On Wed, 2 Sep 2009, Peter Zijlstra wrote: > There's a number of lockdep annotations to help out. ... Thanks for the detailed explanation; I appreciate it. > Now the particular issue at hand is that the device tree is a free form > tree with (afaiu) unlimited lock nesting depth -- if you were to plug in > an already daisy chained usb hub with actual devices on the 7th deep hub > device discovery will hold the device locks for the root hub, all 7 > intermediate hubs and the child device, leading to a total of 9 locks. That's true in principle but not in practice because of the way the USB hub driver is designed. Children aren't discovered and registered when the parent is probed; they are handled later. Of course, this kind of scenario absolutely could occur with other device types. > And there is nothing fundamental -- other than the usb chain depth -- > that limits this scenario, imagine the device to be an i2c bus with yet > more devices ;-) > > [ There used to be the additional complexity that on suspend/resume > we would hold _ALL_ device locks, which would exceed the max we can > track for any one task, this however has long been fixed. ] > > > So the proposal I currently have to solve this is to allocate 48 lock > classes: > > struct lock_class_key device_tree_depth[MAX_LOCK_DEPTH]; > > and when creating a new device node, set the lock class corresponding > the depth in the tree: > > mutex_lock_init(&device->lock); > BUG_ON(device->depth >= MAX_LOCK_DEPTH); // surely we're not that deep > lockdep_set_class(&device->lock, device_tree_depth + device->depth); > ... > mutex_lock(&device->lock); /* already have parent locked */ > device_attach(device, parent); > > and take multiple child locks using: > > mutex_lock_nest_lock(&device->lock, &device->parent->lock); > > Which, I think should work for most cases out there. I agree. It would be rather surprising to find a chain of devices nested more than 48 deep. > Alan had some funny corner cases, but I think he wasn't sure whether > those would indeed show up in reality. Yes; there's no point worrying about them now. This sounds like a good approach. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
