Re: [driver-core PATCH v5 4/9] driver core: Move async_synchronize_full call

[Alexander Duyck]

On Mon, 2018-11-05 at 17:04 -0800, Bart Van Assche wrote:
> On Mon, 2018-11-05 at 13:11 -0800, Alexander Duyck wrote:
> > This patch moves the async_synchronize_full call out of
> > __device_release_driver and into driver_detach.
> > 
> > The idea behind this is that the async_synchronize_full call will only
> > guarantee that any existing async operations are flushed. This doesn't do
> > anything to guarantee that a hotplug event that may occur while we are
> > doing the release of the driver will not be asynchronously scheduled.
> > 
> > By moving this into the driver_detach path we can avoid potential deadlocks
> > as we aren't holding the device lock at this point and we should not have
> > the driver we want to flush loaded so the flush will take care of any
> > asynchronous events the driver we are detaching might have scheduled.
> > 
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/base/dd.c |    6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > index 76c40fe69463..e74cefeb5b69 100644
> > --- a/drivers/base/dd.c
> > +++ b/drivers/base/dd.c
> > @@ -975,9 +975,6 @@ static void __device_release_driver(struct device *dev, struct device *parent)
> >  
> >  	drv = dev->driver;
> >  	if (drv) {
> > -		if (driver_allows_async_probing(drv))
> > -			async_synchronize_full();
> > -
> >  		while (device_links_busy(dev)) {
> >  			__device_driver_unlock(dev, parent);
> >  
> > @@ -1087,6 +1084,9 @@ void driver_detach(struct device_driver *drv)
> >  	struct device_private *dev_prv;
> >  	struct device *dev;
> >  
> > +	if (driver_allows_async_probing(drv))
> > +		async_synchronize_full();
> > +
> >  	for (;;) {
> >  		spin_lock(&drv->p->klist_devices.k_lock);
> >  		if (list_empty(&drv->p->klist_devices.k_list)) {
> 
> Have you considered to move that async_synchronize_full() call into
> bus_remove_driver()? Verifying the correctness of this patch requires to
> check whether the async_synchronize_full() comes after the
> klist_remove(&drv->p->knode_bus) call. That verification is easier when
> the async_synchronize_full() call occurs in bus_remove_driver() instead
> of in driver_detach().
> 
> Thanks,
> 
> Bart.

I considered it, however it ends up with things being more symmetric to
have use take care of synchronizing things in driver_detach since after
this patch set we are scheduling thing asynchronously in driver_attach.

Also I don't think it would be any great risk simply because calling
driver_detach with the driver still associated with the bus would be a
blatent error as it could easily lead to issues where you unbind a
driver but have it get hotplugged to a device while that is going on.

- Alex