Re: [Lse-tech] Re: RFC: patch to allow lock-free traversal of lists with insertion

[Rusty Russell <rusty@rustcorp.com.au>]

In message <Pine.LNX.4.33.0110120948540.31692-100000@penguin.transmeta.com> you
 write:
> Yes. With maybe
> 
> 	non_preempt()
> 	..
> 	preempt()
> 
> around it for the pre-emption patches.

Sure, if they want pre-emption on SMP.  Of course, then they'll want
priority inheritence.

> However, you also need to make your free _free_ be aware of the count.
> Which means that the current RCU patch is really unusable for this. You
> need to have the "count" always in a generic place (put it with the hash),
> and your schedule-time free needs to do
> 
> 	if (atomic_read(&count))
> 		skip_this_do_it_next_time

WTF?  I'll spell it out for you again:

	static inline void foo_put(struct foo *foo)
	{
		if (atomic_dec_and_test(foo->use))
			kfree(foo);
	}

Write side normal:

	lock
	unhash(foo)
	unlock
	foo_put(foo)

Write side RCU:

	lock
	unhash(foo)
	unlock
	rcu_call(foo_put, foo);
		/* ie. call foo_put(foo) "later". */

That's all.  Really.

> Do that, and the RCU patches may start looking usable for the real world.

I know you're under strain, but think harder please.

Rusty.
--
Premature optmztion is rt of all evl. --DK