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

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Message-ID: <20210330154921.GC980260@rowland.harvard.edu>
Date:   Tue, 30 Mar 2021 11:49:21 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Benson Leung <bleung@...gle.com>,
        Prashant Malani <pmalani@...omium.org>,
        Guenter Roeck <linux@...ck-us.net>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/6] usb: Iterator for ports

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@...ux.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@...land.harvard.edu>

Alan Stern