Re: Question about fileset unregister and update codes

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi,

Could you please have a look at this mail? Thanks.

Regards,
Xiaoguang Wang

> hi,
> 
> Look at below function:
> static bool io_register_op_must_quiesce(int op)
> {
>      switch (op) {
>      case IORING_UNREGISTER_FILES:
>      case IORING_REGISTER_FILES_UPDATE:
>      case IORING_REGISTER_PROBE:
>      case IORING_REGISTER_PERSONALITY:
>      case IORING_UNREGISTER_PERSONALITY:
>          return false;
>      default:
>          return true;
>      }
> }
> 
> IORING_REGISTER_FILES will quiesces the ctx, but IORING_UNREGISTER_FILES
> and IORING_REGISTER_FILES_UPDATE won't, so I wonder how userspace applications
> can sure when they can unregister or update the registered fileset.
> Imagine below application behaviour:
>       ThreadA                     |      ThreadB
>                                   |
>      1, register a file           |     while (1) {
>      2, prepare a batch of sqes   |         wait a cqe and handle this cqe.
>      3, submit prepared sqes      |     }
>      4, unregister or update file |
>                                   |
> 
> If IORING_SETUP_SQPOLL is not enabled, I think step4 is safe, because step3
> will ensure that all sqes will be prepared by io_req_set_file(), then corresponding
> struct file will be resolved and ctx->file_data->refs will be increased, we know
> unless requst is completed, struct file will not be put, finally we know step4
> is safe.
> 
> But if IORING_SETUP_SQPOLL is enabled, step3 will complete quickly, if kernel
> thread io_sq_thread submit sqes before step4, everything is ok, but if step4
> starts to run, holding uring_lock, later io_sq_thread starts to handle sqes and
> all previously submitted sqes will be returned with EBADF.
> 
> I'm not sure whether should make application ensure that all sqes against registered
> file to complete, then app can unregister or update fileset, if so, I think it'll
> introduce extra programming overhead to programmer.
> 
> Or if IORING_SETUP_SQPOLL is enabled, we call io_submit_sqes in unregister/update codes,
> then applications will not need to worry about above race.
> 
> Regards,
> Xiaoguang Wang