Re: [PATCH 1/2] srcu-3: RCU variant permitting read-side blocking

Matt Helsley
2006-07-07T21:16:40Z

On Fri, 2006-07-07 at 15:59 -0400, Alan Stern wrote:
> On Fri, 7 Jul 2006, Paul E. McKenney wrote:

<snip>

> > So, a fourth possibility -- can a call from start_kernel() invoke some
> > function in yours and Matt's code invoke init_srcu_struct() to get a
> > statically allocated srcu_struct initialized? Or, if this is part of
> > a module, can the module initialization function do this work?
> >
> > (Hey, I had to ask!)
>
> That is certainly a viable approach: just force everyone to use dynamic
> initialization. Changes to existing code would be relatively few.

Works for me. I've been working on patches for Andrew's multi-chain
proposal and I could use an init function there anyway. Should be faster
too -- dynamically-allocated per-cpu memory can take advantage of
node-local memory whereas, to my knowledge, statically-allocated cannot.

> I'm not sure where the right place would be to add these initialization
> calls. After kmalloc is working but before the relevant notifier chains
> get used at all. Is there such a place? I guess it depends on which
> notifier chains we convert.
>
> We might want to leave some chains using the existing rw-semaphore API.
> It's more appropriate when there's a high frequency of write-locking
> (i.e., things registering or unregistering on the notifier chain). The
> SRCU approach is more appropriate when the chain is called a lot and
> needs to have low overhead, but (un)registration is uncommon. Matt's task
> notifiers are a good example.

Yes, it is an excellent example.

> Alan Stern

Cheers,
-Matt Helsley