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    SubjectRe: [PATCH tip/core/rcu 0/21] v6 add lockdep-based diagnostics to rcu_dereference()
    On Tue, Feb 23, 2010 at 08:15:44AM -0500, Mathieu Desnoyers wrote:
    > * Arnd Bergmann ( 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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