Messages in this thread | | | Date | Tue, 23 Feb 2010 08:16:02 -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 04:54:09PM +0100, Arnd Bergmann wrote: > On Tuesday 23 February 2010, Mathieu Desnoyers wrote: > > 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. > > Like the annotations adding __rcu to each rcu protected pointer, we'd > also have to add annotations to static initialization of those. E.g. > something like > > #define DEFINE_RCU_VAR(type, name, pointer) \ > type __rcu *name = (__force type)pointer > > should do for simple initializations, and I guess I can come up > with similar solutions if we need something more fancy.
We would also need something for initialization of structure fields. Does __force work in that case as well?
> > 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. > > I assume that it's never wrong to do a pointer assignment using > rcu_assign_pointer, or to do a dereference using rcu_dereference, > even if you hold a mutex or stop_machine. I would also guess that > the performance impact of doing so is not measurable. If both are > true for all corner cases, we could just use the rcu primitives > for any access on those variables for consistency reasons.
Seems like a good default position, but please see below.
> If there are cases where it does not work, we need to come up with > names for new primitives that just do the assignment or dereference > with __force but no actual synchronization.
Some data structures are shared by RCU and non-RCU code, with struct list_head being the most prominent example. Making the "next" pointer as __rcu might be OK, but there are a -lot- of non-RCU uses of struct list_head. Would we really want to introduce rcu_dereference() to all non-RCU list-traversal primitives, or do we need to do something else?
Thanx, Paul
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