netdev - Re: [PATCH net 3/3] ipv6: Fix dst_entry refcnt bugs in ip6

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Date:	Wed, 02 Sep 2015 16:04:36 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	kafai@...com, netdev@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH net 3/3] ipv6: Fix dst_entry refcnt bugs in ip6_tunnel

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 02 Sep 2015 15:48:57 -0700

> On Wed, 2015-09-02 at 14:52 -0700, Martin KaFai Lau wrote:
>> On Wed, Sep 02, 2015 at 02:30:45PM -0700, Eric Dumazet wrote:
>> > Object cannot be freed until all cpus have exited their RCU sections.
>> You meant the dst_destroy() here will wait for all cpus exited their RCU sections?
>> 
>> static inline void dst_free(struct dst_entry *dst)
>> {
>> 	if (dst->obsolete > 0)
>> 		return;
>> 	if (!atomic_read(&dst->__refcnt)) {
>> 		dst = dst_destroy(dst);
>> 		if (!dst)
>> 			return;
>> 	}
>> 	__dst_free(dst);
>> }
> 
> dst_free() is called after RCU grace period, in the case you are
> interested in.
> 
> Look at dst_rcu_free() and rt_free()

For ipv4, this is true, but in ipv6, it is not necessarily done in
this way.  And I think that is the point Martin is trying to make.

If you look, the dst_free() calls in ipv6 are basically synchronous,
it does not use dst_rcu_free().

And thus, the fix is to make ipv6 properly RCU free route entries.
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