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Date:   Tue, 14 Aug 2018 09:35:14 -0700
From:   Stephen Hemminger <stephen@networkplumber.org>
To:     "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>
Cc:     KY Srinivasan <kys@microsoft.com>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "olaf@aepfle.de" <olaf@aepfle.de>,
        "apw@canonical.com" <apw@canonical.com>,
        "jasowang@redhat.com" <jasowang@redhat.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "vkuznets@redhat.com" <vkuznets@redhat.com>
Subject: Re: [PATCH 2/5] vmbus: add driver_override support
Message-ID: <20180814093514.7febc8d5@xeon-e3>
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References: <20180810230520.2055-1-kys@linuxonhyperv.com>
        <20180810230611.2114-1-kys@linuxonhyperv.com>
        <20180810230611.2114-2-kys@linuxonhyperv.com>
        <CY4PR21MB07739595C31B9B9349604C0ADC390@CY4PR21MB0773.namprd21.prod.outlook.com>
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On Mon, 13 Aug 2018 19:30:50 +0000
"Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com> wrote:

> > +/*
> > + * Return a matching hv_vmbus_device_id pointer.
> > + * If there is no match, return NULL.
> > + */
> > +static const struct hv_vmbus_device_id *hv_vmbus_get_id(struct hv_driver *drv,
> > +							struct hv_device *dev)
> > +{
> > +	const uuid_le *guid = &dev->dev_type;
> > +	const struct hv_vmbus_device_id *id;
> > 
> > -	return NULL;
> > +	/* When driver_override is set, only bind to the matching driver */
> > +	if (dev->driver_override && strcmp(dev->driver_override, drv->name))
> > +		return NULL;  
> 
> This function needs to be covered by the device lock, so that
> dev->driver_override can't be set to NULL and the memory freed
> during the above 'if' statement.  When called from vmbus_probe(),
> the device lock is held, so it's good. But when called from
> vmbus_match(), the device lock may not be held: consider the path
> __driver_attach() -> driver_match_device() -> vmbus_match().

The function hv_vmbus_get_id is called from that path.
i.e. __device_attach -> driver-match_device -> vmbus_match.
and __device_attach always does:
	device_lock(dev);

The code in driver _override_store uses the same device_lock 
when storing the new value.

This is same locking as is done in pci-sysfs.c