linux-kernel - Re: device struct bloat

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Date:	Tue, 6 Nov 2007 10:36:51 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Greg KH <greg@...ah.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Oliver Neukum <oneukum@...e.de>,
	Stephen Hemminger <shemminger@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>, apw <apw@...dowen.org>,
	Ingo Molnar <mingo@...e.hu>,
	<linux-usb-devel@...ts.sourceforge.net>
Subject: Re: device struct bloat

On Mon, 5 Nov 2007, Greg KH wrote:

> On Mon, Nov 05, 2007 at 11:57:14AM +0100, Peter Zijlstra wrote:
> > Hmm, the problem seems to be stuff like:
> > 
> > add usb driver to pci
> >   scan pci devices
> >      add usb host controller device
> >         scan usb devices
> >           add usb hub device
> >             scan usb devices
> >               add usb .....
> > 
> > This seems to be able to go on forever, as long as one can cascade usb
> > hubs.
> 
> USB hubs only work 7 deep, so there is a limit.

In fact things don't work this way.  The list above stops short after
"add usb host controller devices"; the probe routines for host
controllers do not scan for USB hubs or other USB devices.  Instead
they are detected by a completely separate thread (khubd).

> > Doesn't seem like an ideal thing to do from a stack space POV either.
> > 
> > Would it be possible to break at the second scan, that is the device
> > probe and stick that into a workqueue or something. Then we'd only ever
> > have driver->device nesting.
> 
> Alan and Oliver have done some work in this area I think, combined with
> the suspend/bind/unbind issues.  I'll let them comment on your patch :)

I gather the idea is to convert dev->sem to a mutex.  This idea had 
occurred to me a long time ago but I didn't pursue it because of the 
sheer number of places where dev->sem gets used, not to mention the 
lockdep problems.

You can't possibly solve the lockdep problems here with a simple-minded
approach like your DRIVER_NORMAL, DRIVER_PARENT, etc.  The device tree
is arbitrarily deep & wide, and there is at least one routine that
acquires the semaphores for _all_ the devices in the tree.  This fact
alone seems to preclude using lockdep for device locks.  (If there was 
a form of mutex_lock() that bypassed the lockdep checks, you could use 
it and avoid these issues.)

Deadlock is a serious consideration.  For the most part, routines 
locking devices do so along a single path in the tree.  For this simple 
case the rule is: Never acquire a parent's lock while holding the 
child's lock.

The routine that locks all the devices acquires the locks in order of 
device registration.  The idea here is that children are always 
registered _after_ their parents, so this should be compatible with the 
previous rule.  But there is a potential problem: device_move() can 
move an older child under a younger parent!

Right now we have no way to deal with this.  There has been some 
discussion of reordering the dpm_active list when a device is moved, 
but so far nothing has been done about it.

Alan Stern

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