From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932391Ab3B1SYt (ORCPT ); Thu, 28 Feb 2013 13:24:49 -0500 Received: from mx1.redhat.com ([209.132.183.28]:53610 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752846Ab3B1SYr (ORCPT ); Thu, 28 Feb 2013 13:24:47 -0500 Date: Thu, 28 Feb 2013 19:20:11 +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: <20130228182011.GA4373@redhat.com> References: <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> <20130227192551.GA8333@redhat.com> <20130228180007.GA3537@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130228180007.GA3537@redhat.com> 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 02/28, Oleg Nesterov wrote: > On 02/28, Michel Lespinasse wrote: > > > > On Thu, Feb 28, 2013 at 3:25 AM, Oleg Nesterov wrote: > > > 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. > > > > Hmmm, I was thinking that this was safe because while interrupts might > > modify local_refcnt to acquire a nested read lock, they are expected > > to release that lock as well which would set local_refcnt back to its > > original value ??? > > Yes, yes, this is correct. > > I meant that (in general, x86 is fine) __this_cpu_inc() itself is not > irq-safe. It simply does "pcp += 1". > > this_cpu_inc() is fine, _this_cpu_generic_to_op() does cli/sti around. Just in case, it is not that I really understand why __this_cpu_inc() can race with irq in this particular case (given that irq handler should restore the counter). So perhaps I am wrong again. The comments in include/linux/percpu.h look confusing to me, and I simply know nothing about !x86 architectures. But since, say, preempt_disable() doesn't do anything special then probably __this_cpu_inc() is fine too. In short: please ignore me ;) 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 ACDDE2C02C3 for ; Fri, 1 Mar 2013 05:24:40 +1100 (EST) Date: Thu, 28 Feb 2013 19:20:11 +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: <20130228182011.GA4373@redhat.com> References: <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> <20130227192551.GA8333@redhat.com> <20130228180007.GA3537@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20130228180007.GA3537@redhat.com> 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 02/28, Oleg Nesterov wrote: > On 02/28, Michel Lespinasse wrote: > > > > On Thu, Feb 28, 2013 at 3:25 AM, Oleg Nesterov wrote: > > > 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. > > > > Hmmm, I was thinking that this was safe because while interrupts might > > modify local_refcnt to acquire a nested read lock, they are expected > > to release that lock as well which would set local_refcnt back to its > > original value ??? > > Yes, yes, this is correct. > > I meant that (in general, x86 is fine) __this_cpu_inc() itself is not > irq-safe. It simply does "pcp += 1". > > this_cpu_inc() is fine, _this_cpu_generic_to_op() does cli/sti around. Just in case, it is not that I really understand why __this_cpu_inc() can race with irq in this particular case (given that irq handler should restore the counter). So perhaps I am wrong again. The comments in include/linux/percpu.h look confusing to me, and I simply know nothing about !x86 architectures. But since, say, preempt_disable() doesn't do anything special then probably __this_cpu_inc() is fine too. In short: please ignore me ;) Oleg. From mboxrd@z Thu Jan 1 00:00:00 1970 From: oleg@redhat.com (Oleg Nesterov) Date: Thu, 28 Feb 2013 19:20:11 +0100 Subject: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks In-Reply-To: <20130228180007.GA3537@redhat.com> References: <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> <20130227192551.GA8333@redhat.com> <20130228180007.GA3537@redhat.com> Message-ID: <20130228182011.GA4373@redhat.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 02/28, Oleg Nesterov wrote: > On 02/28, Michel Lespinasse wrote: > > > > On Thu, Feb 28, 2013 at 3:25 AM, Oleg Nesterov wrote: > > > 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. > > > > Hmmm, I was thinking that this was safe because while interrupts might > > modify local_refcnt to acquire a nested read lock, they are expected > > to release that lock as well which would set local_refcnt back to its > > original value ??? > > Yes, yes, this is correct. > > I meant that (in general, x86 is fine) __this_cpu_inc() itself is not > irq-safe. It simply does "pcp += 1". > > this_cpu_inc() is fine, _this_cpu_generic_to_op() does cli/sti around. Just in case, it is not that I really understand why __this_cpu_inc() can race with irq in this particular case (given that irq handler should restore the counter). So perhaps I am wrong again. The comments in include/linux/percpu.h look confusing to me, and I simply know nothing about !x86 architectures. But since, say, preempt_disable() doesn't do anything special then probably __this_cpu_inc() is fine too. In short: please ignore me ;) Oleg.