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

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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