Re: [RFC][PATCH] module: Optimize __module_address() using a latched RB-tree

[Peter Zijlstra]

On Thu, Feb 26, 2015 at 11:41:44AM -0800, Paul E. McKenney wrote:
> > As per the above argument; without doing the whole READ/WRITE_ONCE for
> > the rb tree primitives, I think we're fine. We don't actually need the
> > read_barrier_depends() I think.
> > 
> > I think this because raw_read_seqcount() will issue an rmb, which should
> > be enough for the 'stable' tree. For the unstable one we don't care as
> > long as we don't see split loads/stores.
> 
> I agree that raw_read_seqcount() will issue an rmb(), but that won't help.
> For Alpha, you need the smp_rmb() to be between the load of any given
> pointer and the first dereference of that same pointer.

OK, it seems I'm confused on Alpha, perhaps you can clarify.

My confusion stems from the fact that when things are properly
serialized with locks we do not need these additional barriers.

Then why would we need them to correctly iterate the stable copy of the
tree?

I appreciate we would need them to correctly iterate an true lockless
data structure, but our in-flight copy of the tree cannot be correctly
iterated anyway, all we want is for that iteration to:

  1) only observe valid nodes -- ie. no pointers out into the wild due
  to split load/stores.

  2) terminate -- ie. not get stuck in loops.

And I don't see that read_barrier_depends() helping with either for the
unstable tree; 1) is guaranteed by my patch making the modifiers user
WRITE_ONCE(), and 2) we agreed is guaranteed by the modifiers not having
loops in program order, any cache effects will disappear over time.

> > No, I think someone is 'hoping' sync_rcu() implies sync_sched(). But
> > yes, I should look harder at this. I was assuming the existing code was
> > OK here.
> 
> I am not blaming you for the code itself, but rather just blaming you
> for causing me to notice that the code was broken.  ;-)
> 
> How about if I make something that allows overlapping grace periods,
> so that we could be safe without latency penalty?  One approach would
> be a synchronize_rcu_mult() with a bitmask indicating which to wait for.
> Another would be to make variants of get_state_synchronize_rcu() and
> cond_synchronize_rcu() for RCU-sched as well as for RCU, but also to
> make get_state_synchronize_rcu() force a grace period to start if
> one was not already in progress.

This is module unload, not a fast path by anyones measure I think.
However if you do go about making such a primitive I know of at least
one other place this could be used; we have the following in
_cpu_down():

#ifdef CONFIG_PREEMPT
	synchronize_sched();
#endif
	synchronize_rcu();
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