Re: [driver-core PATCH v4 4/6] driver core: Probe devices asynchronously instead of the driver

[Alexander Duyck]

On Thu, 2018-10-18 at 19:31 -0700, Bart Van Assche wrote:
> On 10/18/18 7:20 PM, Alexander Duyck wrote:
> > I see what you are talking about now. Actually I think this was an
> > existing issue before my patch even came into play. Basically the
> > code
> > as it currently stands is device specific in terms of the attach
> > and
> > release code.
> > 
> > I wonder if we shouldn't have the async_synchronize_full call in
> > __device_release_driver moved down and into driver_detach before we
> > even start the for loop. Assuming the driver is no longer
> > associated
> > with the bus that should flush out all devices so that we can then
> > pull them out of the devices list at least. I may look at adding an
> > additional bitflag to the device struct to indicate that it has a
> > driver attach pending. Then for things like races between any
> > attach
> > and detach calls the logic becomes pretty straight forward. Attach
> > will set the bit and provide driver data, detach will clear the bit
> > and the driver data. If a driver loads in between it should clear
> > the
> > bit as well.
> > 
> > I'll work on it over the next couple days and hopefully have
> > something
> > ready for testing/review early next week.
> 
> Hi Alex,
> 
> How about checking in __driver_attach_async_helper() whether the
> driver 
> pointer is still valid by checking whether bus_for_each_drv(dev-
> >bus, 
> ...) can still find the driver pointer? That approach requires 
> protection with a mutex to avoid races with the driver detach code
> but 
> shouldn't require any new flags in struct device.
> 
> Thanks,
> 
> Bart.

That doesn't solve the problem I was pointing out though.

So the issue you are addressing by rechecking the bus should already be
handled by just calling async_synchronize_full in driver_detach. After
all we can't have a driver that is being added to the bus while it is
also being removed. So if we are detaching the driver calling
async_synchronize_full will flush out any deferred attach calls and
there will be no further calls since the driver has already been
removed from the bus.

The issue I was thinking of is how do we deal with races between
device_attach and device_release_driver. In that case we know the
device we want to remove a driver from, but we may not have information
about the driver. The easiest solution is to basically just disable the
pending enable. I could use the approach I am doing now and just NULL
out the driver_data if dev->driver is NULL. The only thing I am
thinking about is if just dev->driver being NULL is enough to signal
that we are using driver_data to carry a pointer to a pending driver,
or if we should add an extra bit to carry that meaning. It would be
pretty easy to just add a bit and then use that to prevent any false
reads of the deferred driver as driver data, or driver data as a
deferred driver as it would essentially act as a type bit.

Thanks.

- Alex