From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932822Ab3B1SCq (ORCPT ); Thu, 28 Feb 2013 13:02:46 -0500 Received: from mx1.redhat.com ([209.132.183.28]:52461 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753265Ab3B1SCo (ORCPT ); Thu, 28 Feb 2013 13:02:44 -0500 Date: Thu, 28 Feb 2013 19:00:07 +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: <20130228180007.GA3537@redhat.com> References: <512BBAD8.8010006@linux.vnet.ibm.com> <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> <20130227192551.GA8333@redhat.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 02/28, 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. I know this only because I did the same mistake recently, and Srivatsa explained the problem to 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 F343D2C029A for ; Fri, 1 Mar 2013 05:02:38 +1100 (EST) Date: Thu, 28 Feb 2013 19:00:07 +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: <20130228180007.GA3537@redhat.com> References: <512BBAD8.8010006@linux.vnet.ibm.com> <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> <20130227192551.GA8333@redhat.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 02/28, 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. I know this only because I did the same mistake recently, and Srivatsa explained the problem to me ;) Oleg. From mboxrd@z Thu Jan 1 00:00:00 1970 From: oleg@redhat.com (Oleg Nesterov) Date: Thu, 28 Feb 2013 19:00:07 +0100 Subject: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks In-Reply-To: References: <512BBAD8.8010006@linux.vnet.ibm.com> <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> <20130227192551.GA8333@redhat.com> Message-ID: <20130228180007.GA3537@redhat.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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. I know this only because I did the same mistake recently, and Srivatsa explained the problem to me ;) Oleg.