On Wed, 9 Jan 2008 20:46:37 +1100
Herbert Xu <[EMAIL PROTECTED]> wrote:
> 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.
You are absolutely right Herbert, so I changed the patch to :
[NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt_cache
In rt_cache_get_next(), no need to guard seq->private by a rcu_dereference()
since seq is private to the thread running this function. Reading seq.private
once (as guaranted bu rcu_dereference()) or several time if compiler really is
dumb enough wont change the result.
But we miss real spots where rcu_dereference() are needed, both in
rt_cache_get_first() and rt_cache_get_next()
Signed-off-by: Eric Dumazet <[EMAIL PROTECTED]>
Signed-off-by: Herbert Xu <[EMAIL PROTECTED]>
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index d337706..28484f3 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -283,12 +283,12 @@ static struct rtable *rt_cache_get_first(struct seq_file
*seq)
break;
rcu_read_unlock_bh();
}
- return r;
+ return rcu_dereference(r);
}
static struct rtable *rt_cache_get_next(struct seq_file *seq, struct rtable *r)
{
- struct rt_cache_iter_state *st = rcu_dereference(seq->private);
+ struct rt_cache_iter_state *st = seq->private;
r = r->u.dst.rt_next;
while (!r) {
@@ -298,7 +298,7 @@ static struct rtable *rt_cache_get_next(struct seq_file
*seq, struct rtable *r)
rcu_read_lock_bh();
r = rt_hash_table[st->bucket].chain;
}
- return r;
+ return rcu_dereference(r);
}
static struct rtable *rt_cache_get_idx(struct seq_file *seq, loff_t pos)
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