Re: [PATCH net 0/3] net: nexthop: fix refcount issues when replacing groups

On Sun, Nov 21, 2021 at 05:24:50PM +0200, Nikolay Aleksandrov wrote:

From: Nikolay Aleksandrov <redacted>

Hi,
This set fixes a refcount bug when replacing nexthop groups and
modifying routes. It is complex because the objects look valid when
debugging memory dumps, but we end up having refcount dependency between
unlinked objects which can never be released, so in turn they cannot
free their resources and refcounts. The problem happens because we can
have stale IPv6 per-cpu dsts in nexthops which were removed from a
group. Even though the IPv6 gen is bumped, the dsts won't be released
until traffic passes through them or the nexthop is freed, that can take
arbitrarily long time, and even worse we can create a scenario[1] where it
can never be released. The fix is to release the IPv6 per-cpu dsts of
replaced nexthops after an RCU grace period so no new ones can be
created. To do that we add a new IPv6 stub - fib6_nh_release_dsts, which
is used by the nexthop code only when necessary. We can further optimize
group replacement, but that is more suited for net-next as these patches
would have to be backported to stable releases.

Will run regression with these patches tonight and report tomorrow