Message-ID: <4560D2CA.9050000@s5r6.in-berlin.de>
Date: Sun, 19 Nov 2006 22:55:22 +0100
From: Stefan Richter <stefanr@s5r6.in-berlin.de>
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To: Alan Stern <stern@rowland.harvard.edu>
CC: linux1394-devel@lists.sourceforge.net, Greg Kroah-Hartman <gregkh@suse.de>,
       linux-kernel@vger.kernel.org
Subject: Re: deadlock in "modprobe -r ohci1394" shortly after "modprobe ohci1394"
References: <Pine.LNX.4.44L0.0611191431120.15059-100000@netrider.rowland.org>
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Alan Stern wrote:
> On Sun, 19 Nov 2006, Stefan Richter wrote:
>> @@ -1873,8 +1873,19 @@ static void nodemgr_remove_host(struct h
...
>>  	if (hi) {
>> +		/* up(&host->device.sem);	--- apparently not required */
>> +		if (host->device.parent)
>> +			up(&host->device.parent->sem);
>>  		kthread_stop(hi->thread);
>> +		if (host->device.parent)
>> +			down(&host->device.parent->sem);
>> +		/* down(&host->device.sem);	--- apparently not required */
>>  		nodemgr_remove_host_dev(&host->device);
>>  	}
>>  }
> 
> Obviously this patch isn't pretty.  It's also incorrect, because it 
> reacquires the parent's semaphore while holding the child's -- that's 
> another recipe for deadlock.
> 
> 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:

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.
-- 
Stefan Richter
-=====-=-==- =-== =--==
http://arcgraph.de/sr/
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