internal reference counters, UMA_ZONE_NOFREE. This way, those slabs
(with their ref counts) will be effectively type-stable, then using
a trick like this on the refcount is no longer dangerous:
MEXT_REM_REF(m);
if (atomic_cmpset_int(m->m_ext.ref_cnt, 0, 1)) {
if (m->m_ext.ext_type == EXT_PACKET) {
uma_zfree(zone_pack, m);
return;
} else if (m->m_ext.ext_type == EXT_CLUSTER) {
uma_zfree(zone_clust, m->m_ext.ext_buf);
m->m_ext.ext_buf = NULL;
} else {
(*(m->m_ext.ext_free))(m->m_ext.ext_buf,
m->m_ext.ext_args);
if (m->m_ext.ext_type != EXT_EXTREF)
free(m->m_ext.ref_cnt, M_MBUF);
}
}
uma_zfree(zone_mbuf, m);
Previously, a second thread hitting the above cmpset might
actually read the refcnt AFTER it has already been freed. A very
rare occurance. Now we'll know that it won't be freed, though.
Spotted by: julian, pjd
e66468ea7a