From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 2/2] KVM: Convert read-only users of vm_list to RCU Date: Thu, 10 Feb 2011 16:26:44 +0200 Message-ID: <4D53F5A4.8050403@redhat.com> References: <4D512EF7.8040409@siemens.com> <4D512F3B.1080107@siemens.com> <4D53BB02.20206@redhat.com> <4D53CCAB.8040204@siemens.com> <4D53DB54.90605@redhat.com> <4D53DDD3.5020704@siemens.com> <4D53E063.1040004@redhat.com> <4D53E0A9.2030501@redhat.com> <4D53E4C0.5000605@siemens.com> <4D53E5FB.5090109@redhat.com> <4D53EC7B.3030903@siemens.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , kvm , Zachary Amsden To: Jan Kiszka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:60668 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756472Ab1BJO0u (ORCPT ); Thu, 10 Feb 2011 09:26:50 -0500 In-Reply-To: <4D53EC7B.3030903@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: On 02/10/2011 03:47 PM, Jan Kiszka wrote: > >> > >> Accept for mmu_shrink, which is write but not delete, thus works without > >> that slow synchronize_rcu. > > > > I don't really see how you can implement list_move_rcu(), it has to be > > atomic or other users will see a partial vm_list. > > Right, even if we synchronized that step cleanly, rcu-protected users > could miss the moving vm during concurrent list walks. > > What about using a separate mutex for protecting vm_list instead? > Unless I missed some detail, mmu_shrink should allow blocking. What else does kvm_lock protect? I think we could simply reduce the amount of time we hold kvm_lock. Pick a vm, ref it, list_move_tail(), unlock, then do the actual shrinking. Of course taking a ref must be done carefully, we might already be in kvm_destroy_vm() at that time. -- error compiling committee.c: too many arguments to function