On Fri, 2009-08-07 at 12:04 -0400, Alan Stern wrote: > On Fri, 7 Aug 2009, Peter Zijlstra wrote: > > > On Fri, 2009-08-07 at 09:46 -0500, Clark Williams wrote: > > > Peter, > > > > > > I'm getting this warning from lockdep when booting on my T60. > > > > > > The two addresses reported (0xffffffff812664a2 and 0xffffffff812664ae) > > > actually bracket one call to mutex_lock() in driver_attach() so I'm not > > > sure what the complaint is. > > > Oh, that's tglx who's gone wild with sem->mutex conversions. > > Is this code available somewhere? Its in the -rt tree, but this patch was posted to lkml at: http://lkml.org/lkml/2009/7/26/36 The -rt tree can be found in various places, but while tglx is out celebrating his holidays the latest can be found through: http://lkml.org/lkml/2009/8/5/406 > > It used to be that _all_ dev->sem instances were taken on suspend or > > something like that, I think that got fixed a long while back. > > > > I'd have to look at what the current locking requirements for dev->sem > > are. > > It is supposed to be locked whenever the driver core invokes a probe, > remove, or PM-related callback. Under some circumstances, the parent's > semaphore is supposed to be locked as well. Individual subsystems may > have their own requirements in addition to these. > > The ordering requirement is: Don't try to acquire a device's lock if > you already hold the lock for a non-ancestor device. More generally > (if more obscurely): If you already hold device A's lock, then don't > try to acquire the lock for device B unless you already hold the lock > for A & B's most recent common ancestor. > > > I remember talking to Alan on several occasions about this, and I just > > went over some of the old emails, but I must say the precise > > requirements stay hidden from me. Also, I'm not sure these emails are > > still representative of the current state. > > I think they are, pretty much. The real problem, of course, is that > lockdep doesn't understand tree-structured lock orderings. Hence it > isn't practical to convert dev->sem into a mutex. Right, well it would if we'd make every instance a class, but since classes should reside in static storage this is far from trivial. If we'd be able to find a mapping such that we can use a limited number of these classes to represent the needed structure then we're good. I think I proposed adding a class to each driver or something, but then you countered that a single driver could register itself at conflicting places in the device tree. Still it might be worth to try that and see where we'll end up and possibly fix up a few drivers to be more intelligent. /me ponders Nested busses would be interesting though, suppose we assign a class to a USB bus driver, and we chain USB hubs, you'd get a nesting of similar classes and that'd upset lockdep again :/ The other proposal was creating a fixed list of classes and register each device at a class corresponding to its depth in the tree. I can't remember what was wrong with that, but I seem to have been persuaded that that was hard too. -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
