Excerpts from Haren Myneni's message of May 21, 2021 7:32 pm:
> +
> + pid = task_ref->pid;
> + tsk = get_pid_task(pid, PIDTYPE_PID);
> + /*
> + * Process closes send window after all pending NX requests are
> + * completed. In multi-thread applications, a child thread can
> + * open a window and can exit without closing it. May be some
> + * requests are pending or this window can be used by other
> + * threads later. We should handle faults if NX encounters
> + * pages faults on these requests. Update CSB with translation
> + * error and fault address. If csb_addr passed by user space is
> + * invalid, send SEGV signal to pid saved in window. If the
> + * child thread is not running, send the signal to tgid.
> + * Parent thread (tgid) will close this window upon its exit.
> + *
> + * pid and mm references are taken when window is opened by
> + * process (pid). So tgid is used only when child thread opens
> + * a window and exits without closing it.
> + */
> + if (!tsk) {
> + pid = task_ref->tgid;
> + tsk = get_pid_task(pid, PIDTYPE_PID);
> + /*
> + * Parent thread (tgid) will be closing window when it
> + * exits. So should not get here.
> + */
> + if (WARN_ON_ONCE(!tsk))
> + return;
> + }
> +
> + /* Return if the task is exiting. */
> + if (tsk->flags & PF_EXITING) {
> + put_task_struct(tsk);
> + return;
> + }
Just as an aside, I know this is existing code, after this series it
might be good to think about factoring out this above chunk of code
(possibly +/- the kthread_use_mm() bit), and put it together with the
rest of the task/mm refcounting stuff.
Thanks,
Nick
> +
> + kthread_use_mm(task_ref->mm);
> + rc = copy_to_user(csb_addr, &csb, sizeof(csb));
> + /*
> + * User space polls on csb.flags (first byte). So add barrier
> + * then copy first byte with csb flags update.
> + */
> + if (!rc) {
> + csb.flags = CSB_V;
> + /* Make sure update to csb.flags is visible now */
> + smp_mb();
> + rc = copy_to_user(csb_addr, &csb, sizeof(u8));
> + }
> + kthread_unuse_mm(task_ref->mm);
> + put_task_struct(tsk);
> +
