Re: [PATCH for-5.15] io_uring: fix lacking of protection for compl_nr

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

On 8/20/21 9:39 PM, Hao Xu wrote:
> 在 2021/8/21 上午2:59, Pavel Begunkov 写道:
>> On 8/20/21 7:40 PM, Hao Xu wrote:
>>> coml_nr in ctx_flush_and_put() is not protected by uring_lock, this
>>> may cause problems when accessing it parallelly.
>>
>> Did you hit any problem? It sounds like it should be fine as is:
>>
>> The trick is that it's only responsible to flush requests added
>> during execution of current call to tctx_task_work(), and those
>> naturally synchronised with the current task. All other potentially
>> enqueued requests will be of someone else's responsibility.
>>
>> So, if nobody flushed requests, we're finely in-sync. If we see
>> 0 there, but actually enqueued a request, it means someone
>> actually flushed it after the request had been added.
>>
>> Probably, needs a more formal explanation with happens-before
>> and so.
> I should put more detail in the commit message, the thing is:
> say coml_nr > 0
> 
>   ctx_flush_and put                  other context
>    if (compl_nr)                      get mutex
>                                       coml_nr > 0
>                                       do flush
>                                           coml_nr = 0
>                                       release mutex
>         get mutex
>            do flush (*)
>         release mutex
> 
> in (*) place, we do a bunch of unnecessary works, moreover, we

I wouldn't care about overhead, that shouldn't be much

> call io_cqring_ev_posted() which I think we shouldn't.

IMHO, users should expect spurious io_cqring_ev_posted(),
though there were some eventfd users complaining before, so
for them we can do

if (state->nr) {
    lock();
    if (state->nr) flush();
    unlock();
}

>>> Fixes: d10299e14aae ("io_uring: inline struct io_comp_state")
>>
>> FWIW, it came much earlier than this commit, IIRC
>>
>> commit 2c32395d8111037ae2cb8cab883e80bcdbb70713
>> Author: Pavel Begunkov <asml.silence@gmail.com>
>> Date:   Sun Feb 28 22:04:53 2021 +0000
>>
>>      io_uring: fix __tctx_task_work() ctx race
>>
>>
>>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>>> ---
>>>   fs/io_uring.c | 7 +++----
>>>   1 file changed, 3 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index c755efdac71f..420f8dfa5327 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -2003,11 +2003,10 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx)
>>>   {
>>>       if (!ctx)
>>>           return;
>>> -    if (ctx->submit_state.compl_nr) {
>>> -        mutex_lock(&ctx->uring_lock);
>>> +    mutex_lock(&ctx->uring_lock);
>>> +    if (ctx->submit_state.compl_nr)
>>>           io_submit_flush_completions(ctx);
>>> -        mutex_unlock(&ctx->uring_lock);
>>> -    }
>>> +    mutex_unlock(&ctx->uring_lock);
>>>       percpu_ref_put(&ctx->refs);
>>>   }
>>>  
>>
> 

-- 
Pavel Begunkov