For more information about SELINUX

For more information about SELINUX, see Linux general technology-Linux programming and kernel. Question 1: How does the system cache new decisions made by the security server to AVC? What is the use of netlink broadcast?

Question 2: in kernel code, why do the security_load_policy function in security/selinux/ss/services. c need to save the old SID table, load the new table, and release the old table? What's wrong with releasing the old table directly without saving it?
Int security_load_policy (void * data, size_t len ){
...
/* Save the old policydb and SID table to free later .*/
Memcpy (& oldpolicydb, & policydb, sizeof policydb );
Sidtab_set (& oldsidtab, & sidtab );

/* Install the new policydb and SID table .*/
POLICY_WRLOCK;
Memcpy (& policydb, & newpolicydb, sizeof policydb );
Sidtab_set (& sidtab, & newsidtab );
Seqno = ++ latest_granting;
Policydb_loaded_version = policydb. policyvers;
POLICY_WRUNLOCK;
LOAD_UNLOCK;

/* Free the old policydb and SID table .*/
Policydb_destroy (& oldpolicydb );
Sidtab_destroy (& oldsidtab );
...
}