From: Stephen Hemminger <[EMAIL PROTECTED]>
Date: Wed, 23 Jan 2008 14:48:47 -0800
> Since fib dump (via netlink) holds the RTNL mutex, it is unnecessary
> to use RCU, and it is impossible to get truncated (-EBUSY) result.
>
> Signed-off-by: Stephen Hemminger <[EMAIL PROTECTED]>
You tested this patch, right? :-/
The whole reason we need the nlk->cb[] state is to hold things across
multiple recvmsg() calls that might be necessary to obtain the full
dump.
rtnetlink goes:
rtnl_lock();
netlink_rcv_skb(skb, &rtnetlink_rcv_msg);
...
static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
{
...
if (kind == 2 && nlh->nlmsg_flags&NLM_F_DUMP) {
struct sock *rtnl;
rtnl_dumpit_func dumpit;
dumpit = rtnl_get_dumpit(family, type);
if (dumpit == NULL)
return -EOPNOTSUPP;
__rtnl_unlock();
rtnl = net->rtnl;
err = netlink_dump_start(rtnl, skb, nlh, dumpit, NULL);
rtnl_lock();
return err;
(NOTE: Drops RTNL semaphore for netlink_dump_start() call)
...
int netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
struct nlmsghdr *nlh,
int (*dump)(struct sk_buff *skb,
struct netlink_callback *),
int (*done)(struct netlink_callback *))
{
...
cb->dump = dump;
cb->done = done;
cb->nlh = nlh;
atomic_inc(&skb->users);
cb->skb = skb;
...
mutex_lock(nlk->cb_mutex);
...
nlk->cb = cb;
mutex_unlock(nlk->cb_mutex);
netlink_dump(sk);
...
static int netlink_dump(struct sock *sk)
{
...
mutex_lock(nlk->cb_mutex);
...
len = cb->dump(skb, cb);
if (len > 0) {
mutex_unlock(nlk->cb_mutex);
skb_queue_tail(&sk->sk_receive_queue, skb);
sk->sk_data_ready(sk, len);
return 0;
}
(NOTE: Therefore cb->dump() runs without RTNL semaphore held)
...
static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
struct msghdr *msg, size_t len,
int flags)
{
...
if (nlk->cb && atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf / 2)
netlink_dump(sk);
...
Therefore, that RTNL assertion you added should have triggered on any
dump you may have tried since ->dump() is always invoked without the
RTNL semaphore since rtnetlink drops it around the ->dump() call and
the call chain for this fib_trie cause would be:
inet_dump_fib()
fn_trie_dump()
and nothing in that code path retakes the RTNL semaphore.
What test did you run to validate this patch for correctness?
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