Re: [PATCH 1/3] io_uring: flush completions for fallbacks

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

在 2021/8/20 下午8:26, Pavel Begunkov 写道:
> On 8/20/21 11:16 AM, Hao Xu wrote:
>> 在 2021/8/20 下午5:49, Pavel Begunkov 写道:
>>> On 8/20/21 10:21 AM, Hao Xu wrote:
>>>> 在 2021/8/18 下午7:42, Pavel Begunkov 写道:
>>>>> io_fallback_req_func() doesn't expect anyone creating inline
>>>>> completions, and no one currently does that. Teach the function to flush
>>>>> completions preparing for further changes.
>>>>>
>>>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>>>> ---
>>>>>     fs/io_uring.c | 5 +++++
>>>>>     1 file changed, 5 insertions(+)
>>>>>
>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>> index 3da9f1374612..ba087f395507 100644
>>>>> --- a/fs/io_uring.c
>>>>> +++ b/fs/io_uring.c
>>>>> @@ -1197,6 +1197,11 @@ static void io_fallback_req_func(struct work_struct *work)
>>>>>         percpu_ref_get(&ctx->refs);
>>>>>         llist_for_each_entry_safe(req, tmp, node, io_task_work.fallback_node)
>>>>>             req->io_task_work.func(req);
>>>>> +
>>>>> +    mutex_lock(&ctx->uring_lock);
>>>>> +    if (ctx->submit_state.compl_nr)
>>>>> +        io_submit_flush_completions(ctx);
>>>>> +    mutex_unlock(&ctx->uring_lock);
>>>> why do we protect io_submit_flush_completions() with uring_lock,
>>>> regarding that it is called in original context. Btw, why not
>>>> use ctx_flush_and_put()
>>>
>>> The fallback thing is called from a workqueue not the submitter task
>>> context. See delayed_work and so.
>>>
>>> Regarding locking, it touches struct io_submit_state, and it's protected by
>>> ->uring_lock. In particular we're interested in ->reqs and ->free_list.
>>> FWIW, there is refurbishment going on around submit state, so if proves
>>> useful the locking may change in coming months.
>>>
>>> ctx_flush_and_put() could have been used, but simpler to hand code it
>>> and avoid the (always messy) conditional ref grabbing and locking.
> 
>> I didn't get it, what do you mean 'avoid the (always messy) conditional
>> ref grabbing and locking'? the code here and in ctx_flush_and_put() are
>> same..though I think in ctx_flush_and_put(), there is a problem that
>> compl_nr should also be protected.
> 
> Ok, the long story. First, notice a ctx check at the beginning of
> ctx_flush_and_put(), that one is conditional. Even though we know
> it's not NULL, it's more confusing and might be a problem for
> static and human analysis.
> 
> Also, locking is never easy, and so IMHO it's preferable to keep
> lock() and a matching unlock (or get/put) in the same function if
> possible, much easier to read. Compare
> 
> ref_get();
> do_something();
> ref_put();
> 
> and
> 
> ref_get();
> do_something();
> flush_ctx();
> 
> I believe, the first one is of less mental overhead.
Thanks, got it.
>