Most of the list-empty-check macros (list_empty(), hlist_empty(),
hlist_bl_empty(), hlist_nulls_empty(), and hlist_nulls_empty()) use
an unadorned load to check the list header.  Given that these macros
are sometimes invoked without the protection of a lock, this is
not sufficient.  This commit therefore adds READ_ONCE() calls to
them.  This commit does not touch llist_empty() because it already
has the needed ACCESS_ONCE().

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

The various RCU list-deletion macros (list_del_rcu(),
hlist_del_init_rcu(), hlist_del_rcu(), hlist_bl_del_init_rcu(),
hlist_bl_del_rcu(), hlist_nulls_del_init_rcu(), and hlist_nulls_del_rcu())
do plain stores into the ->next pointer of the preceding list elemment.
Unfortunately, the compiler is within its rights to (for example) use
byte-at-a-time writes to update the pointer, which would fatally confuse
concurrent readers.  This patch therefore adds the needed WRITE_ONCE()
macros.

KernelThreadSanitizer (KTSAN) reported the __hlist_del() issue, which
is a problem when __hlist_del() is invoked by hlist_del_rcu().

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Fixup below build error:

include/linux/list_nulls.h: In function ‘hlist_nulls_del’:
include/linux/list_nulls.h:84:13: error: ‘LIST_POISON2’ undeclared (first use in this function)

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

In order to allow for wider usage of rhashtable, use a special nulls
marker to terminate each chain. The reason for not using the existing
nulls_list is that the prev pointer usage would not be valid as entries
can be linked in two different buckets at the same time.

The 4 nulls base bits can be set through the rhashtable_params structure
like this:

struct rhashtable_params params = {
        [...]
        .nulls_base = (1U << RHT_BASE_SHIFT),
};

This reduces the hash length from 32 bits to 27 bits.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

This patch adds the hlist_nulls_add_head() function which is
based on hlist_nulls_add_head_rcu() but without the use of
rcu_assign_pointer(). It also adds hlist_nulls_del which is
exactly the same like hlist_nulls_del_rcu().

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

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>