Re: [PATCH 1/4] io-wq: tweak return value of io_wqe_create_worker()

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

在 2021/9/13 上午5:34, Jens Axboe 写道:
> On 9/12/21 1:02 PM, Hao Xu wrote:
>> 在 2021/9/13 上午2:10, Jens Axboe 写道:
>>> On 9/11/21 1:40 PM, Hao Xu wrote:
>>>> The return value of io_wqe_create_worker() should be false if we cannot
>>>> create a new worker according to the name of this function.
>>>>
>>>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>>>> ---
>>>>    fs/io-wq.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/io-wq.c b/fs/io-wq.c
>>>> index 382efca4812b..1b102494e970 100644
>>>> --- a/fs/io-wq.c
>>>> +++ b/fs/io-wq.c
>>>> @@ -267,7 +267,7 @@ static bool io_wqe_create_worker(struct io_wqe *wqe, struct io_wqe_acct *acct)
>>>>    		return create_io_worker(wqe->wq, wqe, acct->index);
>>>>    	}
>>>>    
>>>> -	return true;
>>>> +	return false;
>>>>    }
>>>
>>> I think this is just a bit confusing. It's not an error case, we just
>>> didn't need to create a worker. So don't return failure, or the caller
>>> will think that we failed while we did not.
>> hmm, I think it is an error case----'we failed to create a new worker
>> since nr_worker == max_worker'. nr_worker == max_worker doesn't mean
>> 'no need', we may meet situation describled in 4/4: max_worker is 1,
> 
> But that's not an error case in the sense of "uh oh, we need to handle
> this as an error". If we're at the max worker count, the work simply has
> to wait for another work to be done and process it.
> 
>> currently 1 worker is running, and we return true here:
>>
>>             did_create = io_wqe_create_worker(wqe, acct);
>>
>>                //*******nr_workers changes******//
>>
>>             if (unlikely(!did_create)) {
>>                     raw_spin_lock(&wqe->lock);
>>                     /* fatal condition, failed to create the first worker */
>>                     if (!acct->nr_workers) {
>>                             raw_spin_unlock(&wqe->lock);
>>                             goto run_cancel;
>>                     }
>>                     raw_spin_unlock(&wqe->lock);
>>             }
>>
>> we will miss the next check, but we have to do the check, since
>> number of workers may decrease to 0 in //******// place.
> 
> If that happens, then the work that we have inserted has already been
> run. Otherwise how else could we have dropped to zero workers?
> 
Sorry, I see. I forgot the fix moved the place of nr_workers...
There is no problems now. Thanks for explanation, Jens.

  io_wqe_enqueue                   worker1
                                no work there and timeout
                                nr_workers--(after fix)
                                unlock(wqe->lock)
  ->insert work

  ->io_wqe_create_worker

                                ->io_worker_exit
                                  ->nr_workers--(before fix)