linux-kernel - Re: [PATCH 2/5] vmbus: add driver

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Message-ID: <20180814093514.7febc8d5@xeon-e3>
Date:   Tue, 14 Aug 2018 09:35:14 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>
Cc:     KY Srinivasan <kys@...rosoft.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "apw@...onical.com" <apw@...onical.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>
Subject: Re: [PATCH 2/5] vmbus: add driver_override support

On Mon, 13 Aug 2018 19:30:50 +0000
"Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.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