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Date:   Mon, 1 Jul 2019 14:04:36 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Kirti Wankhede <kwankhede@nvidia.com>
Cc:     <cohuck@redhat.com>, <kvm@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] mdev: Send uevents around parent device registration
Message-ID: <20190701140436.45eabf07@x1.home>
In-Reply-To: <3b338e73-7929-df20-ca2b-3223ba4ead39@nvidia.com>
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        <3b338e73-7929-df20-ca2b-3223ba4ead39@nvidia.com>
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On Mon, 1 Jul 2019 23:20:35 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 7/1/2019 10:54 PM, Alex Williamson wrote:
> > On Mon, 1 Jul 2019 22:43:10 +0530
> > Kirti Wankhede <kwankhede@nvidia.com> wrote:
> >   
> >> On 7/1/2019 8:24 PM, Alex Williamson wrote:  
> >>> This allows udev to trigger rules when a parent device is registered
> >>> or unregistered from mdev.
> >>>
> >>> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> >>> ---
> >>>
> >>> v2: Don't remove the dev_info(), Kirti requested they stay and
> >>>     removing them is only tangential to the goal of this change.
> >>>     
> >>
> >> Thanks.
> >>
> >>  
> >>>  drivers/vfio/mdev/mdev_core.c |    8 ++++++++
> >>>  1 file changed, 8 insertions(+)
> >>>
> >>> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> >>> index ae23151442cb..7fb268136c62 100644
> >>> --- a/drivers/vfio/mdev/mdev_core.c
> >>> +++ b/drivers/vfio/mdev/mdev_core.c
> >>> @@ -146,6 +146,8 @@ int mdev_register_device(struct device *dev, const struct mdev_parent_ops *ops)
> >>>  {
> >>>  	int ret;
> >>>  	struct mdev_parent *parent;
> >>> +	char *env_string = "MDEV_STATE=registered";
> >>> +	char *envp[] = { env_string, NULL };
> >>>  
> >>>  	/* check for mandatory ops */
> >>>  	if (!ops || !ops->create || !ops->remove || !ops->supported_type_groups)
> >>> @@ -197,6 +199,8 @@ int mdev_register_device(struct device *dev, const struct mdev_parent_ops *ops)
> >>>  	mutex_unlock(&parent_list_lock);
> >>>  
> >>>  	dev_info(dev, "MDEV: Registered\n");
> >>> +	kobject_uevent_env(&dev->kobj, KOBJ_CHANGE, envp);
> >>> +
> >>>  	return 0;
> >>>  
> >>>  add_dev_err:
> >>> @@ -220,6 +224,8 @@ EXPORT_SYMBOL(mdev_register_device);
> >>>  void mdev_unregister_device(struct device *dev)
> >>>  {
> >>>  	struct mdev_parent *parent;
> >>> +	char *env_string = "MDEV_STATE=unregistered";
> >>> +	char *envp[] = { env_string, NULL };
> >>>  
> >>>  	mutex_lock(&parent_list_lock);
> >>>  	parent = __find_parent_device(dev);
> >>> @@ -243,6 +249,8 @@ void mdev_unregister_device(struct device *dev)
> >>>  	up_write(&parent->unreg_sem);
> >>>  
> >>>  	mdev_put_parent(parent);
> >>> +
> >>> +	kobject_uevent_env(&dev->kobj, KOBJ_CHANGE, envp);    
> >>
> >> mdev_put_parent() calls put_device(dev). If this is the last instance
> >> holding device, then on put_device(dev) dev would get freed.
> >>
> >> This event should be before mdev_put_parent()  
> > 
> > So you're suggesting the vendor driver is calling
> > mdev_unregister_device() without a reference to the struct device that
> > it's passing to unregister?  Sounds bogus to me.  We take a
> > reference to the device so that it can't disappear out from under us,
> > the caller cannot rely on our reference and the caller provided the
> > struct device.  Thanks,
> >   
> 
> 1. Register uevent is sent after mdev holding reference to device, then
> ideally, unregister path should be mirror of register path, send uevent
> and then release the reference to device.

I don't see the relevance here.  We're marking an event, not unwinding
state of the device from the registration process.  Additionally, the
event we're trying to mark is the completion of each process, so the
notion that we need to mirror the ordering between the two is invalid.

> 2. I agree that vendor driver shouldn't call mdev_unregister_device()
> without holding reference to device. But to be on safer side, if ever
> such case occur, to avoid any segmentation fault in kernel, better to
> send event before mdev release the reference to device.

I know that get_device() and put_device() are GPL symbols and that's a
bit of an issue, but I don't think we should be kludging the code for a
vendor driver that might have problems with that.  A) we're using the
caller provided device  for the uevent, B) we're only releasing our own
reference to the device that was acquired during registration, the
vendor driver must have other references, C) the parent device
generally lives on a bus, with a vendor driver, there's an entire
ecosystem of references to the device below mdev.  Is this a paranoia
request or are you really concerned that your PCI device suddenly
disappears when mdev's reference to it disappears.  Let's flush those
bugs out if they exist, not mask them behind obscure ordering
dependencies.  Thanks,

Alex