From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39168) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZK2CM-0002eI-1w for qemu-devel@nongnu.org; Tue, 28 Jul 2015 06:33:27 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZK2CI-000626-1B for qemu-devel@nongnu.org; Tue, 28 Jul 2015 06:33:25 -0400 Received: from [59.151.112.132] (port=34279 helo=heian.cn.fujitsu.com) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZK2CH-0005z3-Ar for qemu-devel@nongnu.org; Tue, 28 Jul 2015 06:33:21 -0400 References: <55B6E854.9090600@cn.fujitsu.com> <55B75278.80909@redhat.com> <55B7532D.9010506@cn.fujitsu.com> <55B756D8.9090503@redhat.com> From: Wen Congyang Message-ID: <55B75A70.8080203@cn.fujitsu.com> Date: Tue, 28 Jul 2015 18:33:20 +0800 MIME-Version: 1.0 In-Reply-To: <55B756D8.9090503@redhat.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2 for-2.5] rcu: Allow calling rcu_(un)register_thread() during synchronize_rcu() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini , qemu-devl On 07/28/2015 06:18 PM, Paolo Bonzini wrote: > > > On 28/07/2015 12:02, Wen Congyang wrote: >> I have a question about rcu: while do we call wait_for_readers() >> twice for 32-bit host? > > Because there is a very small but non-zero probability of the counter > going up by exactly 2^31 periods (periods are stored in bits 1-31 so you > lose one bit) while the thread is sleeping. This detail of the > implementation comes from URCU. Yes, so you use rcu_gp_ctr ^ RCU_GP_CTR to instead of rcu_gp_ctr + RCU_GP_CTR. The initial value is 1, so rcu_gp_ctr is: 1, 3, 1, 3, ... The rcu_gp_ctr will never be 0. I think calling wait_for_readers() once is enough. Do I miss something? Thanks Wen Congyang > > Paolo >