Messages in this thread | | | Date | Tue, 6 Nov 2007 10:36:51 -0500 (EST) | From | Alan Stern <> | Subject | Re: device struct bloat |
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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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