Re: [PATCH 2/2] io_uring: fix failed linkchain code logic

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

On 8/23/21 12:02 PM, Pavel Begunkov wrote:
> On 8/23/21 4:25 AM, Hao Xu wrote:
>> Given a linkchain like this:
>> req0(link_flag)-->req1(link_flag)-->...-->reqn(no link_flag)
>>
>> There is a problem:
>>  - if some intermediate linked req like req1 's submittion fails, reqs
>>    after it won't be cancelled.
>>
>>    - sqpoll disabled: maybe it's ok since users can get the error info
>>      of req1 and stop submitting the following sqes.
>>
>>    - sqpoll enabled: definitely a problem, the following sqes will be
>>      submitted in the next round.
>>
>> The solution is to refactor the code logic to:
>>  - if a linked req's submittion fails, just mark it and the head(if it
>>    exists) as REQ_F_FAIL. Leverage req->result to indicate whether it
>>    is failed or cancelled.
>>  - submit or fail the whole chain when we come to the end of it.
> 
> This looks good to me, a couple of comments below.
> 
> 
>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>> ---
>>  fs/io_uring.c | 61 +++++++++++++++++++++++++++++++++++++--------------
>>  1 file changed, 45 insertions(+), 16 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 44b1b2b58e6a..9ae8f2a5c584 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -1776,8 +1776,6 @@ static void io_preinit_req(struct io_kiocb *req, struct io_ring_ctx *ctx)
>>  	req->ctx = ctx;
>>  	req->link = NULL;
>>  	req->async_data = NULL;
>> -	/* not necessary, but safer to zero */
>> -	req->result = 0;
> 
> Please leave it. I'm afraid of leaking stack to userspace because
                                         ^^^^^
Don't know why I called it "stack", just kernel memory/data

> ->result juggling looks prone to errors. And preinit is pretty cold
> anyway.
> 

-- 
Pavel Begunkov