hlist uses NULL value to finish a chain.
hlist_nulls variant use the low order bit set to 1 to signal an end-of-list marker.
This allows to store many different end markers, so that some RCU lockless
algos (used in TCP/UDP stack for example) can save some memory barriers in
fast paths.
Two new files are added :
include/linux/list_nulls.h
- mimics hlist part of include/linux/list.h, derived to hlist_nulls variant
include/linux/rculist_nulls.h
- mimics hlist part of include/linux/rculist.h, derived to hlist_nulls variant
Only four helpers are declared for the moment :
hlist_nulls_del_init_rcu(), hlist_nulls_del_rcu(),
hlist_nulls_add_head_rcu() and hlist_nulls_for_each_entry_rcu()
prefetches() were removed, since an end of list is not anymore NULL value.
prefetches() could trigger useless (and possibly dangerous) memory transactions.
Example of use (extracted from __udp4_lib_lookup())
struct sock *sk, *result;
struct hlist_nulls_node *node;
unsigned short hnum = ntohs(dport);
unsigned int hash = udp_hashfn(net, hnum);
struct udp_hslot *hslot = &udptable->hash[hash];
int score, badness;
rcu_read_lock();
begin:
result = NULL;
badness = -1;
sk_nulls_for_each_rcu(sk, node, &hslot->head) {
score = compute_score(sk, net, saddr, hnum, sport,
daddr, dport, dif);
if (score > badness) {
result = sk;
badness = score;
}
}
/*
* if the nulls value we got at the end of this lookup is
* not the expected one, we must restart lookup.
* We probably met an item that was moved to another chain.
*/
if (get_nulls_value(node) != hash)
goto begin;
if (result) {
if (unlikely(!atomic_inc_not_zero(&result->sk_refcnt)))
result = NULL;
else if (unlikely(compute_score(result, net, saddr, hnum, sport,
daddr, dport, dif) < badness)) {
sock_put(result);
goto begin;
}
}
rcu_read_unlock();
return result;
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>
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