Re: [PATCH 3/4] io_uring: tweak iopoll return for REQ_F_CQE_SKIP

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

On 12/4/21 22:21, Jens Axboe wrote:
> On 12/4/21 1:49 PM, Pavel Begunkov wrote:
>> Currently, IOPOLL returns the number of completed requests, but with
>> REQ_F_CQE_SKIP there are not the same thing anymore. That may be
>> confusing as non-iopoll wait cares only about CQEs, so make io_do_iopoll
>> return the number of posted CQEs.
>>
>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>> ---
>>   fs/io_uring.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 64add8260abb..ea7a0daa0b3b 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -2538,10 +2538,10 @@ static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
>>   		/* order with io_complete_rw_iopoll(), e.g. ->result updates */
>>   		if (!smp_load_acquire(&req->iopoll_completed))
>>   			break;
>> +		if (unlikely(req->flags & REQ_F_CQE_SKIP))
>> +			continue;
>>   
>> -		if (!(req->flags & REQ_F_CQE_SKIP))
>> -			__io_fill_cqe(ctx, req->user_data, req->result,
>> -				      io_put_kbuf(req));
>> +		__io_fill_cqe(ctx, req->user_data, req->result, io_put_kbuf(req));
>>   		nr_events++;
>>   	}
>>   
> 
> Not sure I follow the logic behind this change. Places like
> io_iopoll_try_reap_events() just need a "did we find anything" return,
> which is independent on whether or not we actually posted CQEs or not.
> Other callers either don't care what the return value is or if it's < 0
> or not (which this change won't affect).
> 
> I feel like I'm missing something here, or that the commit message
> better needs to explain why this change is done.

I was wrong on how I described it, but it means that the problem is in
a different place.

int io_do_iopoll() {
	return nr_events;
}

int io_iopoll_check() {
	do {
		nr_events += io_do_iopoll();
	while (nr_events < min && ...);
}

And "events" there better to be CQEs, otherwise the semantics
of @min + CQE_SKIP is not very clear and mismatches non-IOPOLL.

-- 
Pavel Begunkov