| Date | Tue, 23 Feb 2010 08:15:44 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH tip/core/rcu 0/21] v6 add lockdep-based diagnostics to rcu_dereference() |
| |
* Arnd Bergmann (arnd@arndb.de) wrote: > On Tuesday 23 February 2010, Paul E. McKenney wrote: > > This patch series adds lockdep-based checking to the rcu_dereference() > > primitive in order to flag misuses of RCU. > > While I haven't looked much at this series, I've been thinking about > adding static diagnostics for rcu_dereference misuse, in the form of > an __rcu address space qualifier for pointers. Such a patch would > obviously conflict with this series, so I'd wait for yours to go > in first, but maybe you like the idea enough to do it yourself ;-). > > The observation is that all accesses to an RCU protected pointer > are either through rcu_dereference, rcu_assign_pointer or one of their > variants. so it should be possible to add a new address space like we > have for __iomem, __user and soon __percpu and have sparse check that > we use RCU consistently on pointers that need it.
Just to make myself the devil's advocate: how should we consider initialization of RCU pointers at boot time that happens before any possible concurrent reader is allowed to run ? I think this case is an example of valid RCU-pointer access that is not done through the RCU primitives. It seems valid to perform these RCU-pointer accesses when serialized by a different exclusion mechanism, in this case being the guarantee that no concurrent reader are running at early boot. The same applies to stop_machine(), and, as I come to think of it, we could probably think of a scheme that dynamically switch from an RCU read-lock to, e.g., a mutex for all users, wait for RCU quiescent state, and then serialize all users on the mutex during the update. So while some of these cases are a bit far-fetched, I think they are valid, and I wonder how the address space validation would take them into account.
Thanks,
Mathieu
> > Arnd
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
|