Re: INFO: rcu detected stall in io_uring_release

From: Pavel Begunkov
Date: Mon Apr 20 2020 - 08:57:22 EST

On 4/20/2020 2:47 PM, Dan Carpenter wrote:
> On Sun, Apr 19, 2020 at 12:06:26PM +0800, 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;
>> }
>
>
> fs/io_uring.c
> 5880 for (i = 0; i < nr; i++) {
> 5881 const struct io_uring_sqe *sqe;
> 5882 struct io_kiocb *req;
> 5883 int err;
> 5884
> 5885 sqe = io_get_sqe(ctx);
> 5886 if (unlikely(!sqe)) {
> 5887 io_consume_sqe(ctx);
> 5888 break;
> 5889 }
> 5890 req = io_alloc_req(ctx, statep);
> 5891 if (unlikely(!req)) {
> 5892 if (!submitted)
> 5893 submitted = -EAGAIN;
> 5894 break;
> 5895 }
> 5896
> 5897 err = io_init_req(ctx, req, sqe, statep, async);
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> On the success path io_init_req() takes some references like:
>
> get_cred(req->work.creds);

If a req have got into io_init_req(), than it'll be put at some point
with io_put_req(). io_req_work_drop_env() called from there will clean
up req->work.creds.

>
> That one is probably buggy and should be put if the call to:
>
> return io_req_set_file(state, req, fd, sqe_flags);
>
> fails... But io_req_set_file() takes some other references if it
> succeeds like percpu_ref_get(req->fixed_file_refs); and it's not clear
> that those are released if io_submit_sqe() fails.

The same should happen with req->fixed_file_refs, though I don't
remember in details.

>
> 5898 io_consume_sqe(ctx);
> 5899 /* will complete beyond this point, count as submitted */
> 5900 submitted++;

Regarding, "--submitted" patch -- we take 1 ctx->refs per request, which
is put in io_put_req(). So after a request passes the line above (5900),
it's ref will be eventually dropped in io_put_req() and friends.

And it's a bit more peculiar because io_submit_sqes() batch-takes N refs
first, and then puts unused back at the end.

> 5901
> 5902 if (unlikely(err)) {
> 5903 fail_req:
> 5904 io_cqring_add_event(req, err);
> 5905 io_double_put_req(req);
> 5906 break;
> 5907 }
> 5908
> 5909 trace_io_uring_submit_sqe(ctx, req->opcode, req->user_data,
> 5910 true, async);
> 5911 err = io_submit_sqe(req, sqe, statep, &link);
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> here
>
> 5912 if (err)
> 5913 goto fail_req;
> 5914 }
>
> regards,
> dan carpenter
>

--
Pavel Begunkov