Understanding Linux Network Internals

Global Locks

The routing code uses a few locks for protection against race conditions. The following list includes only global locks; those that are embedded in the data structure (i.e., applied to single entries) will be addressed in the associated data structure descriptions.

fib_hash_lock

This read-write spin lock (rwlock) protects all the routing tables. For instance, the insertion of a new fib_node instance requires the lock to be taken in exclusive mode, and a routing table lookup requires the lock to be acquired just in shared mode. Since there is only one lock for all the routing tables, it means that it is not possible to add two routing entries to two distinct routing tables at the same time. However, this does not really represent a bottleneck, because configuration changes are rare events and the user can live with a shared lock without any major impact on router performance.

fib_info_lock

This rwlock protects all the fib_info data structures. It is used, for instance, when accessing fib_info structures through the hash tables described in the section "Organization of fib_info Structures" in Chapter 34.

fib_rules_lock

This rwlock protects the fib_rules global list of fib_rule data structures.

rt_flush_lock

This spin lock is used by rt_cache_flush to protect the manipulation of the rt_deadline global variable and the rt_flush_timer timer. The cache is protected by the per-bucket locks. See Figure 33-1 and the section "Flushing the Routing Cache," both in Chapter 33.