Question about patch "io_uring: add submission polling"

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

hi，

I have questions about below code comments, which was included in patch
"io_uring: add submission polling",
------------------------------------------------------------
     /*
      * Drop cur_mm before scheduling, we can't hold it for
      * long periods (or over schedule()). Do this before
      * adding ourselves to the waitqueue, as the unuse/drop
      * may sleep.
      */
     if (cur_mm) {
             unuse_mm(cur_mm);
             mmput(cur_mm);
             cur_mm = NULL;
     }

     prepare_to_wait(&ctx->sqo_wait, &wait, TASK_INTERRUPTIBLE);
-------------------------------------------------------------
Stefano submited a patch "io_uring: prevent sq_thread from spinning when it should stop",
I understand what issue Stefano fixed, but don't understand below comments.

Can anyone help to explain why we need to rop cur_mm before scheduling, or
why we can't hold it for long periods (or over schedule()), and if we
unuse/drop mm after adding ourselves to the waitqueue, what issue will
happen when unuse/drop sleeps, thanks.

Regards,
Xiaoguang Wang