From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932659AbdLRIuM (ORCPT ); Mon, 18 Dec 2017 03:50:12 -0500 Received: from mail-wm0-f66.google.com ([74.125.82.66]:41977 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932285AbdLRIuJ (ORCPT ); Mon, 18 Dec 2017 03:50:09 -0500 X-Google-Smtp-Source: ACJfBos6qlfL4FLddu1qENPP1aZFlS2mq3Uiw64qZocNlm8mz/ZmoSKWrEmPKYtEv8pPwgZDUyBdPg== Subject: Re: [PATCH] KVM/Eventfd: Avoid crash when assign and deassign same eventfd in parallel. To: David Hildenbrand , Lan Tianyu Cc: rkrcmar@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, dvyukov@google.com, kernellwp@gmail.com References: <1513554007-12302-1-git-send-email-tianyu.lan@intel.com> From: Paolo Bonzini Message-ID: Date: Mon, 18 Dec 2017 09:50:04 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18/12/2017 09:30, David Hildenbrand wrote: > The ugly thing in kvm_irqfd_assign() is that we access irqfd without > holding a lock. I think that should rather be fixed than working around > that issue. (e.g. lock() -> lookup again -> verify still in list -> > unlock()) I wonder if it's even simpler: diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c index f2ac53ab8243..17ed298bd66f 100644 --- a/virt/kvm/eventfd.c +++ b/virt/kvm/eventfd.c @@ -387,7 +387,6 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args) idx = srcu_read_lock(&kvm->irq_srcu); irqfd_update(kvm, irqfd); - srcu_read_unlock(&kvm->irq_srcu, idx); list_add_tail(&irqfd->list, &kvm->irqfds.items); @@ -420,10 +419,12 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args) irqfd->consumer.token, ret); } #endif + srcu_read_unlock(&kvm->irq_srcu, idx); return 0; fail: + /* irq_srcu is *not* held here. */ if (irqfd->resampler) irqfd_resampler_shutdown(irqfd); Thanks, Paolo