From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gerald Schaefer Subject: Re: [PATCH 1/2] iommu/s390: Fix IOMMU groups Date: Fri, 28 Apr 2017 15:20:17 +0200 Message-ID: <20170428152017.5e99d67f@thinkpad> References: <1493306905-32334-1-git-send-email-joro@8bytes.org> <1493306905-32334-2-git-send-email-joro@8bytes.org> <20170427201142.18d467a3@thinkpad> <20170427211232.GF1332@8bytes.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20170427211232.GF1332-zLv9SwRftAIdnm+yROfE0A@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Joerg Roedel Cc: iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Joerg Roedel , Sebastian Ott , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: iommu@lists.linux-foundation.org On Thu, 27 Apr 2017 23:12:32 +0200 Joerg Roedel wrote: > On Thu, Apr 27, 2017 at 08:11:42PM +0200, Gerald Schaefer wrote: > > > +void zpci_destroy_iommu(struct zpci_dev *zdev) > > > +{ > > > + iommu_group_put(zdev->group); > > > + zdev->group = NULL; > > > +} > > > > While the rest of this patch doesn't seem to make much of a difference to > > the current behavior, I'm wondering where this extra iommu_group_put() > > comes from. It either was erroneously missing before this patch, or it > > is erroneously introduced by this patch. > > This is the way to free an iommu-group. It was missing before probably > because it was unclear whether the add_device function allocated a group > or not. So there was no way to know if it needs to be put again in the > remove_device function. Hmm, for the reference count it should not matter whether a new group was allocated or an existing group found with iommu_group_get(). Our add_device callback always gets one reference either from iommu_group_get or _alloc, and then another one from iommu_group_add_device(), after which the first reference is put again. So there should always be one reference more after a successful add_device. Now I'm wondering where this one reference is put again, and I thought that happened in the remove_device callback, where we call iommu_group_remove_device(). Is this not correct? Just want to make sure that we don't have a refcount issue in the current code. Regards, Gerald