diff options
| author | Daniel Borkmann | 2014-01-22 02:29:39 +0100 |
|---|---|---|
| committer | David S. Miller | 2014-01-21 23:17:20 -0800 |
| commit | f337db64af059c9a94278a8b0ab97d87259ff62f (patch) | |
| tree | ef7db2258d5929760f2a22b74779d374916634bd /include | |
| parent | 6cd28f044b47aeeba91807d97d6f3ea5a048e88d (diff) | |
random32: add prandom_u32_max and convert open coded users
Many functions have open coded a function that returns a random
number in range [0,N-1]. 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.
Lets go with Joe and simply call it prandom_u32_max(), although
technically we have an right open interval endpoint, but that we
have documented. Other users can further be migrated to the new
prandom_u32_max() function later on; for now, we need to make sure
to migrate reciprocal_divide() users for the reciprocal_divide()
follow-up fixup since their function signatures are going to change.
Joint work with Hannes Frederic Sowa.
Cc: Jakub Zawadzki <darkjames-ws@darkjames.pl>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'include')
| -rw-r--r-- | include/linux/random.h | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/include/linux/random.h b/include/linux/random.h index 4002b3df4c85..1cfce0e24dbd 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 +37,23 @@ 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_max - returns a pseudo-random number in interval [0, ep_ro) + * @ep_ro: right open interval endpoint + * + * Returns a pseudo-random number that is in interval [0, ep_ro). Note + * that the result depends on PRNG being well distributed in [0, ~0U] + * u32 space. Here we use maximally equidistributed combined Tausworthe + * generator, that is, prandom_u32(). This is useful when requesting a + * random index of an array containing ep_ro elements, for example. + * + * Returns: pseudo-random number in interval [0, ep_ro) + */ +static inline u32 prandom_u32_max(u32 ep_ro) +{ + return (u32)(((u64) prandom_u32() * ep_ro) >> 32); +} + /* * Handle minimum values for seeds */ |