From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756886AbaKTPh0 (ORCPT ); Thu, 20 Nov 2014 10:37:26 -0500 Received: from mx1.redhat.com ([209.132.183.28]:37916 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754076AbaKTPhZ (ORCPT ); Thu, 20 Nov 2014 10:37:25 -0500 Message-ID: <1416497837.27937.362.camel@ul30vt.home> Subject: Re: [PATCH 1/1] vfio: put off the allocation of "minor" in vfio_create_group From: Alex Williamson To: Zhen Lei Cc: KVM , linux-kernel , Zefan Li , Xinwei Hu , Tianhong Ding , Kefeng Wang Date: Thu, 20 Nov 2014 08:37:17 -0700 In-Reply-To: <1416482702-10820-1-git-send-email-thunder.leizhen@huawei.com> References: <1416482702-10820-1-git-send-email-thunder.leizhen@huawei.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2014-11-20 at 19:25 +0800, Zhen Lei wrote: > The next code fragment "list_for_each_entry" is not depend on "minor". With this > patch, the free of "minor" in "list_for_each_entry" can be reduced, and there is > no functional change. A reasonable micro-optimization, but I'm curious if you're actually seeing some measurable overhead from this. It seems like we'd need to have multiple devices, all within the same IOMMU group, all probed by vfio-pci at the same time to exercise the race condition. Thanks, Alex > Signed-off-by: Zhen Lei > --- > drivers/vfio/vfio.c | 13 ++++++------- > 1 file changed, 6 insertions(+), 7 deletions(-) > > diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c > index f018d8d..737eb468 100644 > --- a/drivers/vfio/vfio.c > +++ b/drivers/vfio/vfio.c > @@ -225,22 +225,21 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group) > > mutex_lock(&vfio.group_lock); > > - minor = vfio_alloc_group_minor(group); > - if (minor < 0) { > - vfio_group_unlock_and_free(group); > - return ERR_PTR(minor); > - } > - > /* Did we race creating this group? */ > list_for_each_entry(tmp, &vfio.group_list, vfio_next) { > if (tmp->iommu_group == iommu_group) { > vfio_group_get(tmp); > - vfio_free_group_minor(minor); > vfio_group_unlock_and_free(group); > return tmp; > } > } > > + minor = vfio_alloc_group_minor(group); > + if (minor < 0) { > + vfio_group_unlock_and_free(group); > + return ERR_PTR(minor); > + } > + > dev = device_create(vfio.class, NULL, > MKDEV(MAJOR(vfio.group_devt), minor), > group, "%d", iommu_group_id(iommu_group)); > -- > 1.8.0 > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/