Re: [PATCH 2/3] io-wq: fix no lock protection of acct->nr_worker

[Jens Axboe <axboe@kernel.dk>]

On 8/7/21 3:56 AM, Hao Xu wrote:
> 在 2021/8/6 下午10:27, Jens Axboe 写道:
>> On Thu, Aug 5, 2021 at 4:05 AM Hao Xu <haoxu@linux.alibaba.com> wrote:
>>>
>>> There is an acct->nr_worker visit without lock protection. Think about
>>> the case: two callers call io_wqe_wake_worker(), one is the original
>>> context and the other one is an io-worker(by calling
>>> io_wqe_enqueue(wqe, linked)), on two cpus paralelly, this may cause
>>> nr_worker to be larger than max_worker.
>>> Let's fix it by adding lock for it, and let's do nr_workers++ before
>>> create_io_worker. There may be a edge cause that the first caller fails
>>> to create an io-worker, but the second caller doesn't know it and then
>>> quit creating io-worker as well:
>>>
>>> say nr_worker = max_worker - 1
>>>          cpu 0                        cpu 1
>>>     io_wqe_wake_worker()          io_wqe_wake_worker()
>>>        nr_worker < max_worker
>>>        nr_worker++
>>>        create_io_worker()         nr_worker == max_worker
>>>           failed                  return
>>>        return
>>>
>>> But the chance of this case is very slim.
>>>
>>> Fixes: 685fe7feedb9 ("io-wq: eliminate the need for a manager thread")
>>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>>> ---
>>>   fs/io-wq.c | 17 ++++++++++++-----
>>>   1 file changed, 12 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/fs/io-wq.c b/fs/io-wq.c
>>> index cd4fd4d6268f..88d0ba7be1fb 100644
>>> --- a/fs/io-wq.c
>>> +++ b/fs/io-wq.c
>>> @@ -247,9 +247,14 @@ static void io_wqe_wake_worker(struct io_wqe *wqe, struct io_wqe_acct *acct)
>>>          ret = io_wqe_activate_free_worker(wqe);
>>>          rcu_read_unlock();
>>>
>>> -       if (!ret && acct->nr_workers < acct->max_workers) {
>>> -               atomic_inc(&acct->nr_running);
>>> -               atomic_inc(&wqe->wq->worker_refs);
>>> +       if (!ret) {
>>> +               raw_spin_lock_irq(&wqe->lock);
>>> +               if (acct->nr_workers < acct->max_workers) {
>>> +                       atomic_inc(&acct->nr_running);
>>> +                       atomic_inc(&wqe->wq->worker_refs);
>>> +                       acct->nr_workers++;
>>> +               }
>>> +               raw_spin_unlock_irq(&wqe->lock);
>>>                  create_io_worker(wqe->wq, wqe, acct->index);
>>>          }
>>>   }
>>
>> There's a pretty grave bug in this patch, in that you no call
>> create_io_worker() unconditionally. This causes obvious problems with
>> misaccounting, and stalls that hit the idle timeout...
>>
> This is surely a silly mistake, I'll check this patch and the 3/3 again.

Please do - and please always run the full set of tests before sending
out changes like this, you would have seen the slower runs and/or
timeouts from the regression suite. I ended up wasting time on this
thinking it was a change I made that broke it, before then debugging
this one.

-- 
Jens Axboe