Re: [PATCH v2 5/6] usb: Iterator for ports

Alan Stern
2021-03-30T15:50:11Z

On Tue, Mar 30, 2021 at 12:07:35PM +0300, Heikki Krogerus wrote:
> On Mon, Mar 29, 2021 at 02:49:46PM -0400, Alan Stern wrote:
> > On Mon, Mar 29, 2021 at 11:44:25AM +0300, Heikki Krogerus wrote:
> > > Introducing usb_for_each_port(). It works the same way as
> > > usb_for_each_dev(), but instead of going through every USB
> > > device in the system, it walks through the USB ports in the
> > > system.
> > >
> > > Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> > > ---
> > > drivers/usb/core/usb.c | 46 ++++++++++++++++++++++++++++++++++++++++++
> > > include/linux/usb.h | 1 +
> > > 2 files changed, 47 insertions(+)
> > >
> > > diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
> > > index 2ce3667ec6fae..62368c4ed37af 100644
> > > --- a/drivers/usb/core/usb.c
> > > +++ b/drivers/usb/core/usb.c
> > > @@ -398,6 +398,52 @@ int usb_for_each_dev(void *data, int (*fn)(struct usb_device *, void *))
> > > }
> > > EXPORT_SYMBOL_GPL(usb_for_each_dev);
> > >
> > > +struct each_hub_arg {
> > > + void *data;
> > > + int (*fn)(struct device *, void *);
> > > +};
> > > +
> > > +static int __each_hub(struct usb_device *hdev, void *data)
> > > +{
> > > + struct each_hub_arg *arg = (struct each_hub_arg *)data;
> > > + struct usb_hub *hub;
> > > + int ret = 0;
> > > + int i;
> > > +
> > > + hub = usb_hub_to_struct_hub(hdev);
> > > + if (!hub)
> > > + return 0;
> >
> > What happens if the hub is removed exactly now? Although hdev is
> > reference-counted (and the loop iterator does take a reference to it),
> > usb_hub_to_struct_hub doesn't take a reference to hub. And hub->ports
> > isn't refcounted at all.
>
> If the hub is removed right now, and if hub_disconnect() also manages
> to remove the ports before we have time to take the lock below, then
> hdev->maxchild will be 0 by the time we can take the lock. In that
> case nothing happens here.

Okay, good.

> If on the other hand we manage to acquire the usb_port_peer_mutex
> before hub_disconnect(), then hub_disconnect() will simply have to
> wait until we are done, and only after that remove the ports.
>
> > > + mutex_lock(&usb_port_peer_mutex);
> > > +
> > > + for (i = 0; i < hdev->maxchild; i++) {
> > > + ret = arg->fn(&hub->ports[i]->dev, arg->data);
> > > + if (ret)
> > > + break;
> > > + }
> > > +
> > > + mutex_unlock(&usb_port_peer_mutex);
> >
> > I have a feeling that it would be better to take and release this mutex
> > in usb_for_each_port (or its caller), so that it is held over the whole
> > loop.
>
> I disagree. The lock is for the ports, not the hubs. We should take
> the lock when we are going through the ports of a hub, but release it
> between the hubs. Otherwise we will be only keeping things on hold for
> a long period of time for no good reason (I for example have to
> evaluate the _PLD of every single port which takes a lot of time). We
> don't need to prevent other things from happening to the hubs at the
> same time.

All right, you convinced me.

Acked-by: Alan Stern <stern@rowland.harvard.edu>

Alan Stern