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

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

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.
>