On Wed, Jan 09, 2008 at 11:37:27AM +0100, Eric Dumazet wrote:
> 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.
Agreed -- as long as you don't try to dereference the pointer before
passing it through rcu_dereference(), and as long as both the initial
fetch of the pointer, the rcu_dereference(), and the actual dereferencing
of the pointer are all within the same RCU read-side critical section.
> 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);
> }
Would it be possible to tag rt_cache_get_first() with an __acquires(RCU)
to help out sparse?
> 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);
> }
Ditto for rt_cache_get_next()?
> static struct rtable *rt_cache_get_idx(struct seq_file *seq, loff_t pos)
There would need to be a __releases(RCU) somewhere -- possibly
in rt_cache_seq_stop(), but need to defer to you guys on this one.
Thanx, Paul
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