From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37282) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZK3LR-000553-2P for qemu-devel@nongnu.org; Tue, 28 Jul 2015 07:46:54 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZK3LO-0001UE-CE for qemu-devel@nongnu.org; Tue, 28 Jul 2015 07:46:52 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33614) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZK3LO-0001U2-7X for qemu-devel@nongnu.org; Tue, 28 Jul 2015 07:46:50 -0400 References: <55B6E854.9090600@cn.fujitsu.com> <55B75278.80909@redhat.com> <55B7532D.9010506@cn.fujitsu.com> <55B756D8.9090503@redhat.com> <55B75A70.8080203@cn.fujitsu.com> From: Paolo Bonzini Message-ID: <55B76BA3.4080804@redhat.com> Date: Tue, 28 Jul 2015 13:46:43 +0200 MIME-Version: 1.0 In-Reply-To: <55B75A70.8080203@cn.fujitsu.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: Wen Congyang , qemu-devl On 28/07/2015 12:33, Wen Congyang wrote: > 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? If you call it just once, you have the same problem as before. In fact, it's worse because instead of having an overflow every 2^31 periods, you have one every 2 periods. Instead, by checking that rcu_reader went through 1 _and_ 3 (or that it was at least once 0, i.e. the thread was quiescent), you are sure that the thread went through _at least one_ grace period. Paolo