Re: INFO: rcu detected stall in io_uring_release

[Jens Axboe]

On 4/18/20 10:06 PM, Hillf Danton wrote:
> 
> Sat, 18 Apr 2020 11:59:13 -0700
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:    8f3d9f35 Linux 5.7-rc1
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=115720c3e00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=5d351a1019ed81a2
>> dashboard link: https://syzkaller.appspot.com/bug?extid=66243bb7126c410cefe6
>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>>
>> Unfortunately, I don't have any reproducer for this crash yet.
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+66243bb7126c410cefe6@xxxxxxxxxxxxxxxxxxxxxxxxx
>>
>> rcu: INFO: rcu_preempt self-detected stall on CPU
>> rcu: 	0-....: (10500 ticks this GP) idle=57e/1/0x4000000000000002 softirq=44329/44329 fqs=5245 
>> 	(t=10502 jiffies g=79401 q=2096)
>> NMI backtrace for cpu 0
>> CPU: 0 PID: 23184 Comm: syz-executor.5 Not tainted 5.7.0-rc1-syzkaller #0
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> Call Trace:
>>  <IRQ>
>>  __dump_stack lib/dump_stack.c:77 [inline]
>>  dump_stack+0x188/0x20d lib/dump_stack.c:118
>>  nmi_cpu_backtrace.cold+0x70/0xb1 lib/nmi_backtrace.c:101
>>  nmi_trigger_cpumask_backtrace+0x231/0x27e lib/nmi_backtrace.c:62
>>  trigger_single_cpu_backtrace include/linux/nmi.h:164 [inline]
>>  rcu_dump_cpu_stacks+0x19b/0x1e5 kernel/rcu/tree_stall.h:254
>>  print_cpu_stall kernel/rcu/tree_stall.h:475 [inline]
>>  check_cpu_stall kernel/rcu/tree_stall.h:549 [inline]
>>  rcu_pending kernel/rcu/tree.c:3225 [inline]
>>  rcu_sched_clock_irq.cold+0x55d/0xcfa kernel/rcu/tree.c:2296
>>  update_process_times+0x25/0x60 kernel/time/timer.c:1727
>>  tick_sched_handle+0x9b/0x180 kernel/time/tick-sched.c:176
>>  tick_sched_timer+0x4e/0x140 kernel/time/tick-sched.c:1320
>>  __run_hrtimer kernel/time/hrtimer.c:1520 [inline]
>>  __hrtimer_run_queues+0x5ca/0xed0 kernel/time/hrtimer.c:1584
>>  hrtimer_interrupt+0x312/0x770 kernel/time/hrtimer.c:1646
>>  local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1113 [inline]
>>  smp_apic_timer_interrupt+0x15b/0x600 arch/x86/kernel/apic/apic.c:1138
>>  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
>>  </IRQ>
>> RIP: 0010:io_ring_ctx_wait_and_kill+0x98/0x5a0 fs/io_uring.c:7301
>> Code: 01 00 00 4d 89 f4 48 b8 00 00 00 00 00 fc ff df 4c 89 ed 49 c1 ec 03 48 c1 ed 03 49 01 c4 48 01 c5 eb 1c e8 3a ea 9d ff f3 90 <41> 80 3c 24 00 0f 85 53 04 00 00 48 83 bb 10 01 00 00 00 74 21 e8
>> RSP: 0018:ffffc9000897fdf0 EFLAGS: 00000293 ORIG_RAX: ffffffffffffff13
>> RAX: ffff888024082080 RBX: ffff88808df8e000 RCX: 1ffff9200112ffab
>> RDX: 0000000000000000 RSI: ffffffff81d549c6 RDI: ffff88808df8e300
>> RBP: ffffed1011bf1c2c R08: 0000000000000001 R09: ffffed1011bf1c61
>> R10: ffff88808df8e307 R11: ffffed1011bf1c60 R12: ffffed1011bf1c22
>> R13: ffff88808df8e160 R14: ffff88808df8e110 R15: ffffffff81d54ed0
>>  io_uring_release+0x3e/0x50 fs/io_uring.c:7324
>>  __fput+0x33e/0x880 fs/file_table.c:280
>>  task_work_run+0xf4/0x1b0 kernel/task_work.c:123
>>  tracehook_notify_resume include/linux/tracehook.h:188 [inline]
>>  exit_to_usermode_loop+0x2fa/0x360 arch/x86/entry/common.c:165
>>  prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
>>  syscall_return_slowpath arch/x86/entry/common.c:279 [inline]
>>  do_syscall_64+0x6b1/0x7d0 arch/x86/entry/common.c:305
>>  entry_SYSCALL_64_after_hwframe+0x49/0xb3
> 
> Make io ring ctx's percpu_ref balanced.
> 
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -5904,6 +5904,7 @@ static int io_submit_sqes(struct io_ring
>  fail_req:
>  			io_cqring_add_event(req, err);
>  			io_double_put_req(req);
> +			--submitted;
>  			break;
>  		}

Not sure this is right, need to look closer. But if we post a completion
event, the event has been consumed and should be accounted as such.

-- 
Jens Axboe