Re: [PATCH 4/6] io_uring: let fast poll support multishot

[Hao Xu <haoxu@linux.alibaba.com>]

在 2021/9/9 下午3:01, Hao Xu 写道:
> 在 2021/9/8 下午8:03, Pavel Begunkov 写道:
>> On 9/8/21 12:21 PM, Hao Xu wrote:
>>> 在 2021/9/7 下午2:48, Hao Xu 写道:
>>>> 在 2021/9/7 上午3:04, Pavel Begunkov 写道:
>>>>> On 9/3/21 12:00 PM, Hao Xu wrote:
>>>>>> For operations like accept, multishot is a useful feature, since 
>>>>>> we can
>>>>>> reduce a number of accept sqe. Let's integrate it to fast poll, it 
>>>>>> may
>>>>>> be good for other operations in the future.
>>>>>>
>>>>>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>>>>>> ---
>>>>>>    fs/io_uring.c | 15 ++++++++++++---
>>>>>>    1 file changed, 12 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>>> index d6df60c4cdb9..dae7044e0c24 100644
>>>>>> --- a/fs/io_uring.c
>>>>>> +++ b/fs/io_uring.c
>>>>>> @@ -5277,8 +5277,15 @@ static void io_async_task_func(struct 
>>>>>> io_kiocb *req, bool *locked)
>>>>>>            return;
>>>>>>        }
>>>>>> -    hash_del(&req->hash_node);
>>>>>> -    io_poll_remove_double(req);
>>>>>> +    if (READ_ONCE(apoll->poll.canceled))
>>>>>> +        apoll->poll.events |= EPOLLONESHOT;
>>>>>> +    if (apoll->poll.events & EPOLLONESHOT) {
>>>>>> +        hash_del(&req->hash_node);
>>>>>> +        io_poll_remove_double(req);
>>>>>> +    } else {
>>>>>> +        add_wait_queue(apoll->poll.head, &apoll->poll.wait);
>>>>>
>>>>> It looks like it does both io_req_task_submit() and adding back
>>>>> to the wq, so io_issue_sqe() may be called in parallel with
>>>>> io_async_task_func(). If so, there will be tons of all kind of
>>>>> races.
>>>> IMHO, io_async_task_func() is called in original context one by
>>>> one(except PF_EXITING is set, it is also called in system-wq), so
>>>> shouldn't be parallel case there.
>>> ping...
>>
>> fwiw, the case we're talking about:
>>
>> CPU0                            | CPU1
>> io_async_task_func()            |
>> -> add_wait_queue();            |
>> -> io_req_task_submit();        |
>>                 /* no tw run happened in between */
>>                                  | io_async_task_func()
>>                                  | --> io_req_task_submit()
>>
>> We called io_req_task_submit() twice without running tw in-between,
>> both of the calls use the same req->io_task_work.node field in the
>> request for accounting, and so the second call will screw
>> tctx->task_list and not only by not considering that
>> req->io_task_work.node is already taken/enqueued.
>>
>> io_req_task_work_add() {
>>          wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
>> }
>>
> I guess you mean io_req_task_work_add() called by async_wake() two times:
> io_async_task_func()
> -> add_wait_queue()
>                              async_wake()
>                              ->io_req_task_work_add()
>                              this one mess up the running task_work list
>                              since req->io_task_work.node is in use.
> 
> It seems the current poll_add + multishot logic has this issue too, I'll
> give it a shot(simply clean req->io_task_work.node before running
> req->io_task_work.func should work)
Similar issue for double wait entry since we didn't remove double entry
in interrupt handler:
async_wake() --> io_req_task_work_add()
io_poll_double_wake()-->async_wake()-->io_req_task_work_add()

>>>>>
>>>>>> +    }
>>>>>> +
>>>>>>        spin_unlock(&ctx->completion_lock);
>>>>>>        if (!READ_ONCE(apoll->poll.canceled))
>>>>>> @@ -5366,7 +5373,7 @@ static int io_arm_poll_handler(struct 
>>>>>> io_kiocb *req)
>>>>>>        struct io_ring_ctx *ctx = req->ctx;
>>>>>>        struct async_poll *apoll;
>>>>>>        struct io_poll_table ipt;
>>>>>> -    __poll_t ret, mask = EPOLLONESHOT | POLLERR | POLLPRI;
>>>>>> +    __poll_t ret, mask = POLLERR | POLLPRI;
>>>>>>        int rw;
>>>>>>        if (!req->file || !file_can_poll(req->file))
>>>>>> @@ -5388,6 +5395,8 @@ static int io_arm_poll_handler(struct 
>>>>>> io_kiocb *req)
>>>>>>            rw = WRITE;
>>>>>>            mask |= POLLOUT | POLLWRNORM;
>>>>>>        }
>>>>>> +    if (!(req->flags & REQ_F_APOLL_MULTISHOT))
>>>>>> +        mask |= EPOLLONESHOT;
>>>>>>        /* if we can't nonblock try, then no point in arming a poll 
>>>>>> handler */
>>>>>>        if (!io_file_supports_nowait(req, rw))
>>>>>>
>>>>>
>>>
>>