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

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

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.

-- 
Pavel Begunkov