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

Crash report:
======================================================
WARNING: possible circular locking dependency detected
6.14.0-syzkaller-05877-g1a9239bb4253 #0 Not tainted
------------------------------------------------------
syz-executor192/5823 is trying to acquire lock:
ffff888144302cd8 (&q->elevator_lock){+.+.}-{4:4}, at: elv_iosched_store+0x201/0x5f0 block/elevator.c:735

but task is already holding lock:
ffff8881443027a8 (&q->q_usage_counter(io)#58){++++}-{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)#58){++++}-{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:3853 [inline]
       fs_reclaim_acquire+0x102/0x150 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+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:587 [inline]
       __mutex_lock+0x19a/0xb00 kernel/locking/mutex.c:732
       elv_iosched_store+0x201/0x5f0 block/elevator.c:735
       queue_attr_store+0x270/0x310 block/blk-sysfs.c:799
       sysfs_kf_write+0x117/0x170 fs/sysfs/file.c:139
       kernfs_fop_write_iter+0x349/0x510 fs/kernfs/file.c:334
       iter_file_splice_write+0x91c/0x1150 fs/splice.c:738
       do_splice_from fs/splice.c:935 [inline]
       direct_splice_actor+0x18f/0x6c0 fs/splice.c:1158
       splice_direct_to_actor+0x342/0xa30 fs/splice.c:1102
       do_splice_direct_actor fs/splice.c:1201 [inline]
       do_splice_direct+0x174/0x240 fs/splice.c:1227
       do_sendfile+0xafd/0xe50 fs/read_write.c:1368
       __do_sys_sendfile64 fs/read_write.c:1429 [inline]
       __se_sys_sendfile64 fs/read_write.c:1415 [inline]
       __x64_sys_sendfile64+0x1d8/0x220 fs/read_write.c:1415
       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)#58

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&q->q_usage_counter(io)#58);
                               lock(fs_reclaim);
                               lock(&q->q_usage_counter(io)#58);
  lock(&q->elevator_lock);

 *** DEADLOCK ***

5 locks held by syz-executor192/5823:
 #0: ffff888035bd2420 (sb_writers#7){.+.+}-{0:0}, at: splice_direct_to_actor+0x342/0xa30 fs/splice.c:1102
 #1: ffff888033e17088 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x287/0x510 fs/kernfs/file.c:325
 #2: ffff8880258933c8 (kn->active#47){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2aa/0x510 fs/kernfs/file.c:326
 #3: ffff8881443027a8 (&q->q_usage_counter(io)#58){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:215
 #4: ffff8881443027e0 (&q->q_usage_counter(queue)#10){+.+.}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:215

stack backtrace:
CPU: 0 UID: 0 PID: 5823 Comm: syz-executor192 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+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:587 [inline]
 __mutex_lock+0x19a/0xb00 kernel/locking/mutex.c:732
 elv_iosched_store+0x201/0x5f0 block/elevator.c:735
 queue_attr_store+0x270/0x310 block/blk-sysfs.c:799
 sysfs_kf_write+0x117/0x170 fs/sysfs/file.c:139
 kernfs_fop_write_iter+0x349/0x510 fs/kernfs/file.c:334
 iter_file_splice_write+0x91c/0x1150 fs/splice.c:738
 do_splice_from fs/splice.c:935 [inline]
 direct_splice_actor+0x18f/0x6c0 fs/splice.c:1158
 splice_direct_to_actor+0x342/0xa30 fs/splice.c:1102
 do_splice_direct_actor fs/splice.c:1201 [inline]
 do_splice_direct+0x174/0x240 fs/splice.c:1227
 do_sendfile+0xafd/0xe50 fs/read_write.c:1368
 __do_sys_sendfile64 fs/read_write.c:1429 [inline]
 __se_sys_sendfile64 fs/read_write.c:1415 [inline]
 __x64_sys_sendfile64+0x1d8/0x220 fs/read_write.c:1415
 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: 0033:0x7ff2c3abb2e9
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:00007fffc4e88b88 EFLAGS: 00000246 ORIG_RAX: 0000000000000028
RAX: ffffffffffffffda RBX: 00007fffc4e88d58 RCX: 00007ff2c3abb2e9
RDX: 0000000000000000 RSI: 0000000000000003 RDI: 0000000000000003
RBP: 00007ff2c3b2e610 R08: 0000000000000000 R09: 00007fffc4e88d58
R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fffc4e88d48 R14: 0000000000000001 R15: 0000