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Sun, 01 Nov 2026 19:47:38 -0800 (PST) Date: Sun, 01 Nov 2026 19:47:38 -0800 X-Google-Appengine-App-Id: s~syzkaller X-Google-Appengine-App-Id-Alias: syzkaller Message-ID: <6ae807da.c8ffc79a.e3697.002b.GAE@google.com> Subject: [syzbot] [block?] INFO: task hung in blk_mq_freeze_queue_nomemsave (3) From: syzbot To: axboe@kernel.dk, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com Content-Type: text/plain; charset="UTF-8" Hello, syzbot found the following issue on: HEAD commit: 7a19f0cc94ad Merge branch 'for-next/core' into for-kernelci git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci console output: https://syzkaller.appspot.com/x/log.txt?x=10d2b5e1580000 kernel config: https://syzkaller.appspot.com/x/.config?x=6602d83d2f34eb80 dashboard link: https://syzkaller.appspot.com/bug?extid=a31cda410a4ccd419712 compiler: Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8 userspace arch: arm64 syz repro: https://syzkaller.appspot.com/x/repro.syz?x=17c11a8d580000 C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10211a8d580000 Downloadable assets: disk image: https://storage.googleapis.com/syzbot-assets/26f2e4202da9/disk-7a19f0cc.raw.xz vmlinux: https://storage.googleapis.com/syzbot-assets/1e23a9081d19/vmlinux-7a19f0cc.xz kernel image: https://storage.googleapis.com/syzbot-assets/b51f551829ee/Image-7a19f0cc.gz.xz AI-generated root cause summary: The loop driver's backing file validation in loop_validate_file() and loop_check_backing_file() only checks for the presence of read_iter/write_iter operations, S_ISREG/S_ISBLK inode mode, and direct loop-to-loop device recursion. Because sysfs/kernfs attribute files are regular files (S_IFREG) and implement read_iter/write_iter, a loop device can be configured with its own sysfs queue attribute (such as /sys/block/loop0/queue/scheduler) as its backing file. When an I/O request is submitted to the loop device, loop_workfn processes the request while holding a reference on the queue's q->q_usage_counter and writes to the sysfs file via lo_rw_aio(). The sysfs write handler (elv_iosched_store -> elevator_change -> blk_mq_freeze_queue) then attempts to freeze the loop device's own queue, waiting indefinitely in blk_mq_freeze_queue_wait() for q->q_usage_counter to reach zero, causing an unresolvable self-deadlock. full analysis: https://syzkaller.appspot.com/x/ai_root_cause?x=a31cda410a4ccd419712 IMPORTANT: if you fix the issue, please add the following tag to the commit: Reported-by: syzbot+a31cda410a4ccd419712@syzkaller.appspotmail.com INFO: task kworker/u8:1:13 blocked for more than 143 seconds. Not tainted syzkaller #0 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:kworker/u8:1 state:D stack:0 pid:13 tgid:13 ppid:2 task_flags:0x4388060 flags:0x00000010 Workqueue: loop0 loop_workfn Call trace: __switch_to+0x2b4/0x6e4 arch/arm64/kernel/process.c:775 (T) context_switch kernel/sched/core.c:5533 [inline] __schedule+0x135c/0x2b00 kernel/sched/core.c:7284 __schedule_loop kernel/sched/core.c:7361 [inline] schedule+0x13c/0x20c kernel/sched/core.c:7376 blk_mq_freeze_queue_wait+0xe0/0x160 block/blk-mq.c:191 blk_mq_freeze_queue_nomemsave+0x28/0x38 block/blk-mq.c:207 blk_mq_freeze_queue include/linux/blk-mq.h:956 [inline] elevator_change+0x174/0x35c block/elevator.c:668 elv_iosched_store+0x2a8/0x3bc block/elevator.c:826 queue_attr_store+0x1c0/0x24c block/blk-sysfs.c:906 sysfs_kf_write+0xe0/0x108 fs/sysfs/file.c:145 kernfs_fop_write_iter+0x260/0x3b8 fs/kernfs/file.c:345 lo_rw_aio+0xaa0/0xd04 include/linux/percpu-rwsem.h:-1 do_req_filebacked drivers/block/loop.c:-1 [inline] loop_handle_cmd drivers/block/loop.c:1949 [inline] loop_process_work+0x4c8/0x960 drivers/block/loop.c:1984 loop_workfn+0x48/0x58 drivers/block/loop.c:2008 process_one_work kernel/workqueue.c:3396 [inline] process_scheduled_works+0x91c/0x1250 kernel/workqueue.c:3479 worker_thread+0x798/0xbd0 kernel/workqueue.c:3560 kthread+0x304/0x3d4 kernel/kthread.c:436 ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:854 INFO: task syz.0.17:4976 blocked in I/O wait for more than 143 seconds. Not tainted syzkaller #0 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:syz.0.17 state:D stack:0 pid:4976 tgid:4976 ppid:4887 task_flags:0x400140 flags:0x00800001 Call trace: __switch_to+0x2b4/0x6e4 arch/arm64/kernel/process.c:775 (T) context_switch kernel/sched/core.c:5533 [inline] __schedule+0x135c/0x2b00 kernel/sched/core.c:7284 __schedule_loop kernel/sched/core.c:7361 [inline] schedule+0x13c/0x20c kernel/sched/core.c:7376 io_schedule+0x1c/0xe0 kernel/sched/core.c:8220 folio_wait_bit_common+0x6b8/0xa30 mm/filemap.c:1329 folio_wait_bit+0x30/0x40 mm/filemap.c:1473 folio_wait_writeback+0x9c/0xf8 mm/page-writeback.c:3062 __filemap_fdatawait_range+0x168/0x260 mm/filemap.c:532 filemap_write_and_wait_range+0xe0/0x114 mm/filemap.c:692 filemap_write_and_wait include/linux/pagemap.h:65 [inline] sync_blockdev block/bdev.c:275 [inline] bdev_release+0x114/0x328 block/bdev.c:1171 blkdev_release+0x20/0x34 block/fops.c:687 __fput+0x338/0x74c fs/file_table.c:512 ____fput+0x20/0x30 fs/file_table.c:541 task_work_run+0x1cc/0x25c kernel/task_work.c:233 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline] __exit_to_user_mode_loop kernel/entry/common.c:70 [inline] exit_to_user_mode_loop+0x10c/0x17c kernel/entry/common.c:101 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline] syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline] arm64_syscall_exit_to_user_mode arch/arm64/kernel/entry-common.c:146 [inline] el0_svc+0x12c/0x1d8 arch/arm64/kernel/entry-common.c:797 el0t_64_sync_handler+0x44/0x104 arch/arm64/kernel/entry-common.c:815 el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:606 Showing all locks held in the system: locks held by kworker/u8:1/13: 7, on CPU#1: #0: ffff0000c3bd9940 ((wq_completion)loop0){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3371 [inline] #0: ffff0000c3bd9940 ((wq_completion)loop0){+.+.}-{0:0}, at: process_scheduled_works+0x8c8/0x1250 kernel/workqueue.c:3479 #1: ffff80008f997c20 ((work_completion)(&worker->work)){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3371 [inline] #1: ffff80008f997c20 ((work_completion)(&worker->work)){+.+.}-{0:0}, at: process_scheduled_works+0x8d0/0x1250 kernel/workqueue.c:3479 #2: ffff0000d6c59c80 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x1b0/0x3b8 fs/kernfs/file.c:336 #3: ffff0000c77fe878 (kn->active#57){.+.+}-{0:0}, at: kernfs_get_active_of fs/kernfs/file.c:73 [inline] #3: ffff0000c77fe878 (kn->active#57){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x1f0/0x3b8 fs/kernfs/file.c:337 #4: ffff0000c77b83a0 (&set->update_nr_hwq_lock){++++}-{4:4}, at: elv_iosched_store+0x260/0x3bc block/elevator.c:821 #5: ffff0000c7783810 (&q->q_usage_counter(io)#17){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x20/0x38 block/blk-mq.c:206 #6: ffff0000c7783848 (&q->q_usage_counter(queue)){+.+.}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x20/0x38 block/blk-mq.c:206 locks held by khungtaskd/31: 1, last CPU#0: #0: ffff8000899e1820 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline] #0: ffff8000899e1820 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline] #0: ffff8000899e1820 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x4c/0x188 kernel/locking/lockdep.c:6871 locks held by kworker/0:2/875: 3, on CPU#0: #0: ffff0000c002b540 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3371 [inline] #0: ffff0000c002b540 ((wq_completion)events){+.+.}-{0:0}, at: process_scheduled_works+0x8c8/0x1250 kernel/workqueue.c:3479 #1: ffff800094497c20 ((work_completion)(&data->fib_event_work)){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3371 [inline] #1: ffff800094497c20 ((work_completion)(&data->fib_event_work)){+.+.}-{0:0}, at: process_scheduled_works+0x8d0/0x1250 kernel/workqueue.c:3479 #2: ffff0000e922e250 (&data->fib_lock){+.+.}-{4:4}, at: nsim_fib_event_work+0x198/0x320 drivers/net/netdevsim/fib.c:1490 locks held by getty/4522: 2, on CPU#1: #0: ffff0000cf9db0a0 (&tty->ldisc_sem){++++}-{0:0}, at: ldsem_down_read+0x3c/0x4c drivers/tty/tty_ldsem.c:340 #1: ffff8000931fe2e8 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x348/0xf78 drivers/tty/n_tty.c:2211 ============================================= --- This report is generated by a bot. It may contain errors. See https://goo.gl/tpsmEJ for more information about syzbot. syzbot engineers can be reached at syzkaller@googlegroups.com. syzbot will keep track of this issue. See: https://goo.gl/tpsmEJ#status for how to communicate with syzbot. If the report is already addressed, let syzbot know by replying with: #syz fix: exact-commit-title 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. If you want to overwrite report's subsystems, reply with: #syz set subsystems: new-subsystem (See the list of subsystem names on the web dashboard) If the report is a duplicate of another one, reply with: #syz dup: exact-subject-of-another-report If you want to undo deduplication, reply with: #syz undup