From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755363Ab3GQN0M (ORCPT ); Wed, 17 Jul 2013 09:26:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50235 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754502Ab3GQN0J (ORCPT ); Wed, 17 Jul 2013 09:26:09 -0400 Date: Wed, 17 Jul 2013 16:25:03 +0300 From: Gleb Natapov To: Raghavendra K T Cc: mingo@redhat.com, jeremy@goop.org, x86@kernel.org, konrad.wilk@oracle.com, hpa@zytor.com, pbonzini@redhat.com, linux-doc@vger.kernel.org, habanero@linux.vnet.ibm.com, xen-devel@lists.xensource.com, peterz@infradead.org, mtosatti@redhat.com, stefano.stabellini@eu.citrix.com, andi@firstfloor.org, ouyang@cs.pitt.edu, agraf@suse.de, chegu_vinod@hp.com, torvalds@linux-foundation.org, avi.kivity@gmail.com, tglx@linutronix.de, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, riel@redhat.com, drjones@redhat.com, virtualization@lists.linux-foundation.org, srivatsa.vaddagiri@gmail.com Subject: Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor Message-ID: <20130717132503.GA13732@redhat.com> References: <20130714131241.GA11772@redhat.com> <51E3C5CE.7000009@linux.vnet.ibm.com> <20130715103648.GN11772@redhat.com> <51E4C011.4060803@linux.vnet.ibm.com> <20130716060215.GE11772@redhat.com> <51E5941B.3090300@linux.vnet.ibm.com> <20130717093420.GU11772@redhat.com> <51E66C71.6020605@linux.vnet.ibm.com> <20130717124511.GW11772@redhat.com> <51E69429.7040309@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <51E69429.7040309@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 17, 2013 at 06:25:05PM +0530, Raghavendra K T wrote: > On 07/17/2013 06:15 PM, Gleb Natapov wrote: > >On Wed, Jul 17, 2013 at 03:35:37PM +0530, Raghavendra K T wrote: > >>>>Instead of halt we started with a sleep hypercall in those > >>>> versions. Changed to halt() once Avi suggested to reuse existing sleep. > >>>> > >>>>If we use older hypercall with few changes like below: > >>>> > >>>>kvm_pv_wait_for_kick_op(flags, vcpu, w->lock ) > >>>>{ > >>>> // a0 reserved for flags > >>>>if (!w->lock) > >>>>return; > >>>>DEFINE_WAIT > >>>>... > >>>>end_wait > >>>>} > >>>> > >>>How would this help if NMI takes lock in critical section. The thing > >>>that may happen is that lock_waiting->want may have NMI lock value, but > >>>lock_waiting->lock will point to non NMI lock. Setting of want and lock > >>>have to be atomic. > >> > >>True. so we are here > >> > >> non NMI lock(a) > >> w->lock = NULL; > >> smp_wmb(); > >> w->want = want; > >> NMI > >> <--------------------- > >> NMI lock(b) > >> w->lock = NULL; > >> smp_wmb(); > >> w->want = want; > >> smp_wmb(); > >> w->lock = lock; > >> ----------------------> > >> smp_wmb(); > >> w->lock = lock; > >> > >>so how about fixing like this? > >> > >>again: > >> w->lock = NULL; > >> smp_wmb(); > >> w->want = want; > >> smp_wmb(); > >> w->lock = lock; > >> > >>if (!lock || w->want != want) goto again; > >> > >NMI can happen after the if() but before halt and the same situation > >we are trying to prevent with IRQs will occur. > > True, we can not fix that. I thought to fix the inconsistency of > lock,want pair. > But NMI could happen after the first OR condition also. > /me thinks again > lock_spinning() can check that it is called in nmi context and bail out. How often this will happens anyway. -- Gleb.