linux-kernel - Re: [PATCH v3 03/11] iommufd: Introduce IOMMUFD_OBJ

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Message-ID: <CABQgh9HP1M8GAXMwf0ZNn5EpMBAvJU3JLRWcN5H1wfGrqXSe1Q@mail.gmail.com>
Date: Mon, 14 Oct 2024 15:58:55 +0800
From: Zhangfei Gao <zhangfei.gao@...aro.org>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...dia.com, kevin.tian@...el.com, will@...nel.org, joro@...tes.org, 
	suravee.suthikulpanit@....com, robin.murphy@....com, dwmw2@...radead.org, 
	baolu.lu@...ux.intel.com, shuah@...nel.org, linux-kernel@...r.kernel.org, 
	iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org, 
	linux-kselftest@...r.kernel.org, eric.auger@...hat.com, 
	jean-philippe@...aro.org, mdf@...nel.org, mshavit@...gle.com, 
	shameerali.kolothum.thodi@...wei.com, smostafa@...gle.com, yi.l.liu@...el.com, 
	aik@....com, patches@...ts.linux.dev
Subject: Re: [PATCH v3 03/11] iommufd: Introduce IOMMUFD_OBJ_VIOMMU and its
 related struct

Hi, Nico

On Sat, 12 Oct 2024 at 18:18, Zhangfei Gao <zhangfei.gao@...aro.org> wrote:
>
> On Sat, 12 Oct 2024 at 12:49, Nicolin Chen <nicolinc@...dia.com> wrote:
> >
> > On Sat, Oct 12, 2024 at 11:23:07AM +0800, Zhangfei Gao wrote:
> >
> > > > diff --git a/drivers/iommu/iommufd/viommu_api.c b/drivers/iommu/iommufd/viommu_api.c
> > > > new file mode 100644
> > > > index 000000000000..c1731f080d6b
> > > > --- /dev/null
> > > > +++ b/drivers/iommu/iommufd/viommu_api.c
> > > > @@ -0,0 +1,57 @@
> > > > +// SPDX-License-Identifier: GPL-2.0-only
> > > > +/* Copyright (c) 2024, NVIDIA CORPORATION & AFFILIATES
> > > > + */
> > > > +
> > > > +#include "iommufd_private.h"
> > > > +
> > > > +struct iommufd_object *iommufd_object_alloc_elm(struct iommufd_ctx *ictx,
> > > > +                                               size_t size,
> > > > +                                               enum iommufd_object_type type)
> > > > +{
> > > > +       struct iommufd_object *obj;
> > > > +       int rc;
> > > > +
> > > > +       obj = kzalloc(size, GFP_KERNEL_ACCOUNT);
> > > > +       if (!obj)
> > > > +               return ERR_PTR(-ENOMEM);
> > > > +       obj->type = type;
> > > > +       /* Starts out bias'd by 1 until it is removed from the xarray */
> > > > +       refcount_set(&obj->shortterm_users, 1);
> > > > +       refcount_set(&obj->users, 1);
> > >
> > > here set refcont 1
> > >
> > > iommufd_device_bind -> iommufd_object_alloc(ictx, idev,
> > > IOMMUFD_OBJ_DEVICE): refcont -> 1
> > > refcount_inc(&idev->obj.users); refcount -> 2
> > > will cause iommufd_device_unbind fail.
> > >
> > > May remove refcount_inc(&idev->obj.users) in iommufd_device_bind
> >
> > Hmm, why would it fail? Or is it failing on your system?
>
> Not sure, still in check, it may only be on my platform.
>
> it hit
> iommufd_object_remove:
> if (WARN_ON(obj != to_destroy))
>
> iommufd_device_bind refcount=2
> iommufd_device_attach refcount=3
> //still not sure which operation inc the count?
> iommufd_device_detach refcount=4
>

Have a question,
when should iommufd_vdevice_destroy be called, before or after
iommufd_device_unbind.

Now iommufd_vdevice_destroy (ref--) is after unbind, hits the if
(!refcount_dec_if_one(&obj->users)) check.

iommufd_device_bind
iommufd_device_attach
iommufd_vdevice_alloc_ioctl

iommufd_device_detach
iommufd_device_unbind // refcount check fail
iommufd_vdevice_destroy ref--

Thanks