Re: [PATCH 2/2] io_uring: add memory barrier to synchronize io_kiocb's result and iopoll_completed

Jens Axboe
2020-06-15T15:35:14Z

On 6/15/20 9:32 AM, Xiaoguang Wang wrote:
> hi,
>
>> On 6/15/20 8:48 AM, Xiaoguang Wang wrote:
>>> hi,
>>>
>>>> On 6/15/20 3:24 AM, Xiaoguang Wang wrote:
>>>>> In io_complete_rw_iopoll(), stores to io_kiocb's result and iopoll
>>>>> completed are two independent store operations, to ensure that once
>>>>> iopoll_completed is ture and then req->result must been perceived by
>>>>> the cpu executing io_do_iopoll(), proper memory barrier should be used.
>>>>>
>>>>> And in io_do_iopoll(), we check whether req->result is EAGAIN, if it is,
>>>>> we'll need to issue this io request using io-wq again. In order to just
>>>>> issue a single smp_rmb() on the completion side, move the re-submit work
>>>>> to io_iopoll_complete().
>>>>
>>>> Did you actually test this one?
>>> I only run test cases in liburing/test in a vm.
>>>
>>>>
>>>>> @@ -1736,11 +1748,20 @@ static void io_iopoll_complete(struct io_ring_ctx *ctx, unsigned int *nr_events,
>>>>> {
>>>>> struct req_batch rb;
>>>>> struct io_kiocb *req;
>>>>> + LIST_HEAD(again);
>>>>> +
>>>>> + /* order with ->result store in io_complete_rw_iopoll() */
>>>>> + smp_rmb();
>>>>>
>>>>> rb.to_free = rb.need_iter = 0;
>>>>> while (!list_empty(done)) {
>>>>> int cflags = 0;
>>>>>
>>>>> + if (READ_ONCE(req->result) == -EAGAIN) {
>>>>> + req->iopoll_completed = 0;
>>>>> + list_move_tail(&req->list, &again);
>>>>> + continue;
>>>>> + }
>>>>> req = list_first_entry(done, struct io_kiocb, list);
>>>>> list_del(&req->list);
>>>>>
>>>>
>>>> You're using 'req' here before you initialize it...
>>> Sorry, next time when I submit patches, I'll construct test cases which
>>> will cover my codes changes.
>>
>> I'm surprised the compiler didn't complain, or that the regular testing
>> didn't barf on it.
> I'm also surprised, will try to find the reason.
> And indeed the iopoll test case failed, but below command displayed nothing:
> [lege@localhost test]$ sudo ./iopoll
> Then I considered this test case pass wrongly.
>
> dmesg show errors:
> [ 127.806945] ==================================================================
> [ 127.806983] BUG: KASAN: use-after-free in io_iopoll_complete+0xbb/0x980
> [ 127.806989] Read of size 4 at addr ffff8886e3e98808 by task io_uring-sq/1643
>
> [ 127.806999] CPU: 16 PID: 1643 Comm: io_uring-sq Not tainted 5.7.0+ #501
> [ 127.807013] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.1-0-g0551a4be2c-prebuilt.qemu-project.org 04/01/2014
> [ 127.807021] Call Trace:
> [ 127.807040] dump_stack+0x75/0xa0
> [ 127.807047] ? io_iopoll_complete+0xbb/0x980
> [ 127.807062] print_address_description.constprop.6+0x1a/0x220
> [ 127.807086] ? _raw_write_lock_irqsave+0xd0/0xd0
> [ 127.807092] ? io_free_req_many.part.79+0x208/0x2a0
> [ 127.807107] ? __rcu_read_unlock+0x37/0x200
> [ 127.807112] ? io_iopoll_complete+0xbb/0x980
> [ 127.807117] ? io_iopoll_complete+0xbb/0x980
> [ 127.807122] kasan_report.cold.9+0x1f/0x42
> [ 127.807128] ? io_iopoll_complete+0xbb/0x980
> [ 127.807133] io_iopoll_complete+0xbb/0x980
> [ 127.807138] ? io_timeout_fn+0x140/0x140
> [ 127.807150] ? __switch_to+0x2e9/0x5a0
> [ 127.807157] io_iopoll_getevents+0x287/0x310
> [ 127.807163] ? io_iopoll_complete+0x980/0x980
> [ 127.807172] ? finish_wait+0xcb/0xf0
> [ 127.807179] io_sq_thread+0x1c1/0x600
> [ 127.807185] ? __ia32_sys_io_uring_enter+0x450/0x450
> [ 127.807194] ? preempt_count_add+0x77/0xd0
> [ 127.807200] ? _raw_spin_lock_irqsave+0x84/0xd0
> [ 127.807206] ? _raw_write_lock_irqsave+0xd0/0xd0
> [ 127.807210] ? finish_wait+0xf0/0xf0
> [ 127.807215] ? preempt_count_sub+0x18/0xc0
> [ 127.807224] ? __kthread_parkme+0xaf/0xd0
> [ 127.807231] ? __ia32_sys_io_uring_enter+0x450/0x450
> [ 127.807235] kthread+0x1e4/0x210
> [ 127.807241] ? kthread_create_on_node+0xa0/0xa0
> [ 127.807246] ret_from_fork+0x22/0x30

There you go, so it did fail, just didn't register as a failure. I should
probably add a dmesg check for the liburing tests, and fail a test if
we trigger a WARNING or BUG condition. I'll look into that.

--
Jens Axboe