On 9/8/21 1:03 PM, Pavel Begunkov wrote:
> 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@xxxxxxxxxxxxxxxxx>
>>>>> ---
>>>>> 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);
> }
fwiw, can be just 1 CPU, just
io_req_task_work_add();
io_req_task_work_add();
task_work_run(); // first one
is buggy in current constraints.
>
>>>>
>>>>> + }
>>>>> +
>>>>> 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
