ipv6: Refactor fib6_drop_pcpu_from

Move the existing pcpu walk in fib6_drop_pcpu_from to a new
helper, __fib6_drop_pcpu_from, that can be invoked per fib6_nh with a
reference to the from entries that need to be evicted. If the passed
in 'from' is non-NULL then only entries associated with that fib6_info
are removed (e.g., case where fib entry is deleted); if the 'from' is
NULL are entries are flushed (e.g., fib6_nh is deleted).

For fib6_info entries with builtin fib6_nh (ie., current code) there
is no change in behavior.
Signed-off-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -874,21 +874,15 @@ static struct fib6_node *fib6_add_1(struct net *net,
 	return ln;
 }
 
-static void fib6_drop_pcpu_from(struct fib6_info *f6i,
-				const struct fib6_table *table)
+static void __fib6_drop_pcpu_from(struct fib6_nh *fib6_nh,
+				  const struct fib6_info *match,
+				  const struct fib6_table *table)
 {
-	struct fib6_nh *fib6_nh = &f6i->fib6_nh;
 	int cpu;
 
 	if (!fib6_nh->rt6i_pcpu)
 		return;
 
-	/* Make sure rt6_make_pcpu_route() wont add other percpu routes
-	 * while we are cleaning them here.
-	 */
-	f6i->fib6_destroying = 1;
-	mb(); /* paired with the cmpxchg() in rt6_make_pcpu_route() */
-
 	/* release the reference to this fib entry from
 	 * all of its cached pcpu routes
 	 */
@@ -898,7 +892,13 @@ static void fib6_drop_pcpu_from(struct fib6_info *f6i,
 
 		ppcpu_rt = per_cpu_ptr(fib6_nh->rt6i_pcpu, cpu);
 		pcpu_rt = *ppcpu_rt;
-		if (pcpu_rt) {
+
+		/* only dropping the 'from' reference if the cached route
+		 * is using 'match'. The cached pcpu_rt->from only changes
+		 * from a fib6_info to NULL (ip6_dst_destroy); it can never
+		 * change from one fib6_info reference to another
+		 */
+		if (pcpu_rt && rcu_access_pointer(pcpu_rt->from) == match) {
 			struct fib6_info *from;
 
 			from = xchg((__force struct fib6_info **)&pcpu_rt->from, NULL);
@@ -907,6 +907,21 @@ static void fib6_drop_pcpu_from(struct fib6_info *f6i,
 	}
 }
 
+static void fib6_drop_pcpu_from(struct fib6_info *f6i,
+				const struct fib6_table *table)
+{
+	struct fib6_nh *fib6_nh;
+
+	/* Make sure rt6_make_pcpu_route() wont add other percpu routes
+	 * while we are cleaning them here.
+	 */
+	f6i->fib6_destroying = 1;
+	mb(); /* paired with the cmpxchg() in rt6_make_pcpu_route() */
+
+	fib6_nh = &f6i->fib6_nh;
+	__fib6_drop_pcpu_from(fib6_nh, f6i, table);
+}
+
 static void fib6_purge_rt(struct fib6_info *rt, struct fib6_node *fn,
 			  struct net *net)
 {