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

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

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