On 1/31/21 7:39 AM, Hao Xu wrote:
> Abaci reported this issue:
>
> #[ 605.170872] INFO: task kworker/u4:1:53 blocked for more than 143 seconds.
> [ 605.172123] Not tainted 5.10.0+ #1
> [ 605.172811] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 605.173915] task:kworker/u4:1 state:D stack: 0 pid: 53 ppid: 2 flags:0x00004000
> [ 605.175130] Workqueue: events_unbound io_ring_exit_work
> [ 605.175931] Call Trace:
> [ 605.176334] __schedule+0xe0e/0x25a0
> [ 605.176971] ? firmware_map_remove+0x1a1/0x1a1
> [ 605.177631] ? write_comp_data+0x2a/0x80
> [ 605.178272] schedule+0xd0/0x270
> [ 605.178811] schedule_timeout+0x6b6/0x940
> [ 605.179415] ? mark_lock.part.0+0xca/0x1420
> [ 605.180062] ? usleep_range+0x170/0x170
> [ 605.180684] ? wait_for_completion+0x16d/0x280
> [ 605.181392] ? mark_held_locks+0x9e/0xe0
> [ 605.182079] ? rwlock_bug.part.0+0x90/0x90
> [ 605.182853] ? lockdep_hardirqs_on_prepare+0x286/0x400
> [ 605.183817] wait_for_completion+0x175/0x280
> [ 605.184713] ? wait_for_completion_interruptible+0x340/0x340
> [ 605.185611] ? _raw_spin_unlock_irq+0x24/0x30
> [ 605.186307] ? migrate_swap_stop+0x9c0/0x9c0
> [ 605.187046] kthread_park+0x127/0x1c0
> [ 605.187738] io_sq_thread_stop+0xd5/0x530
> [ 605.188459] io_ring_exit_work+0xb1/0x970
> [ 605.189207] process_one_work+0x92c/0x1510
> [ 605.189947] ? pwq_dec_nr_in_flight+0x360/0x360
> [ 605.190682] ? rwlock_bug.part.0+0x90/0x90
> [ 605.191430] ? write_comp_data+0x2a/0x80
> [ 605.192207] worker_thread+0x9b/0xe20
> [ 605.192900] ? process_one_work+0x1510/0x1510
> [ 605.193599] kthread+0x353/0x460
> [ 605.194154] ? _raw_spin_unlock_irq+0x24/0x30
> [ 605.194910] ? kthread_create_on_node+0x100/0x100
> [ 605.195821] ret_from_fork+0x1f/0x30
> [ 605.196605]
> [ 605.196605] Showing all locks held in the system:
> [ 605.197598] 1 lock held by khungtaskd/25:
> [ 605.198301] #0: ffffffff8b5f76a0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire.constprop.0+0x0/0x30
> [ 605.199914] 3 locks held by kworker/u4:1/53:
> [ 605.200609] #0: ffff888100109938 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x82a/0x1510
> [ 605.202108] #1: ffff888100e47dc0 ((work_completion)(&ctx->exit_work)){+.+.}-{0:0}, at: process_one_work+0x85e/0x1510
> [ 605.203681] #2: ffff888116931870 (&sqd->lock){+.+.}-{3:3}, at: io_sq_thread_park.part.0+0x19/0x50
> [ 605.205183] 3 locks held by systemd-journal/161:
> [ 605.206037] 1 lock held by syslog-ng/254:
> [ 605.206674] 2 locks held by agetty/311:
> [ 605.207292] #0: ffff888101097098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x27/0x80
> [ 605.208715] #1: ffffc900000332e8 (&ldata->atomic_read_lock){+.+.}-{3:3}, at: n_tty_read+0x222/0x1bb0
> [ 605.210131] 2 locks held by bash/677:
> [ 605.210723] #0: ffff88810419a098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x27/0x80
> [ 605.212105] #1: ffffc900000512e8 (&ldata->atomic_read_lock){+.+.}-{3:3}, at: n_tty_read+0x222/0x1bb0
> [ 605.213777]
> [ 605.214151] =============================================
>
> I believe this is caused by the follow race:
>
> (ctx_list is empty now)
> => io_put_sq_data |
> ==> kthread_park(sqd->thread); |
> ====> set KTHREAD_SHOULD_PARK |
> ====> wake_up_process(k) | sq thread is running
> |
> |
> | needs_sched is true since no ctx,
> | so TASK_INTERRUPTIBLE set and schedule
> | out then never wake up again
> |
> ====> wait_for_completion |
> (stuck here)
>
> So check if sqthread gets park flag right before schedule().
> since ctx_list is always empty when this problem happens, here I put
> kthread_should_park() before setting the wakeup flag(ctx_list is empty
> so this for loop is fast), where is close enough to schedule(). The
> problem doesn't show again in my repro testing after this fix.
Applied, thanks.
--
Jens Axboe
