ipv6: Fix a potential deadlock when creating pcpu rt

rt6_make_pcpu_route() is called under read_lock(&table->tb6_lock).
rt6_make_pcpu_route() calls ip6_rt_pcpu_alloc(rt) which then
calls dst_alloc().  dst_alloc() _may_ call ip6_dst_gc() which takes
the write_lock(&tabl->tb6_lock).  A visualized version:

read_lock(&table->tb6_lock);
rt6_make_pcpu_route();
=> ip6_rt_pcpu_alloc();
=> dst_alloc();
=> ip6_dst_gc();
=> write_lock(&table->tb6_lock); /* oops */

The fix is to do a read_unlock first before calling ip6_rt_pcpu_alloc().

A reported stack:

[141625.537638] INFO: rcu_sched self-detected stall on CPU { 27}  (t=60000 jiffies g=4159086 c=4159085 q=2139)
[141625.547469] Task dump for CPU 27:
[141625.550881] mtr             R  running task        0 22121  22081 0x00000008
[141625.558069]  0000000000000000 ffff88103f363d98 ffffffff8106e488 000000000000001b
[141625.565641]  ffffffff81684900 ffff88103f363db8 ffffffff810702b0 0000000008000000
[141625.573220]  ffffffff81684900 ffff88103f363de8 ffffffff8108df9f ffff88103f375a00
[141625.580803] Call Trace:
[141625.583345]  <IRQ>  [<ffffffff8106e488>] sched_show_task+0xc1/0xc6
[141625.589650]  [<ffffffff810702b0>] dump_cpu_task+0x35/0x39
[141625.595144]  [<ffffffff8108df9f>] rcu_dump_cpu_stacks+0x6a/0x8c
[141625.601320]  [<ffffffff81090606>] rcu_check_callbacks+0x1f6/0x5d4
[141625.607669]  [<ffffffff810940c8>] update_process_times+0x2a/0x4f
[141625.613925]  [<ffffffff8109fbee>] tick_sched_handle+0x32/0x3e
[141625.619923]  [<ffffffff8109fc2f>] tick_sched_timer+0x35/0x5c
[141625.625830]  [<ffffffff81094a1f>] __hrtimer_run_queues+0x8f/0x18d
[141625.632171]  [<ffffffff81094c9e>] hrtimer_interrupt+0xa0/0x166
[141625.638258]  [<ffffffff8102bf2a>] local_apic_timer_interrupt+0x4e/0x52
[141625.645036]  [<ffffffff8102c36f>] smp_apic_timer_interrupt+0x39/0x4a
[141625.651643]  [<ffffffff8140b9e8>] apic_timer_interrupt+0x68/0x70
[141625.657895]  <EOI>  [<ffffffff81346ee8>] ? dst_destroy+0x7c/0xb5
[141625.664188]  [<ffffffff813d45b5>] ? fib6_flush_trees+0x20/0x20
[141625.670272]  [<ffffffff81082b45>] ? queue_write_lock_slowpath+0x60/0x6f
[141625.677140]  [<ffffffff8140aa33>] _raw_write_lock_bh+0x23/0x25
[141625.683218]  [<ffffffff813d4553>] __fib6_clean_all+0x40/0x82
[141625.689124]  [<ffffffff813d45b5>] ? fib6_flush_trees+0x20/0x20
[141625.695207]  [<ffffffff813d6058>] fib6_clean_all+0xe/0x10
[141625.700854]  [<ffffffff813d60d3>] fib6_run_gc+0x79/0xc8
[141625.706329]  [<ffffffff813d0510>] ip6_dst_gc+0x85/0xf9
[141625.711718]  [<ffffffff81346d68>] dst_alloc+0x55/0x159
[141625.717105]  [<ffffffff813d09b5>] __ip6_dst_alloc.isra.32+0x19/0x63
[141625.723620]  [<ffffffff813d1830>] ip6_pol_route+0x36a/0x3e8
[141625.729441]  [<ffffffff813d18d6>] ip6_pol_route_output+0x11/0x13
[141625.735700]  [<ffffffff813f02c8>] fib6_rule_action+0xa7/0x1bf
[141625.741698]  [<ffffffff813d18c5>] ? ip6_pol_route_input+0x17/0x17
[141625.748043]  [<ffffffff81357c48>] fib_rules_lookup+0xb5/0x12a
[141625.754050]  [<ffffffff81141628>] ? poll_select_copy_remaining+0xf9/0xf9
[141625.761002]  [<ffffffff813f0535>] fib6_rule_lookup+0x37/0x5c
[141625.766914]  [<ffffffff813d18c5>] ? ip6_pol_route_input+0x17/0x17
[141625.773260]  [<ffffffff813d008c>] ip6_route_output+0x7a/0x82
[141625.779177]  [<ffffffff813c44c8>] ip6_dst_lookup_tail+0x53/0x112
[141625.785437]  [<ffffffff813c45c3>] ip6_dst_lookup_flow+0x2a/0x6b
[141625.791604]  [<ffffffff813ddaab>] rawv6_sendmsg+0x407/0x9b6
[141625.797423]  [<ffffffff813d7914>] ? do_ipv6_setsockopt.isra.8+0xd87/0xde2
[141625.804464]  [<ffffffff8139d4b4>] inet_sendmsg+0x57/0x8e
[141625.810028]  [<ffffffff81329ba3>] sock_sendmsg+0x2e/0x3c
[141625.815588]  [<ffffffff8132be57>] SyS_sendto+0xfe/0x143
[141625.821063]  [<ffffffff813dd551>] ? rawv6_setsockopt+0x5e/0x67
[141625.827146]  [<ffffffff8132c9f8>] ? sock_common_setsockopt+0xf/0x11
[141625.833660]  [<ffffffff8132c08c>] ? SyS_setsockopt+0x81/0xa2
[141625.839565]  [<ffffffff8140ac17>] entry_SYSCALL_64_fastpath+0x12/0x6a

Fixes: d52d3997 ("pv6: Create percpu rt6_info")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
CC: Hannes Frederic Sowa <hannes@stressinduktion.org>
Reported-by: Steinar H. Gunderson <sgunderson@bigfoot.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -172,6 +172,8 @@ static void rt6_free_pcpu(struct rt6_info *non_pcpu_rt)
 			*ppcpu_rt = NULL;
 		}
 	}
+
+	non_pcpu_rt->rt6i_pcpu = NULL;
 }
 
 static void rt6_release(struct rt6_info *rt)

net/ipv6/route.c

@@ -1007,16 +1007,19 @@ static struct rt6_info *rt6_get_pcpu_route(struct rt6_info *rt)
 
 static struct rt6_info *rt6_make_pcpu_route(struct rt6_info *rt)
 {
+	struct fib6_table *table = rt->rt6i_table;
 	struct rt6_info *pcpu_rt, *prev, **p;
 
 	pcpu_rt = ip6_rt_pcpu_alloc(rt);
 	if (!pcpu_rt) {
 		struct net *net = dev_net(rt->dst.dev);
 
-		pcpu_rt = net->ipv6.ip6_null_entry;
-		goto done;
+		dst_hold(&net->ipv6.ip6_null_entry->dst);
+		return net->ipv6.ip6_null_entry;
 	}
 
+	read_lock_bh(&table->tb6_lock);
+	if (rt->rt6i_pcpu) {
 		p = this_cpu_ptr(rt->rt6i_pcpu);
 		prev = cmpxchg(p, NULL, pcpu_rt);
 		if (prev) {
@@ -1024,10 +1027,19 @@ static struct rt6_info *rt6_make_pcpu_route(struct rt6_info *rt)
 			dst_destroy(&pcpu_rt->dst);
 			pcpu_rt = prev;
 		}
-
-done:
+	} else {
+		/* rt has been removed from the fib6 tree
+		 * before we have a chance to acquire the read_lock.
+		 * In this case, don't brother to create a pcpu rt
+		 * since rt is going away anyway.  The next
+		 * dst_check() will trigger a re-lookup.
+		 */
+		dst_destroy(&pcpu_rt->dst);
+		pcpu_rt = rt;
+	}
 	dst_hold(&pcpu_rt->dst);
 	rt6_dst_from_metrics_check(pcpu_rt);
+	read_unlock_bh(&table->tb6_lock);
 	return pcpu_rt;
 }
 
@@ -1103,11 +1115,21 @@ static struct rt6_info *ip6_pol_route(struct net *net, struct fib6_table *table,
 		rt->dst.__use++;
 		pcpu_rt = rt6_get_pcpu_route(rt);
 
-		if (!pcpu_rt)
+		if (pcpu_rt) {
+			read_unlock_bh(&table->tb6_lock);
+		} else {
+			/* We have to do the read_unlock first
+			 * because rt6_make_pcpu_route() may trigger
+			 * ip6_dst_gc() which will take the write_lock.
+			 */
+			dst_hold(&rt->dst);
+			read_unlock_bh(&table->tb6_lock);
 			pcpu_rt = rt6_make_pcpu_route(rt);
+			dst_release(&rt->dst);
+		}
 
-		read_unlock_bh(&table->tb6_lock);
 		return pcpu_rt;
+
 	}
 }