Many functions have open coded a function that return a random number in range [0,N-1]. Also, only because we have a function that is named reciprocal_divide(), it has not much to do with the pupose where it is being used when a previous reciprocal_value() has not been obtained.
Under the assumption that we have a PRNG such as taus113 with being well distributed in [0, ~0U] space, we can implement such a function as uword t = (n*m')>>32, where m' is a random number obtained from PRNG, n the right open interval border and t our resulting random number, with n,m',t in u32 universe. Other users can further be migrated to the new prandom_u32_lt_N() function later on; for now, we need to make sure to migrate "misuses" of reciprocal_divide() for the reciprocal_divide() follow-up fixup. Joint work with Hannes Frederic Sowa. Cc: Jakub Zawadzki <[email protected]> Cc: Eric Dumazet <[email protected]> Cc: [email protected] Signed-off-by: Hannes Frederic Sowa <[email protected]> Signed-off-by: Daniel Borkmann <[email protected]> --- drivers/net/team/team_mode_random.c | 8 +------- include/linux/random.h | 19 ++++++++++++++++++- include/net/codel.h | 4 +--- net/packet/af_packet.c | 5 ++--- net/sched/sch_choke.c | 9 +-------- 5 files changed, 23 insertions(+), 22 deletions(-) diff --git a/drivers/net/team/team_mode_random.c b/drivers/net/team/team_mode_random.c index 7f032e2..f2d22a5 100644 --- a/drivers/net/team/team_mode_random.c +++ b/drivers/net/team/team_mode_random.c @@ -13,20 +13,14 @@ #include <linux/module.h> #include <linux/init.h> #include <linux/skbuff.h> -#include <linux/reciprocal_div.h> #include <linux/if_team.h> -static u32 random_N(unsigned int N) -{ - return reciprocal_divide(prandom_u32(), N); -} - static bool rnd_transmit(struct team *team, struct sk_buff *skb) { struct team_port *port; int port_index; - port_index = random_N(team->en_port_count); + port_index = prandom_u32_lt_N(team->en_port_count); port = team_get_port_by_index_rcu(team, port_index); if (unlikely(!port)) goto drop; diff --git a/include/linux/random.h b/include/linux/random.h index 4002b3d..cc3f006 100644 --- a/include/linux/random.h +++ b/include/linux/random.h @@ -8,7 +8,6 @@ #include <uapi/linux/random.h> - extern void add_device_randomness(const void *, unsigned int); extern void add_input_randomness(unsigned int type, unsigned int code, unsigned int value); @@ -38,6 +37,24 @@ struct rnd_state { u32 prandom_u32_state(struct rnd_state *state); void prandom_bytes_state(struct rnd_state *state, void *buf, int nbytes); +/** + * prandom_u32_lt_N - returns a random number in interval [0, N), the + * interval is right-open. This is useful when an + * requesting random index of an array containing + * N elements, for example. + * + * @N: higher interval endpoint + * + * Returns a pseduo-random number that is in interval [0, N). Note + * that the result depends on PRNG being well distributed in [0, ~0U] + * space. Here, we use maximally equidistributed combined Tausworthe + * generator, that is, prandom_u32(). + */ +static inline u32 prandom_u32_lt_N(u32 N) +{ + return (u32)(((u64) prandom_u32() * N) >> 32); +} + /* * Handle minimum values for seeds */ diff --git a/include/net/codel.h b/include/net/codel.h index 3b04ff5..7705a72 100644 --- a/include/net/codel.h +++ b/include/net/codel.h @@ -46,7 +46,6 @@ #include <linux/skbuff.h> #include <net/pkt_sched.h> #include <net/inet_ecn.h> -#include <linux/reciprocal_div.h> /* Controlling Queue Delay (CoDel) algorithm * ========================================= @@ -211,10 +210,9 @@ static codel_time_t codel_control_law(codel_time_t t, codel_time_t interval, u32 rec_inv_sqrt) { - return t + reciprocal_divide(interval, rec_inv_sqrt << REC_INV_SQRT_SHIFT); + return t + (u32)(((u64) interval * (rec_inv_sqrt << REC_INV_SQRT_SHIFT)) >> 32); } - static bool codel_should_drop(const struct sk_buff *skb, struct Qdisc *sch, struct codel_vars *vars, diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c index 279467b..f976474 100644 --- a/net/packet/af_packet.c +++ b/net/packet/af_packet.c @@ -88,7 +88,6 @@ #include <linux/virtio_net.h> #include <linux/errqueue.h> #include <linux/net_tstamp.h> -#include <linux/reciprocal_div.h> #ifdef CONFIG_INET #include <net/inet_common.h> #endif @@ -1220,7 +1219,7 @@ static unsigned int fanout_demux_hash(struct packet_fanout *f, struct sk_buff *skb, unsigned int num) { - return reciprocal_divide(skb->rxhash, num); + return (u32)(((u64) skb->rxhash * num) >> 32); } static unsigned int fanout_demux_lb(struct packet_fanout *f, @@ -1247,7 +1246,7 @@ static unsigned int fanout_demux_rnd(struct packet_fanout *f, struct sk_buff *skb, unsigned int num) { - return reciprocal_divide(prandom_u32(), num); + return prandom_u32_lt_N(num); } static unsigned int fanout_demux_rollover(struct packet_fanout *f, diff --git a/net/sched/sch_choke.c b/net/sched/sch_choke.c index ddd73cb..556b6f6 100644 --- a/net/sched/sch_choke.c +++ b/net/sched/sch_choke.c @@ -14,7 +14,6 @@ #include <linux/types.h> #include <linux/kernel.h> #include <linux/skbuff.h> -#include <linux/reciprocal_div.h> #include <linux/vmalloc.h> #include <net/pkt_sched.h> #include <net/inet_ecn.h> @@ -77,12 +76,6 @@ struct choke_sched_data { struct sk_buff **tab; }; -/* deliver a random number between 0 and N - 1 */ -static u32 random_N(unsigned int N) -{ - return reciprocal_divide(prandom_u32(), N); -} - /* number of elements in queue including holes */ static unsigned int choke_len(const struct choke_sched_data *q) { @@ -233,7 +226,7 @@ static struct sk_buff *choke_peek_random(const struct choke_sched_data *q, int retrys = 3; do { - *pidx = (q->head + random_N(choke_len(q))) & q->tab_mask; + *pidx = (q->head + prandom_u32_lt_N(choke_len(q))) & q->tab_mask; skb = q->tab[*pidx]; if (skb) return skb; -- 1.8.3.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
