Re: Multitude of dst obsolescense race conditions

[Eric Dumazet]

On Wed, 2014-05-14 at 02:57 -0700, dormando wrote:
> Hi,
> 
> Given a machine with frequently changing routes (ie; a router with an
> active internet BGP table and multiple interfaces), there're at least
> several places where obsolete dst's are handled improperly. If I pause the
> route changes, the crashes appear to stop. This first one has a crash
> utility we've made, so I was able to more quickly find a patch and test
> it. The others take time to reproduce.
> 
> I'm testing against 3.10.39, but I think if these were fixed they'd be
> backported to stable? I've also had recent 3.12's running that have
> crashed in the same spots. Anyway correct me if I'm wrong...

Is this a vanilla kernel ? I never had any issues like that.

I wonder if you have some RCU issues.

static inline struct dst_entry *
sk_dst_get(struct sock *sk)
{
        struct dst_entry *dst;

        rcu_read_lock();
        dst = rcu_dereference(sk->sk_dst_cache);
        if (dst)
                dst_hold(dst);
        rcu_read_unlock();
        return dst;
}

static inline void
__sk_dst_set(struct sock *sk, struct dst_entry *dst)
{
        struct dst_entry *old_dst;

        sk_tx_queue_clear(sk);
        /*
         * This can be called while sk is owned by the caller only,
         * with no state that can be checked in a rcu_dereference_check() cond
         */
        old_dst = rcu_dereference_raw(sk->sk_dst_cache);
        rcu_assign_pointer(sk->sk_dst_cache, dst);
        dst_release(old_dst);
}

static inline void
sk_dst_set(struct sock *sk, struct dst_entry *dst)
{
        spin_lock(&sk->sk_dst_lock);
        __sk_dst_set(sk, dst);
        spin_unlock(&sk->sk_dst_lock);
}



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