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SubjectRe: deadlock in "modprobe -r ohci1394" shortly after "modprobe ohci1394"
On Sun, 19 Nov 2006, Stefan Richter wrote:

> > Knowing nothing at all about ieee1394, I get the feeling that the culprit
> > here is a strange subsystem design. In fact, I don't understand exactly
> > what's going wrong. Evidently the rmmod thread owns the locks for both
> > the host being removed and its parent, and it wants to stop knodemgrd,
> > which is waiting to acquire the host's parent's lock because it is
> > attempting to rescan the parent. Is that right?
>
> That's right.
>
> > It doesn't make sense. If knodemgrd is rescanning the parent then the
> > parent must not have a driver. If it doesn't have a driver, how can it
> > have children?
>
> Well, I don't fully understand the reasoning behind it. (But even less
> do I grasp the driver core.) Short story: It may indeed be possible to
> get rid of the parent relationship to the host device and of the
> rescanning of the host device. Long story:

Wait a minute. Above you agreed that the problem was caused by knodemgrd
attempting to rescan the host's _parent_. So which is the focus of the
deadlock: the host or its parent?

> ieee1394 has:
> - struct hpsb_host (contains a struct device and a struct class_device)
> ieee1394's nodemgr in addition has:
> - struct node_entry (ditto)
> - struct unit_directory (ditto)
>
> nodemgr also provides different .release callbacks for each of these
> devices and class_devices, an .uevent callback for unit_directory
> class_devices and the struct bus_type for the three kinds of device.
>
> All of the existing protocol drivers bind only to unit_directories, so
> these are the most important ones as far as driver matching is
> concerned. (There is only a little out-of-tree driver, mem1394 for
> forensics, which binds differently because it doesn't require any actual
> protocol on a remote node.)
>
> A hpsb_host is merely access to controllers; each one controls a bus. On
> a bus live a few node_entries, as many as there are nodes with an active
> link layer controller. This includes the local host, therefore there is
> always at least one node_entry per bus. A node may implement 0, 1 or
> more units and presents them in a hierarchical manner. A unit talks a
> protocol; and our protocol drivers access such a unit and, if the
> protocol has this necessity, the unit's node. (While doing so, it also
> accesses the hpsb_host in front of the node.)
>
> If support for eth1394 is configured in, our drivers add a unit to each
> of the local nodes which indicate IP over 1394 capability to external
> nodes. This unit_directory also matches the eth1394 when the driver core
> scans the ieee1394_bus_type.
>
> So in short, I think we could actually live with a minimum sysfs
> implementation which exposes only unit directories. (But I may be
> missing something --- I'm not quite familiar with the sysfs interfacing
> tools, that is udev and hald.) At least device file naming of SBP-2
> units wouldn't suffer because udev takes the unique name of SBP-2 units
> from a sysfs attribute of the SCSI device, not from the unit_directory's
> device.
>
> The /sys/class/net/eth?/device links of eth1394 class devices currently
> point to the hpsb_host devices i.e. fw-host devices; maybe they could as
> well point to respective unit directories.
>
> So maybe a simple, flat sysfs representation without node_entry devices,
> perhaps also without fw-host devices --- or at least a representation
> (1) without parent relationship to the fw-host devices and (2) without
> ieee1394_bus_type in fw-host devices --- is possible without breaking
> userspace. It probably would eliminate the problem which we are
> discussing here. This would mean that we loose the ability to bind
> protocol drivers to a hpsb_host (and depending on how far the
> respresentation is cut down, also the ability to bind protocol drivers
> to a node_entry). But as I said I see no actual need for this ability.

If I understand all this, you've got an hpsb_host, directly below which
are one or more node_entry's, below each of which may be some
unit_directory's. Right?

But how is this relevant if the problem is caused by knodemgrd trying to
rescan the hpsb_host's parent?

Is the problem caused by the fact that some of these struct device's
aren't bound to a driver? Remember, bus_rescan_devices() will skip over
anything that already has a driver. Could you solve your problem by
adding a do_nothing driver that would bind to these otherwise unused
devices?

Alan Stern

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