On Wed, Jan 09, 2008 at 08:38:56AM +0100, Eric Dumazet wrote:
>
> I am not sure this is valid, since it will do this :
>
> r = rt_hash_table[st->bucket].chain;
> if (r)
> return rcu_dereference(r);
>
> So compiler might be dumb enough do dereference
> &rt_hash_table[st->bucket].chain two times.
That wouldn't be a problem at all. The key is to add a barrier between
reading the pointer:
r = rt_hash_table[st->bucket].chain
and dereferencing it later, e.g.,
r->u.dst.rt_next
The barrier is there so that when we dereference r we don't read
stale cache that was there before the memory at r was initialised.
How many times you read the pointer value before the barrier is
irrelevant to the effectiveness of the barrier preceding the
dereference.
Cheers,
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