Re: [PATCH io_uring-5.14 v2] io_uring: remove double poll wait entry for pure poll

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

在 2021/7/23 下午10:31, Pavel Begunkov 写道:
> On 7/23/21 10:22 AM, Hao Xu wrote:
>> For pure poll requests, we should remove the double poll wait entry.
>> And io_poll_remove_double() is good enough for it compared with
>> io_poll_remove_waitqs().
> 
> 5.14 in the subject hints me that it's a fix. Is it?
> Can you add what it fixes or expand on why it's better?
Hi Pavel, I found that for poll_add() requests, it doesn't remove the
double poll wait entry when it's done, neither after vfs_poll() or in
the poll completion handler. The patch is mainly to fix it.

> 
> 
>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>> ---
>>
>> v1-->v2
>>    delete redundant io_poll_remove_double()
>>
>>   fs/io_uring.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index f2fe4eca150b..c5fe8b9e26b4 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -4903,7 +4903,6 @@ static bool io_poll_complete(struct io_kiocb *req, __poll_t mask)
>>   	if (req->poll.events & EPOLLONESHOT)
>>   		flags = 0;
>>   	if (!io_cqring_fill_event(ctx, req->user_data, error, flags)) {
>> -		io_poll_remove_waitqs(req);
Currently I only see it does that with io_poll_remove_waitqs() when
cqring overflow and then ocqe allocation failed. Using
io_poll_remove_waitqs() here is not very suitable since (1) it calls
__io_poll_remove_one() which set poll->cancelled = true, why do we set
poll->cancelled and poll->done to true at the same time though I think
that doesn't cause any problem. (2) it does
list_del_init(&poll->wait.entry) and hash_del(&req->hash_node) which
has been already done.
Correct me if I'm wrong since I may misunderstand the code.

Regards,
Hao
>>   		req->poll.done = true;
>>   		flags = 0;
>>   	}
>> @@ -4926,6 +4925,7 @@ static void io_poll_task_func(struct io_kiocb *req)
>>   
>>   		done = io_poll_complete(req, req->result);
>>   		if (done) {
>> +			io_poll_remove_double(req);
>>   			hash_del(&req->hash_node);
>>   		} else {
>>   			req->result = 0;
>> @@ -5113,7 +5113,7 @@ static __poll_t __io_arm_poll_handler(struct io_kiocb *req,
>>   		ipt->error = -EINVAL;
>>   
>>   	spin_lock_irq(&ctx->completion_lock);
>> -	if (ipt->error)
>> +	if (ipt->error || (mask && (poll->events & EPOLLONESHOT)))
>>   		io_poll_remove_double(req);
>>   	if (likely(poll->head)) {
>>   		spin_lock(&poll->head->lock);
>> @@ -5185,7 +5185,6 @@ static int io_arm_poll_handler(struct io_kiocb *req)
>>   	ret = __io_arm_poll_handler(req, &apoll->poll, &ipt, mask,
>>   					io_async_wake);
>>   	if (ret || ipt.error) {
>> -		io_poll_remove_double(req);
>>   		spin_unlock_irq(&ctx->completion_lock);
>>   		if (ret)
>>   			return IO_APOLL_READY;
>>
>