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

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);
+