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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