From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932182Ab3B0T2s (ORCPT ); Wed, 27 Feb 2013 14:28:48 -0500 Received: from mx1.redhat.com ([209.132.183.28]:13771 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751398Ab3B0T2o (ORCPT ); Wed, 27 Feb 2013 14:28:44 -0500 Date: Wed, 27 Feb 2013 20:25:51 +0100 From: Oleg Nesterov To: Michel Lespinasse Cc: "Srivatsa S. Bhat" , Lai Jiangshan , linux-doc@vger.kernel.org, peterz@infradead.org, fweisbec@gmail.com, linux-kernel@vger.kernel.org, namhyung@kernel.org, mingo@kernel.org, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, rusty@rustcorp.com.au, rostedt@goodmis.org, rjw@sisk.pl, vincent.guittot@linaro.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, sbw@mit.edu, tj@kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks Message-ID: <20130227192551.GA8333@redhat.com> References: <51226F91.7000108@linux.vnet.ibm.com> <512BBAD8.8010006@linux.vnet.ibm.com> <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/27, Michel Lespinasse wrote: > > +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw) > +{ > + preempt_disable(); > + > + if (__this_cpu_read(*lgrw->local_refcnt) || > + arch_spin_trylock(this_cpu_ptr(lgrw->lglock->lock))) { > + __this_cpu_inc(*lgrw->local_refcnt); Please look at __this_cpu_generic_to_op(). You need this_cpu_inc() to avoid the race with irs. The same for _read_unlock. But otherwise I agree, looks like a clever and working idea to me. And simple! > There is an interesting case where lg_rwlock_local_read_lock could be > interrupted after getting the local lglock but before incrementing > local_refcnt to 1; if that happens any nested readers within that > interrupt will have to take the global rwlock read side. I think this > is perfectly acceptable Agreed. Or interrupt can do spin_trylock(percpu-lock) after we take the global ->fallback_rwlock (if we race with write_lock + write_unlock), but I do not see any possible deadlock in this case. Oleg. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by ozlabs.org (Postfix) with ESMTP id C9D242C0085 for ; Thu, 28 Feb 2013 06:28:36 +1100 (EST) Date: Wed, 27 Feb 2013 20:25:51 +0100 From: Oleg Nesterov To: Michel Lespinasse Subject: Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks Message-ID: <20130227192551.GA8333@redhat.com> References: <51226F91.7000108@linux.vnet.ibm.com> <512BBAD8.8010006@linux.vnet.ibm.com> <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: Cc: Lai Jiangshan , linux-doc@vger.kernel.org, peterz@infradead.org, fweisbec@gmail.com, mingo@kernel.org, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, rusty@rustcorp.com.au, rostedt@goodmis.org, rjw@sisk.pl, namhyung@kernel.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, vincent.guittot@linaro.org, sbw@mit.edu, "Srivatsa S. Bhat" , tj@kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 02/27, Michel Lespinasse wrote: > > +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw) > +{ > + preempt_disable(); > + > + if (__this_cpu_read(*lgrw->local_refcnt) || > + arch_spin_trylock(this_cpu_ptr(lgrw->lglock->lock))) { > + __this_cpu_inc(*lgrw->local_refcnt); Please look at __this_cpu_generic_to_op(). You need this_cpu_inc() to avoid the race with irs. The same for _read_unlock. But otherwise I agree, looks like a clever and working idea to me. And simple! > There is an interesting case where lg_rwlock_local_read_lock could be > interrupted after getting the local lglock but before incrementing > local_refcnt to 1; if that happens any nested readers within that > interrupt will have to take the global rwlock read side. I think this > is perfectly acceptable Agreed. Or interrupt can do spin_trylock(percpu-lock) after we take the global ->fallback_rwlock (if we race with write_lock + write_unlock), but I do not see any possible deadlock in this case. Oleg. From mboxrd@z Thu Jan 1 00:00:00 1970 From: oleg@redhat.com (Oleg Nesterov) Date: Wed, 27 Feb 2013 20:25:51 +0100 Subject: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks In-Reply-To: References: <51226F91.7000108@linux.vnet.ibm.com> <512BBAD8.8010006@linux.vnet.ibm.com> <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> Message-ID: <20130227192551.GA8333@redhat.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 02/27, Michel Lespinasse wrote: > > +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw) > +{ > + preempt_disable(); > + > + if (__this_cpu_read(*lgrw->local_refcnt) || > + arch_spin_trylock(this_cpu_ptr(lgrw->lglock->lock))) { > + __this_cpu_inc(*lgrw->local_refcnt); Please look at __this_cpu_generic_to_op(). You need this_cpu_inc() to avoid the race with irs. The same for _read_unlock. But otherwise I agree, looks like a clever and working idea to me. And simple! > There is an interesting case where lg_rwlock_local_read_lock could be > interrupted after getting the local lglock but before incrementing > local_refcnt to 1; if that happens any nested readers within that > interrupt will have to take the global rwlock read side. I think this > is perfectly acceptable Agreed. Or interrupt can do spin_trylock(percpu-lock) after we take the global ->fallback_rwlock (if we race with write_lock + write_unlock), but I do not see any possible deadlock in this case. Oleg.