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

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Message-ID: <20150902205802.GA78846@kafai-mba.local>
Date:	Wed, 2 Sep 2015 13:58:02 -0700
From:	Martin KaFai Lau <kafai@...com>
To:	Eric Dumazet <eric.dumazet@...il.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 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.  A spinlock is
needed to stop the ip6_tnl_dst_set() side from removing the refcnt.
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