire lock:
ffff8880261b2318 (&q->elevator_lock){+.+.}-{4:4}, at: blk_mq_elv_switch_none block/blk-mq.c:4942 [inline]
ffff8880261b2318 (&q->elevator_lock){+.+.}-{4:4}, at: __blk_mq_update_nr_hw_queues block/blk-mq.c:5022 [inline]
ffff8880261b2318 (&q->elevator_lock){+.+.}-{4:4}, at: blk_mq_update_nr_hw_queues+0x4a9/0x1360 block/blk-mq.c:5074

but task is already holding lock:
ffff8880261b1de8 (&q->q_usage_counter(io)#49){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave block/blk-mq.c:215 [inline]
ffff8880261b1de8 (&q->q_usage_counter(io)#49){++++}-{0:0}, at: __blk_mq_update_nr_hw_queues block/blk-mq.c:5014 [inline]
ffff8880261b1de8 (&q->q_usage_counter(io)#49){++++}-{0:0}, at: blk_mq_update_nr_hw_queues+0x263/0x1360 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}:
       blk_alloc_queue+0x619/0x760 block/blk-core.c:461
       blk_mq_alloc_queue+0x179/0x290 block/blk-mq.c:4349
       __blk_mq_alloc_disk+0x29/0x120 block/blk-mq.c:4396
       nbd_dev_add+0x49d/0xbb0 drivers/block/nbd.c:1933
       nbd_init+0x181/0x320 drivers/block/nbd.c:2670
       do_one_initcall+0x120/0x6e0 init/main.c:1257
       do_initcall_level init/main.c:1319 [inline]
       do_initcalls init/main.c:1335 [inline]
       do_basic_setup init/main.c:1354 [inline]
       kernel_init_freeable+0x5c2/0x900 init/main.c:1567
       kernel_init+0x1c/0x2b0 init/main.c:1457
       ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:153
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #1 (fs_reclaim){+.+.}-{0:0}:
       __fs_reclaim_acquire mm/page_alloc.c:4028 [inline]
       fs_reclaim_acquire+0x102/0x150 mm/page_alloc.c:4042
       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+0x503/0x11c0 block/blk-mq.c:4457
       blk_mq_realloc_hw_ctxs+0x8f6/0xc00 block/blk-mq.c:4486
       blk_mq_init_allocated_queue+0x3af/0x1230 block/blk-mq.c:4540
       blk_mq_alloc_queue+0x1c2/0x290 block/blk-mq.c:4353
       __blk_mq_alloc_disk+0x29/0x120 block/blk-mq.c:4396
       loop_add+0x496/0xb70 drivers/block/loop.c:2067
       loop_init+0x164/0x270 drivers/block/loop.c:2302
       do_one_initcall+0x120/0x6e0 init/main.c:1257
       do_initcall_level init/main.c:1319 [inline]
       do_initcalls init/main.c:1335 [inline]
       do_basic_setup init/main.c:1354 [inline]
       kernel_init_freeable+0x5c2/0x900 init/main.c:1567
       kernel_init+0x1c/0x2b0 init/main.c:1457
       ret_from_fork+0x45/0x80 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 kernel/locking/lockdep.c:3909 [inline]
       __lock_acquire+0x1173/0x1ba0 kernel/locking/lockdep.c:5235
       lock_acquire kernel/locking/lockdep.c:5866 [inline]
       lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5823
       __mutex_lock_common kernel/locking/mutex.c:601 [inline]
       __mutex_lock+0x199/0xb90 kernel/locking/mutex.c:746
       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+0x4a9/0x1360 block/blk-mq.c:5074
       nbd_start_device+0x172/0xcd0 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+0x219/0xda0 drivers/block/nbd.c:1642
       blkdev_ioctl+0x274/0x6d0 block/ioctl.c:698
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:906 [inline]
       __se_sys_ioctl fs/ioctl.c:892 [inline]
       __x64_sys_ioctl+0x190/0x200 fs/ioctl.c:892
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xcd/0x260 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-executor876/5939:
 #0: ffff8880261c3998 (&nbd->config_lock){+.+.}-{4:4}, at: nbd_ioctl+0x150/0xda0 drivers/block/nbd.c:1635
 #1: ffff8880261c38d8 (&set->tag_list_lock){+.+.}-{4:4}, at: blk_mq_update_nr_hw_queues+0x8e/0x1360 block/blk-mq.c:5073
 #2: ffff8880261b1de8 (&q->q_usage_counter(io)#49){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave block/blk-mq.c:215 [inline]
 #2: ffff8880261b1de8 (&q->q_usage_counter(io)#49){++++}-{0:0}, at: __blk_mq_update_nr_hw_queues block/blk-mq.c:5014 [inline]
 #2: ffff8880261b1de8 (&q->q_usage_counter(io)#49){++++}-{0:0}, at: blk_mq_update_nr_hw_queues+0x263/0x1360 block/blk-mq.c:5074
 #3: ffff8880261b1e20 (&q->q_usage_counter(queue)){+.+.}-{0:0}, at: blk_mq_freeze_queue_nomemsave block/blk-mq.c:215 [inline]
 #3: ffff8880261b1e20 (&q->q_usage_counter(queue)){+.+.}-{0:0}, at: __blk_mq_update_nr_hw_queues block/blk-mq.c:5014 [inline]
 #3: ffff8880261b1e20 (&q->q_usage_counter(queue)){+.+.}-{0:0}, at: blk_mq_update_nr_hw_queues+0x263/0x1360 block/blk-mq.c:5074

stack backtrace:
CPU: 3 UID: 0 PID: 5939 Comm: syz-executor876 Not tainted 6.14.0-syzkaller-12966-ga2cc6ff5ec8f #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
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:2079
 check_noncircular+0x14c/0x170 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 kernel/locking/lockdep.c:3909 [inline]
 __lock_acquire+0x1173/0x1ba0 kernel/locking/lockdep.c:5235
 lock_acquire kernel/locking/lockdep.c:5866 [inline]
 lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5823
 __mutex_lock_common kernel/locking/mutex.c:601 [inline]
 __mutex_lock+0x199/0xb90 kernel/locking/mutex.c:746
 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+0x4a9/0x1360 block/blk-mq.c:5074
 nbd_start_device+0x172/0xcd0 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+0x219/0xda0 drivers/block/nbd.c:1642
 blkdev_ioctl+0x274/0x6d0 block/ioctl.c:698
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl fs/ioctl.c:892 [inline]
 __x64_sys_ioctl+0x190/0x200 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0x260 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fe4217ab4a9
Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 00 00 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 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe73ad6c28 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007ffe73ad6df8 RCX: 00007fe4217ab4a9
RDX: 0000000000000000 RSI: 000000000000ab03 RDI: 0000000000000003
RBP: 00007fe42181e610 R08: 00007ffe73ad6df8 R09: 00007ffe73ad6df8
R10: 00007ffe73ad6df8 R11: 0000000000000246 R12: 0000000000000001
R13: 00007ffe73ad6de8 R14: 0000000000000001 R15: 0000000000000001
 </TASK>


---
If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

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