[PATCH v4 05/16] powerpc/vas: Move update_csb/dump_crb to common book3s platform

[Nicholas Piggin]

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);
> +