From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arjan van de Ven Subject: Re: Question about usage of RCU in the input layer Date: Thu, 19 Mar 2009 07:02:21 -0700 Message-ID: <20090319070221.6aca7245@infradead.org> References: <20090318215812.15496a86@infradead.org> <200903190023.56929.dmitry.torokhov@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: Received: from casper.infradead.org ([85.118.1.10]:52618 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755379AbZCSOB1 (ORCPT ); Thu, 19 Mar 2009 10:01:27 -0400 In-Reply-To: <200903190023.56929.dmitry.torokhov@gmail.com> Sender: linux-input-owner@vger.kernel.org List-Id: linux-input@vger.kernel.org To: Dmitry Torokhov Cc: linux-input@vger.kernel.org, dipankar@in.ibm.com, linux-kernel@vger.kernel.org On Thu, 19 Mar 2009 00:23:56 -0700 Dmitry Torokhov wrote: > Hi Arjan, > > On Wednesday 18 March 2009 21:58:12 Arjan van de Ven wrote: > > Hi, > > > > the input layer does a "synchronize_rcu()" after a > > list_add_tail_rcu(), which is costing me 1 second of boot time..... > > And based on my understanding of the RCU concept, you only need to > > synchronize on delete, not on addition... so I think the > > synchronize is entirely redundant here... > > > It is there to guarantee that once we registered the handle all > subsequent input events will be delivered through it. afaik rcu already guarantees that even without a synchronize; the only reason you would need a synchronize is to guarantee that people STOPPED using your memory. Or am I now totally misunderstanding RCU ?