Re: [syzbot] [block?] possible deadlock in queue_wb_lat_store

syzbot
2025-04-14T11:07:25Z

syzbot has found a reproducer for the following issue on:

HEAD commit: 8ffd015db85f Linux 6.15-rc2
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11a66a3f980000
kernel config: https://syzkaller.appspot.com/x/.config?x=ae323da55fbd6522
dashboard link: https://syzkaller.appspot.com/bug?extid=5a9d5d7faa80fa15e3db
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=13ecdf4c580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=123420cc580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b7a7f5609ad8/disk-8ffd015d.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c8cb2488367a/vmlinux-8ffd015d.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c99e36835459/bzImage-8ffd015d.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+5a9d5d7faa80fa15e3db@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
6.15.0-rc2-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor134/5835 is trying to acquire lock:
ffff8880260c0f98 (&q->elevator_lock){+.+.}-{4:4}, at: queue_wb_lat_store+0x187/0x3d0 block/blk-sysfs.c:596

but task is already holding lock:
ffff8880260c0a68 (&q->q_usage_counter(io)#62){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:215

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (&q->q_usage_counter(io)#62){++++}-{0:0}:
blk_alloc_queue+0x619/0x760 block/blk-core.c:461
blk_mq_alloc_queue+0x179/0x290 block/blk-mq.c:4348
__blk_mq_alloc_disk+0x29/0x120 block/blk-mq.c:4395
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]
slab_pre_alloc_hook mm/slub.c:4102 [inline]
slab_alloc_node mm/slub.c:4180 [inline]
kmem_cache_alloc_noprof+0x53/0x3b0 mm/slub.c:4207
__kernfs_new_node+0xd2/0x8a0 fs/kernfs/dir.c:637
kernfs_new_node+0x13c/0x1e0 fs/kernfs/dir.c:713
kernfs_create_dir_ns+0x4c/0x1a0 fs/kernfs/dir.c:1081
sysfs_create_dir_ns+0x13a/0x2b0 fs/sysfs/dir.c:59
create_dir lib/kobject.c:73 [inline]
kobject_add_internal+0x2c4/0x9b0 lib/kobject.c:240
kobject_add_varg lib/kobject.c:374 [inline]
kobject_add+0x16e/0x240 lib/kobject.c:426
elv_register_queue+0xd3/0x2a0 block/elevator.c:462
blk_register_queue+0x37e/0x500 block/blk-sysfs.c:874
add_disk_fwnode+0x911/0x13a0 block/genhd.c:505
add_disk include/linux/blkdev.h:779 [inline]
nbd_dev_add+0x78e/0xbb0 drivers/block/nbd.c:1963
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

-> #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
queue_wb_lat_store+0x187/0x3d0 block/blk-sysfs.c:596
queue_attr_store+0x270/0x310 block/blk-sysfs.c:799
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:591 [inline]
vfs_write+0x5ba/0x1180 fs/read_write.c:684
ksys_write+0x12a/0x240 fs/read_write.c:736
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)#62

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&q->q_usage_counter(io)#62);
lock(fs_reclaim);
lock(&q->q_usage_counter(io)#62);
lock(&q->elevator_lock);

*** DEADLOCK ***

6 locks held by syz-executor134/5835:
#0: ffff88802ae609b8 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x2a2/0x370 fs/file.c:1213
#1: ffff888024454420 (sb_writers#7){.+.+}-{0:0}, at: ksys_write+0x12a/0x240 fs/read_write.c:736
#2: ffff888033450088 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x28f/0x510 fs/kernfs/file.c:325
#3: ffff888140fb55a8 (kn->active#47){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2b2/0x510 fs/kernfs/file.c:326
#4: ffff8880260c0a68 (&q->q_usage_counter(io)#62){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:215
#5: ffff8880260c0aa0 (&q->q_usage_counter(queue)#14){+.+.}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:215

stack backtrace:
CPU: 1 UID: 0 PID: 5835 Comm: syz-executor134 Not tainted 6.15.0-rc2-syzkaller #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+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
queue_wb_lat_store+0x187/0x3d0 block/blk-sysfs.c:596
queue_attr_store+0x270/0x310 block/blk-sysfs.c:799
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:591 [inline]
vfs_write+0x5ba/0x1180 fs/read_write.c:684
ksys_write+0x12a/0x240 fs/read_write.c:736
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: 0033:0x7f217454ee39
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 c1 17 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:00007f21744e8218 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f21745d8318 RCX: 00007f217454ee39
RDX: 00000000fffffdef RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007f21745d8310 R08: 00007ffee7802e27 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f21745a6004
R13: 0000200000000150 R14: 0000200000000140 R15: 00000000fffffdef
</TASK>

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