Understanding Linux Network Internals

Major Cache Operations

The protocol-independent (DST) part of the cache is a set of dst_entry data structures. Most of the activities in this chapter happen through a dst_entry structure. The IPv4 and IPv6 data structures rtable and rt6_info both include a dst_entry data structure.

The dst_entry structure offers a set of virtual functions in a field named dst_ops, which allows higher-layer protocols to run protocol-specific functions that manipulate the entries. The DST code is located in net/core/dst.c and include/net/dst.h.

All the routines that manipulate dst_entry structures start with a dst_ prefix. Note that even though they operate on dst_entry structures, they actually affect the outer rtable structures, too.

DST is initialized with dst_init, invoked at boot time by net_dev_init (see Chapter 5).

Cache Locking

Read-only operations, such as lookups , use a different locking mechanism from read-write operations such as insertion and deletion, but they naturally have to cooperate. Here is how they are handled:

Read-only operations

These use the routines presented in the section "Cache Lookup" and are protected by a read-copy-update (RCU) read lock, as in the following snapshot:

rcu_read_lock( );
...
perform lookup
...
rcu_read_unlock( );

This code actually does no locking, because read operations can proceed simultaneously without interfering with each other.

Read-write operations

The insertion of an entry (see the section "Adding Elements to the Cache") and the deletion of an entry ...