Re: [PATCH 1/2] io_uring: make the logic clearer for io_sequence_defer

[Jens Axboe <axboe@kernel.dk>]

On 10/10/19 9:17 PM, Jens Axboe wrote:
> On 10/10/19 9:06 PM, Jackie Liu wrote:
>>
>>
>>> 2019年10月11日 10:35，Jens Axboe <axboe@kernel.dk> 写道：
>>>
>>> On 10/10/19 8:24 PM, yangerkun wrote:
>>>>
>>>>
>>>> On 2019/10/9 9:19, Jackie Liu wrote:
>>>>> __io_get_deferred_req is used to get all defer lists, including defer_list
>>>>> and timeout_list, but io_sequence_defer should be only cares about the sequence.
>>>>>
>>>>> Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
>>>>> ---
>>>>>     fs/io_uring.c | 13 +++++--------
>>>>>     1 file changed, 5 insertions(+), 8 deletions(-)
>>>>>
>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>> index 8a0381f1a43b..8ec2443eb019 100644
>>>>> --- a/fs/io_uring.c
>>>>> +++ b/fs/io_uring.c
>>>>> @@ -418,9 +418,7 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
>>>>>     static inline bool io_sequence_defer(struct io_ring_ctx *ctx,
>>>>>     				     struct io_kiocb *req)
>>>>>     {
>>>>> -	/* timeout requests always honor sequence */
>>>>> -	if (!(req->flags & REQ_F_TIMEOUT) &&
>>>>> -	    (req->flags & (REQ_F_IO_DRAIN|REQ_F_IO_DRAINED)) != REQ_F_IO_DRAIN)
>>>>> +	if ((req->flags & (REQ_F_IO_DRAIN|REQ_F_IO_DRAINED)) != REQ_F_IO_DRAIN)
>>>>>     		return false;
>>>>>
>>>>>     	return req->sequence != ctx->cached_cq_tail + ctx->rings->sq_dropped;
>>>>> @@ -435,12 +433,11 @@ static struct io_kiocb *__io_get_deferred_req(struct io_ring_ctx *ctx,
>>>>>     		return NULL;
>>>>>
>>>>>     	req = list_first_entry(list, struct io_kiocb, list);
>>>>> -	if (!io_sequence_defer(ctx, req)) {
>>>>> -		list_del_init(&req->list);
>>>>> -		return req;
>>>>> -	}
>>>>> +	if (!(req->flags & REQ_F_TIMEOUT) && io_sequence_defer(ctx, req))
>>>>> +		return NULL;
>>>> Hi,
>>>>
>>>> For timeout req, we should also compare the sequence to determine to
>>>> return NULL or the req. But now we will return req directly. Actually,
>>>> no need to compare req->flags with REQ_F_TIMEOUT.
>>>
>>> Yes, not sure how I missed this, the patch is broken as-is. I will kill
>>> it, cleanup can be done differently.
>>
>> Sorry for miss it, I don't wanna change the logic, it's not my
>> original meaning.
> 
> No worries, mistakes happen.
> 
>> Personal opinion, timeout list should not be mixed with defer_list,
>> which makes the code more complicated and difficult to understand.
> 
> Not sure why you feel they are mixed? They are in separate lists, but
> they share using the sequence logic. In that respet they are really not
> that different, the sequence will trigger either one of them. Either as
> a timeout, or as a "can now be issued". Hence the code handling them is
> both shared and identical.
> 
> I do agree that the check could be cleaner, which is probably how the
> mistake in this patch happened in the first place.

I think we should just make it clear if the sequence checking is for
one of the paths - we don't want to defer anything based on a timeout,
just the timeout itself. That will also take care of the issue that
yangerkun brought up.

-- 
Jens Axboe