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

[Pavel Begunkov <asml.silence@gmail.com>]

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);
}

>>>
>>>> +    }
>>>> +
>>>>       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))
>>>>
>>>
> 

-- 
Pavel Begunkov