Re: [patch 1/3] radix tree: RCU lockless read-side

[Nick Piggin <nickpiggin@yahoo.com.au>]

Balbir Singh wrote:
> <snip>
> 
>>                if (slot->slots[i]) {
>>-                       results[nr_found++] = slot->slots[i];
>>+                       results[nr_found++] = &slot->slots[i];
>>                        if (nr_found == max_items)
>>                                goto out;
>>                }
> 
> 
> A quick clarification - Shouldn't accesses to slot->slots[i] above be
> protected using rcu_derefence()?
> 

I think we're safe here -- this is the _address_ of the pointer.
However, when dereferencing this address in _gang_lookup,
I think we do need rcu_dereference indeed.

Note that _gang_lookup_slot doesn't do this for us, however --
the caller must do that when dereferencing the pointer to the
item (eg. see page_cache_get_speculative in 2/3).

That said, I'm not 100% sure I have the rcu memory barriers in
the right places (well I'm sure I don't, given the _gang_lookup
bug you exposed!).

Thanks,
Nick

-- 
SUSE Labs, Novell Inc.
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