Re: [RFC,PATCH] RCU: clean up a few remaining synchronize_kernel() calls

[Paul E. McKenney]

On Tue, Jun 28, 2005 at 11:15:55AM -0600, Zwane Mwaikambo wrote:
> On Tue, 28 Jun 2005, Paul E. McKenney wrote:
> > However, you do have a good point -- weakly ordered CPUs would need to
> > have an explicit memory barrier.  This might well already be taken care
> > of by the memory barriers in the locking primitives used by the up()
> > operation invoked at the end of oprofile_start(), but I did not check
> > all the possible ways that these functions can be called.
> 
> I agree, that usage looks safe.

Cool!

> > Given that set_nmi_callback isn't invoked all that often, seems like
> > it might be preferable to insert an smp_wmb() at the beginning of
> > set_nmi_callback(), so that it reads as follows:
> > 
> > 	void set_nmi_callback(nmi_callback_t callback)
> > 	{
> > 		smp_wmb();
> > 		nmi_callback = callback;
> > 	}
> > 
> > Thoughts?
> 
> Andrew (rightly) tends to howls whenever someone adds a memory barrier 
> without a comment ;) So if we were to make that change, how about the 
> following accompanying comment;
> 
> "smp_wmb ensures that all data dependencies for the callback are posted 
> and callback is ready for execution"

Actually, I was guilty of posting email to LKML before being fully
awake...  How about the following instead?

	void set_nmi_callback(nmi_callback_t callback)
	{
		rcu_assign_pointer(nmi_callback, callback);
	}

Similarly:

	void unset_nmi_callback(void)
	{
		rcu_assign_pointer(nmi_callback, dummy_nmi_callback);
	}

This, combined with the rcu_dereference() in do_nmi() seem to me to
make the usage a lot more clear.  If you agree, would you like to
submit the patches, or should I?

> Thanks for elaborating, the examples certainly do help clarify usage.

Glad they help, will clean them up (so that the examples use
rcu_dereference() and rcu_assign_pointer()) and submit them!

							Thanx, Paul
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