Re: [syzbot] [net] [virt] INFO: task hung in __vhost_worker_flush

[syzbot <syzbot+7f3bbe59e8dd2328a990@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
INFO: task hung in __vhost_worker_flush

INFO: task syz.0.17:6484 blocked 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:27928 pid:6484  tgid:6484  ppid:6375   task_flags:0x400040 flags:0x00004004
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5357 [inline]
 __schedule+0x1190/0x5de0 kernel/sched/core.c:6961
 __schedule_loop kernel/sched/core.c:7043 [inline]
 schedule+0xe7/0x3a0 kernel/sched/core.c:7058
 schedule_timeout+0x257/0x290 kernel/time/sleep_timeout.c:75
 do_wait_for_common kernel/sched/completion.c:100 [inline]
 __wait_for_common+0x2fc/0x4e0 kernel/sched/completion.c:121
 __vhost_worker_flush+0x1a8/0x1d0 drivers/vhost/vhost.c:296
 vhost_worker_flush drivers/vhost/vhost.c:303 [inline]
 vhost_dev_flush+0xac/0x110 drivers/vhost/vhost.c:313
 vhost_vsock_flush drivers/vhost/vsock.c:698 [inline]
 vhost_vsock_dev_release+0x19f/0x400 drivers/vhost/vsock.c:750
 __fput+0x3ff/0xb70 fs/file_table.c:468
 task_work_run+0x14d/0x240 kernel/task_work.c:227
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 exit_to_user_mode_loop+0xeb/0x110 kernel/entry/common.c:43
 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
 do_syscall_64+0x3f6/0x4c0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f4ae478ebe9
RSP: 002b:00007ffe922fd5f8 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 00007f4ae49c7da0 RCX: 00007f4ae478ebe9
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 00007f4ae49c7da0 R08: 0000000000000000 R09: 00000008922fd8ef
R10: 00007f4ae49c7cb0 R11: 0000000000000246 R12: 000000000001e366
R13: 00007ffe922fd6f0 R14: ffffffffffffffff R15: 00007ffe922fd710
 </TASK>

Showing all locks held in the system:
1 lock held by khungtaskd/31:
 #0: ffffffff8e5c1260 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
 #0: ffffffff8e5c1260 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:841 [inline]
 #0: ffffffff8e5c1260 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x36/0x1c0 kernel/locking/lockdep.c:6775
2 locks held by kworker/u8:2/36:
1 lock held by klogd/5209:
 #0: ffff8880b853a318 (&rq->__lock){-.-.}-{2:2}, at: raw_spin_rq_lock_nested+0x29/0x130 kernel/sched/core.c:636
2 locks held by getty/5608:
 #0: ffff88814df690a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x24/0x80 drivers/tty/tty_ldisc.c:243
 #1: ffffc900036bb2f0 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x41b/0x14f0 drivers/tty/n_tty.c:2222

=============================================

NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 31 Comm: khungtaskd Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 nmi_cpu_backtrace+0x27b/0x390 lib/nmi_backtrace.c:113
 nmi_trigger_cpumask_backtrace+0x29c/0x300 lib/nmi_backtrace.c:62
 trigger_all_cpu_backtrace include/linux/nmi.h:160 [inline]
 check_hung_uninterruptible_tasks kernel/hung_task.c:328 [inline]
 watchdog+0xf0e/0x1260 kernel/hung_task.c:491
 kthread+0x3c5/0x780 kernel/kthread.c:463
 ret_from_fork+0x5d4/0x6f0 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 5911 Comm: kworker/0:5 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Workqueue:  0x0 (wg-crypt-wg0)
RIP: 0010:schedule_debug kernel/sched/core.c:5917 [inline]
RIP: 0010:__schedule+0x1b2/0x5de0 kernel/sched/core.c:6838
Code: 80 3c 02 00 0f 85 1f 53 00 00 48 8b 85 10 ff ff ff 4c 8b 68 20 48 b8 00 00 00 00 00 fc ff df 4c 89 ea 48 c1 ea 03 80 3c 02 00 <0f> 85 ec 52 00 00 49 81 7d 00 9d 6e ac 57 0f 85 24 53 00 00 48 8b
RSP: 0018:ffffc900041efbc8 EFLAGS: 00000246
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff8b94c176
RDX: 1ffff9200083d000 RSI: ffffffff8c162980 RDI: ffff88802740c8a0
RBP: ffffc900041efd80 R08: 0000000000000000 R09: fffffbfff2156a52
R10: ffffffff90ab5297 R11: 0000000000000000 R12: 0000000000000000
R13: ffffc900041e8000 R14: ffff8880b843a300 R15: ffff888033d59840
FS:  0000000000000000(0000) GS:ffff8881246b8000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005555588e15c8 CR3: 000000000e380000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __schedule_loop kernel/sched/core.c:7043 [inline]
 schedule+0xe7/0x3a0 kernel/sched/core.c:7058
 worker_thread+0x2e5/0xf10 kernel/workqueue.c:3415
 kthread+0x3c5/0x780 kernel/kthread.c:463
 ret_from_fork+0x5d4/0x6f0 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>


Tested on:

commit:         b320789d Linux 6.17-rc4
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16446242580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d4703ac89d9e185a
dashboard link: https://syzkaller.appspot.com/bug?extid=7f3bbe59e8dd2328a990
compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
patch:          https://syzkaller.appspot.com/x/patch.diff?x=127a4662580000