From mboxrd@z Thu Jan 1 00:00:00 1970 From: Henrik Rydberg Subject: Re: [PATCH 1/4] input: Introduce buflock, a one-to-many circular buffer mechanism Date: Fri, 04 Jun 2010 18:36:55 +0200 Message-ID: <4C092BA7.507@bitmath.org> References: <1275552062-8153-1-git-send-email-rydberg@euromail.se> <1275552062-8153-2-git-send-email-rydberg@euromail.se> <20100604065658.GE21239@core.coreip.homeip.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from csmtp1.one.com ([195.47.247.21]:32811 "EHLO csmtp1.one.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754660Ab0FDQhC (ORCPT ); Fri, 4 Jun 2010 12:37:02 -0400 In-Reply-To: <20100604065658.GE21239@core.coreip.homeip.net> Sender: linux-input-owner@vger.kernel.org List-Id: linux-input@vger.kernel.org To: Dmitry Torokhov Cc: linux-input@vger.kernel.org, linux-kernel@vger.kernel.org, Jiri Kosina , Mika Kuoppala , Benjamin Tissoires , Rafi Rubin , Oleg Nesterov >> +#define buflock_write(bw, buf, size, item) \ >> + do { \ >> + bw.next_head = (bw.head + 1) & ((size) - 1); \ >> + smp_wmb(); \ > > Why do we need the write barrier here? I believe my first answer to this question was foggy indeed, so allow me to go again, with a time line: Scenario 1, correct write order: writer store_next_head store_buf store_head reader load_head load_buf load_next_head Result: head != next_head, incoherent read detected Scenario 2, incorrect write order: writer store_buf store_next_head store_head reader load_head load_buf load_next_head Result: head == next_head, incoherent read not detected Based on the assumption that scenario 2 could happen if the smp_wmb() is not present, the barrier is needed. Henrik