Re: [PATCH 2/2] io_uring: add memory barrier to synchronize io_kiocb's result and iopoll_completed

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi,

> On 6/15/20 3:24 AM, Xiaoguang Wang wrote:
>> In io_complete_rw_iopoll(), stores to io_kiocb's result and iopoll
>> completed are two independent store operations, to ensure that once
>> iopoll_completed is ture and then req->result must been perceived by
>> the cpu executing io_do_iopoll(), proper memory barrier should be used.
>>
>> And in io_do_iopoll(), we check whether req->result is EAGAIN, if it is,
>> we'll need to issue this io request using io-wq again. In order to just
>> issue a single smp_rmb() on the completion side, move the re-submit work
>> to io_iopoll_complete().
> 
> Did you actually test this one?
I only run test cases in liburing/test in a vm.

> 
>> @@ -1736,11 +1748,20 @@ static void io_iopoll_complete(struct io_ring_ctx *ctx, unsigned int *nr_events,
>>   {
>>   	struct req_batch rb;
>>   	struct io_kiocb *req;
>> +	LIST_HEAD(again);
>> +
>> +	/* order with ->result store in io_complete_rw_iopoll() */
>> +	smp_rmb();
>>   
>>   	rb.to_free = rb.need_iter = 0;
>>   	while (!list_empty(done)) {
>>   		int cflags = 0;
>>   
>> +		if (READ_ONCE(req->result) == -EAGAIN) {
>> +			req->iopoll_completed = 0;
>> +			list_move_tail(&req->list, &again);
>> +			continue;
>> +		}
>>   		req = list_first_entry(done, struct io_kiocb, list);
>>   		list_del(&req->list);
>>   
> 
> You're using 'req' here before you initialize it...
Sorry, next time when I submit patches, I'll construct test cases which
will cover my codes changes.

Regards,
Xiaoguang Wang
>