Re: [PATCH] io_uring: fix SQPOLL IORING_OP_CLOSE cancelation state

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

On 20/01/2021 02:07, Joseph Qi wrote:
[...]
>> Fix this by moving the IO_WQ_WORK_NO_CANCEL until _after_ we've modified
>> the fdtable. Canceling before this point is totally fine, and running
>> it in the io-wq context _after_ that point is also fine.
>>
>> For 5.12, we'll handle this internally and get rid of the no-cancel
>> flag, as IORING_OP_CLOSE is the only user of it.
>>
>> Fixes: 14587a46646d ("io_uring: enable file table usage for SQPOLL rings")
> 
> As discussed with Pavel, this can not only happen in case sqpoll, but
> also in case async cancel is from io-wq.

Right, and it's handled because execution-during-cancellation can't
anymore get into close_fd_get_file(), either it was already done or
we didn't yet set IO_WQ_WORK_NO_CANCEL. And blkcg is not a problem.

> 
>> Reported-by: Joseph Qi <joseph.qi@linux.alibaba.com>
> 
> In fact, it is reported by "Abaci <abaci@linux.alibaba.com>"
> 
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> Reviewed-and-tested-by: Joseph Qi <joseph.qi@linux.alibaba.com>

Thanks!

>>
>> ---
>>
>> Joseph, can you test this patch and see if this fixes it for you?
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index b76bb50f18c7..5f6f1e48954e 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -4472,7 +4472,6 @@ static int io_close_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>  	 * io_wq_work.flags, so initialize io_wq_work firstly.
>>  	 */
>>  	io_req_init_async(req);
>> -	req->work.flags |= IO_WQ_WORK_NO_CANCEL;
>>  
>>  	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
>>  		return -EINVAL;
>> @@ -4505,6 +4504,8 @@ static int io_close(struct io_kiocb *req, bool force_nonblock,
>>  
>>  	/* if the file has a flush method, be safe and punt to async */
>>  	if (close->put_file->f_op->flush && force_nonblock) {
>> +		/* not safe to cancel at this point */
>> +		req->work.flags |= IO_WQ_WORK_NO_CANCEL;
>>  		/* was never set, but play safe */
>>  		req->flags &= ~REQ_F_NOWAIT;
>>  		/* avoid grabbing files - we don't need the files */
>>

-- 
Pavel Begunkov