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

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

在 2021/9/6 下午11:56, 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.
> 
> __io_arm_poll_handler()         |
>    -> vfs_poll()                 |
>                                  | io_async_task_func() // post CQE
>                                  | ...
>                                  | do_apoll_rewait();
>    -> continues after vfs_poll(),|
>       removing poll->head of     |
>       the second poll attempt.   |
> 
> 
Sorry.. a little bit confused by this case, would you mind explain a bit
more..is the right part a system-workqueue context? and is
do_apoll_rewait() io_poll_rewait() function?
> One of the reasons for forbidding multiple apoll's is that it
> might be racy. I haven't looked into this implementation, but
> we should check if there will be problems from that.
> 
> FWIW, putting aside this patchset, the poll/apoll is not in
> the best shape and can use some refactoring.
> 
> 
>> 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);
>> +	}
>> +
>>   	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))
>>
>