Messages in this thread | | | Date | Tue, 23 Feb 2010 06:35:21 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 0/21] v6 add lockdep-based diagnostics to rcu_dereference() |
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On Tue, Feb 23, 2010 at 08:15:44AM -0500, Mathieu Desnoyers wrote: > * 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.
And this is an excellent exposition of a few of the initialization issues I referred to in my earlier email!
Thanx, Paul
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