From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932353AbbJaNCQ (ORCPT ); Sat, 31 Oct 2015 09:02:16 -0400 Received: from ns.horizon.com ([71.41.210.147]:15610 "HELO ns.horizon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1753123AbbJaNCP (ORCPT ); Sat, 31 Oct 2015 09:02:15 -0400 Date: 31 Oct 2015 09:02:14 -0400 Message-ID: <20151031130214.10598.qmail@ns.horizon.com> From: "George Spelvin" To: bigeasy@linutronix.de, tglx@linutronix.de Subject: Re: [PATCH] ipc/msg: Implement lockless pipelined wakeups Cc: akpm@linux-foundation.org, dave@stgolabs.net, linux-kernel@vger.kernel.org, linux@horizon.com, manfred@colorfullife.com, peterz@infradead.org In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > Don't we need to keep that NULL init? I might be missing something. I wondered the same thing, but on reading it, the cleanup is that he's gotten rid of the need for the entire thing. Previously, there was a mechanism for detecting "wakeup not quite finished" that used a NULL value, but it's no longer needed. The resultant busy-waiting on the part of the woken-up task was the entire problem this patch aims to fix. So it gets rid of a whole lot of code and barriers. And, as you noticed, the comments explaining them. As the old code explained, the issue is that a task may exit as soon as r_msg is set, so the wakeup procedure has to be: - Ensure r_msg is set to NULL (special-case flag) - Do the wake up - Set r_msg to the final value The woken-up task has to spin as long as r_msg is NULL. Ick. However, a wake_q keeps a reference to a task, so exiting is not a danger. As long as wake_q_add precedes setting r_msg, all is well.