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From: Arnd Bergmann <arnd@arndb.de>
To: paulmck@linux.vnet.ibm.com
Subject: Re: [PATCH V4 1/1] rcu: introduce kfree_rcu()
Date: Tue, 15 Mar 2011 13:02:09 +0100
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On Tuesday 15 March 2011, Paul E. McKenney wrote:
> > Another alternative might be to encode the difference between a
> > function pointer and an offset in one of the lower bits of the address.
> 
> We discussed this some time back, and it turned out that there were
> CPUs that could legitimately have any combination of low-order bits
> set -- functions could start at any byte address.
> 
> If this has changed, I would prefer to use the low-order bits, but
> if it has not, we can't.  :-(

Ok, I see.

I just had another idea, which may or may not have new problems:

static inline void *kzalloc_rcu(size_t len, gfp_t flags)
{
	struct rcu_head *head = kzalloc(len + sizeof (struct rcu_head), flags);
	return head + 1;
}

void __kfree_rcu(struct rcu_head *head)
{
	kfree(head);
}

static inline void kfree_rcu(void *p)
{
	struct rcu_head *head = p - sizeof (struct rcu_head);
	call_rcu(head, __kfree_rcu);
}

The only disadvantage I can see right now is that it messes
with the alignment of the structure.

	Arnd