linux-kernel - Re: device struct bloat

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Message-Id: <1194364684.6289.40.camel@twins>
Date:	Tue, 06 Nov 2007 16:58:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Greg KH <greg@...ah.com>, 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 Tue, 2007-11-06 at 10:36 -0500, Alan Stern wrote:

> > > 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

That should never stop us from doing the right thing :-), also dev->sem
isn't used nearly as much as for example work_struct which was changed.

> , not to mention the lockdep problems.

Right, that is the only sort-of valid reason this has not been done yet.

> 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. 

*blink* someone needs to take all locks - why?

>  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.)

I'm sceptical of this, since its a simple tree (as opposed to a balanced
tree) a simple lock-coupling approach should be enough to guarantee
consistency.

> 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.

Sure, but once you have a parent's lock, you could unlock your
grandparent, no? (it having a locked child, your parent, should be
enough to guarantee its continued existence)

> 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!

Seems like a weird rule, a typical tree locking rule would be to lock
them top-down - such a rule can easily cope with moves: lock old parent,
lock child, lock new parent, move child, unlock all in reverse order.

> 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.

Like said, I think the tree locking model should be revisited. A
top-down locking model with lock-coupling should, from my ignorant
perspective, solve your problems.


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