ipv4: disable bh while doing route gc

Further tests revealed that after moving the garbage collector to a work
queue and protecting it with a spinlock may leave the system prone to
soft lockups if bottom half gets very busy.

It was reproced with a set of firewall rules that REJECTed packets. If
the NIC bottom half handler ends up running on the same CPU that is
running the garbage collector on a very large cache, the garbage
collector will not be able to do its job due to the amount of work
needed for handling the REJECTs and also won't reschedule.

The fix is to disable bottom half during the garbage collecting, as it
already was in the first place (most calls to it came from softirqs).
Signed-off-by: Marcelo Ricardo Leitner <mleitner@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/ipv4/route.c

@@ -1000,7 +1000,7 @@ static void __do_rt_garbage_collect(int elasticity, int min_interval)
 	 * do not make it too frequently.
 	 */
 
-	spin_lock(&rt_gc_lock);
+	spin_lock_bh(&rt_gc_lock);
 
 	RT_CACHE_STAT_INC(gc_total);
 
@@ -1103,7 +1103,7 @@ static void __do_rt_garbage_collect(int elasticity, int min_interval)
 	    dst_entries_get_slow(&ipv4_dst_ops) < ipv4_dst_ops.gc_thresh)
 		expire = ip_rt_gc_timeout;
 out:
-	spin_unlock(&rt_gc_lock);
+	spin_unlock_bh(&rt_gc_lock);
 }
 
 static void __rt_garbage_collect(struct work_struct *w)