Re: [PATCH 2/5] vmbus: add driver_override support

[Stephen Hemminger <stephen@networkplumber.org>]

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

> From: kys@linuxonhyperv.com <kys@linuxonhyperv.com>  Sent: Friday, August 10, 2018 4:06 PM
> 
> > From: Stephen Hemminger <stephen@networkplumber.org>
> > 
> > Add support for overriding the default driver for a VMBus device
> > in the same way that it can be done for PCI devices. This patch
> > adds the /sys/bus/vmbus/devices/.../driver_override file
> > and the logic for matching.
> > 
> > This is used by driverctl tool to do driver override.
> > https://gitlab.com/driverctl/driverctl
> > 
> > Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
> > Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
> > ---
> > diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> > index b1b548a21f91..e6d8fdac6d8b 100644
> > --- a/drivers/hv/vmbus_drv.c
> > +++ b/drivers/hv/vmbus_drv.c
> > @@ -498,6 +498,54 @@ static ssize_t device_show(struct device *dev,
> >  }
> >  static DEVICE_ATTR_RO(device);
> > 
> > +static ssize_t driver_override_store(struct device *dev,
> > +				     struct device_attribute *attr,
> > +				     const char *buf, size_t count)
> > +{
> > +	struct hv_device *hv_dev = device_to_hv_device(dev);
> > +	char *driver_override, *old, *cp;
> > +
> > +	/* We need to keep extra room for a newline */
> > +	if (count >= (PAGE_SIZE - 1))
> > +		return -EINVAL;  
> 
> Does 'count' actually have a relationship to PAGE_SIZE, or
> is PAGE_SIZE just used as an arbitrary size limit?  I'm
> wondering what happens on ARM64 with a 64K page size,
> for example.  If it's just arbitrary, coding such a constant
> would be better.

This comes from original code how sysfs works.
Sysfs uses PAGE_SIZE for string buffers on store. This code
snippet was cloned from PCI version of same thing.

> > +/*
> > + * 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 patch clones existing locking model from PCI.
So either both are broken, or somehow vmbus is behaving differently.
I will investigate.