Re: [PATCH v2 4/4] io_uring: get next req on subm ref drop

[Jens Axboe]

On 3/2/20 11:54 PM, Pavel Begunkov wrote:
> On 03/03/2020 07:26, Jens Axboe wrote:
>> On 3/2/20 1:45 PM, Pavel Begunkov wrote:
>>> Get next request when dropping the submission reference. However, if
>>> there is an asynchronous counterpart (i.e. read/write, timeout, etc),
>>> that would be dangerous to do, so ignore them using new
>>> REQ_F_DONT_STEAL_NEXT flag.
>>
>> Hmm, not so sure I like this one. It's not quite clear to me where we
>> need REQ_F_DONT_STEAL_NEXT. If we have an async component, then we set
>> REQ_F_DONT_STEAL_NEXT. So this is generally the case where our
>> io_put_req() for submit is not the last drop. And for the other case,
>> the put is generally in the caller anyway. So I don't really see what
>> this extra flag buys us?
> 
> Because io_put_work() holds a reference, no async handler can achive req->refs
> == 0, so it won't return next upon dropping the submission ref (i.e. by
> put_find_nxt()). And I want to have next before io_put_work(), to, instead of as
> currently:
> 
> run_work(work);
> assign_cur_work(NULL); // spinlock + unlock worker->lock
> new_work = put_work(work);
> assign_cur_work(new_work); // the second time
> 
> do:
> 
> new_work = run_work(work);
> assign_cur_work(new_work); // need new_work here
> put_work(work);
> 
> 
> The other way:
> 
> io_wq_submit_work() // for all async handlers
> {
> 	...
> 	// Drop submission reference.
> 	// One extra ref will be put in io_put_work() right
> 	// after return, and it'll be done in the same thread
> 	if (atomic_dec_and_get(req) == 1)
> 		steal_next(req);
> }
> 
> Maybe cleaner, but looks fragile as well. Would you prefer it?

I think I prefer that, since it doesn't need random setting of a
no-steal flag throughout. And it should be pretty solid, since we know
that we hold one and that can only be our reference. Just needs a nice
comment explaining that fact as well.

>>> @@ -3943,7 +3947,10 @@ static int io_poll_add(struct io_kiocb *req)
>>>  	if (mask) {
>>>  		io_cqring_ev_posted(ctx);
>>>  		io_put_req(req);
>>> +	} else {
>>> +		req->flags |= REQ_F_DONT_STEAL_NEXT;
>>>  	}
>>> +
>>>  	return ipt.error;
>>>  }
>>
>> Is this racy? I guess it doesn't matter since we're still holding the
>> completion reference.
> 
> It's done by the same thread, that uses it. There could be a race if
> the async counterpart is going to change req->flags, but we tolerate
> false negative (i.e.  put_req() will handle it).

It's relying on the fact that it's the task itself that'll run the task
work, which can't be done by this time. Just caught my eye as something
to look out for.

-- 
Jens Axboe