Re: [PATCH 1/3] loop: Use worker per cgroup instead of kworker

Hi Hillf, thanks for the review

On Sat, Apr 03, 2021 at 10:09:02AM +0800, Hillf Danton wrote:

On Fri,  2 Apr 2021 12:16:32 Dan Schatzberg wrote:

quoted

+queue_work:
+	if (worker) {
+		/*
+		 * We need to remove from the idle list here while
+		 * holding the lock so that the idle timer doesn't
+		 * free the worker
+		 */
+		if (!list_empty(&worker->idle_list))
+			list_del_init(&worker->idle_list);

Nit, only queue work if the worker is inactive - otherwise it is taking
care of the cmd_list.

By worker is inactive, you mean worker is on the idle_list? Yes, I
think you're right that queue_work() is unnecessary in that case since
each worker checks empty cmd_list then adds itself to idle_list under
the lock.

quoted

+		work = &worker->work;
+		cmd_list = &worker->cmd_list;
+	} else {
+		work = &lo->rootcg_work;
+		cmd_list = &lo->rootcg_cmd_list;
+	}
+	list_add_tail(&cmd->list_entry, cmd_list);
+	queue_work(lo->workqueue, work);
+	spin_unlock_irq(&lo->lo_work_lock);
 }

[...]

quoted

+	/*
+	 * We only add to the idle list if there are no pending cmds
+	 * *and* the worker will not run again which ensures that it
+	 * is safe to free any worker on the idle list
+	 */
+	if (worker && !work_pending(&worker->work)) {

The empty cmd_list is a good enough reason for worker to become idle.

This is only true with the above change to avoid a gratuitous
queue_work(), right? Otherwise we run the risk of freeing a worker
concurrently with loop_process_work() being invoked.