[PATCH RFC net 2/3] ipv6: Add rt6_make_pcpu_route()

[Martin KaFai Lau]

It is a prep work for the potential deadlock.  The current
rt6_get_pcpu_route() will also create a pcpu rt if one does not exist.
This patch moves the pcpu rt creation logic into another function,
rt6_make_pcpu_route().

Signed-off-by: Martin KaFai Lau <ka...@fb.com>
CC: Hannes Frederic Sowa <han...@stressinduktion.org>
---
 net/ipv6/route.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index c95c319..0a82653 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -993,13 +993,21 @@ static struct rt6_info *ip6_rt_pcpu_alloc(struct rt6_info 
*rt)
 /* It should be called with read_lock_bh(&tb6_lock) acquired */
 static struct rt6_info *rt6_get_pcpu_route(struct rt6_info *rt)
 {
-       struct rt6_info *pcpu_rt, *prev, **p;
+       struct rt6_info *pcpu_rt, **p;
 
        p = this_cpu_ptr(rt->rt6i_pcpu);
        pcpu_rt = *p;
 
-       if (pcpu_rt)
-               goto done;
+       if (pcpu_rt) {
+               dst_hold(&pcpu_rt->dst);
+               rt6_dst_from_metrics_check(pcpu_rt);
+       }
+       return pcpu_rt;
+}
+
+static struct rt6_info *rt6_make_pcpu_route(struct rt6_info *rt)
+{
+       struct rt6_info *pcpu_rt, *prev, **p;
 
        pcpu_rt = ip6_rt_pcpu_alloc(rt);
        if (!pcpu_rt) {
@@ -1009,6 +1017,7 @@ static struct rt6_info *rt6_get_pcpu_route(struct 
rt6_info *rt)
                goto done;
        }
 
+       p = this_cpu_ptr(rt->rt6i_pcpu);
        prev = cmpxchg(p, NULL, pcpu_rt);
        if (prev) {
                /* If someone did it before us, return prev instead */
@@ -1093,8 +1102,11 @@ redo_rt6_select:
                rt->dst.lastuse = jiffies;
                rt->dst.__use++;
                pcpu_rt = rt6_get_pcpu_route(rt);
-               read_unlock_bh(&table->tb6_lock);
 
+               if (!pcpu_rt)
+                       pcpu_rt = rt6_make_pcpu_route(rt);
+
+               read_unlock_bh(&table->tb6_lock);
                return pcpu_rt;
        }
 }
-- 
1.8.1

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