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.
Send instant messages to your online friends http://au.messenger.yahoo.com 
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/