Re: [PATCH tip/core/rcu 0/21] v6 add lockdep-based diagnostics to rcu_dereference()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* 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
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