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

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Date:	Wed, 02 Sep 2015 14:30:45 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Martin KaFai Lau <kafai@...com>
Cc:	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Kernel Team <kernel-team@...com>
Subject: Re: [PATCH net 3/3] ipv6: Fix dst_entry refcnt bugs in ip6_tunnel

On Wed, 2015-09-02 at 13:58 -0700, Martin KaFai Lau wrote:
> On Tue, Sep 01, 2015 at 01:14:20PM -0700, Eric Dumazet wrote:
> > > 2. Use a spinlock to protect the dst_cache operations
> >
> > Well, a seqlock would be better : No need for an atomic operation in
> > fast path.
> >
> seqlock can ensure consistency between idst->dst and idst->cookie.
> However, IPv6 dst destruction is not protected by rcu.  dst_free() is
> directly called, like in ip6_fib.c and a few other places.
> Hence, atomic_inc_not_zero() cannot be used here because the dst may
> have already been kmem_cache_free() when refcnt is 0.

Really ? What about basic rcu rules ?

Object cannot be freed until all cpus have exited their RCU sections.

>   A spinlock is
> needed to stop the ip6_tnl_dst_set() side from removing the refcnt.

Are you telling me RCU should be banished from the kernel ? ;)


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