lib: Add a simple prime number generator
Prime numbers are interesting for testing components that use multiplies
and divides, such as testing DRM's struct drm_mm alignment computations.
v2: Move to lib/, add selftest
v3: Fix initial constants (exclude 0/1 from being primes)
v4: More RCU markup to keep 0day/sparse happy
v5: Fix RCU unwind on module exit, add to kselftests
v6: Tidy computation of bitmap size
v7: for_each_prime_number_from()
v8: Compose small-primes using BIT() for easier verification
v9: Move rcu dance entirely into callers.
v10: Improve quote for Betrand's Postulate (aka Chebyshev's theorem)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222144514.3911-1-chris@chris-wilson.co.uk
diff --git a/lib/prime_numbers.c b/lib/prime_numbers.c
new file mode 100644
index 0000000..c9b3c29
--- /dev/null
+++ b/lib/prime_numbers.c
@@ -0,0 +1,314 @@
+#define pr_fmt(fmt) "prime numbers: " fmt "\n"
+
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/prime_numbers.h>
+#include <linux/slab.h>
+
+#define bitmap_size(nbits) (BITS_TO_LONGS(nbits) * sizeof(unsigned long))
+
+struct primes {
+ struct rcu_head rcu;
+ unsigned long last, sz;
+ unsigned long primes[];
+};
+
+#if BITS_PER_LONG == 64
+static const struct primes small_primes = {
+ .last = 61,
+ .sz = 64,
+ .primes = {
+ BIT(2) |
+ BIT(3) |
+ BIT(5) |
+ BIT(7) |
+ BIT(11) |
+ BIT(13) |
+ BIT(17) |
+ BIT(19) |
+ BIT(23) |
+ BIT(29) |
+ BIT(31) |
+ BIT(37) |
+ BIT(41) |
+ BIT(43) |
+ BIT(47) |
+ BIT(53) |
+ BIT(59) |
+ BIT(61)
+ }
+};
+#elif BITS_PER_LONG == 32
+static const struct primes small_primes = {
+ .last = 31,
+ .sz = 32,
+ .primes = {
+ BIT(2) |
+ BIT(3) |
+ BIT(5) |
+ BIT(7) |
+ BIT(11) |
+ BIT(13) |
+ BIT(17) |
+ BIT(19) |
+ BIT(23) |
+ BIT(29) |
+ BIT(31)
+ }
+};
+#else
+#error "unhandled BITS_PER_LONG"
+#endif
+
+static DEFINE_MUTEX(lock);
+static const struct primes __rcu *primes = RCU_INITIALIZER(&small_primes);
+
+static unsigned long selftest_max;
+
+static bool slow_is_prime_number(unsigned long x)
+{
+ unsigned long y = int_sqrt(x);
+
+ while (y > 1) {
+ if ((x % y) == 0)
+ break;
+ y--;
+ }
+
+ return y == 1;
+}
+
+static unsigned long slow_next_prime_number(unsigned long x)
+{
+ while (x < ULONG_MAX && !slow_is_prime_number(++x))
+ ;
+
+ return x;
+}
+
+static unsigned long clear_multiples(unsigned long x,
+ unsigned long *p,
+ unsigned long start,
+ unsigned long end)
+{
+ unsigned long m;
+
+ m = 2 * x;
+ if (m < start)
+ m = roundup(start, x);
+
+ while (m < end) {
+ __clear_bit(m, p);
+ m += x;
+ }
+
+ return x;
+}
+
+static bool expand_to_next_prime(unsigned long x)
+{
+ const struct primes *p;
+ struct primes *new;
+ unsigned long sz, y;
+
+ /* Betrand's Postulate (or Chebyshev's theorem) states that if n > 3,
+ * there is always at least one prime p between n and 2n - 2.
+ * Equivalently, if n > 1, then there is always at least one prime p
+ * such that n < p < 2n.
+ *
+ * http://mathworld.wolfram.com/BertrandsPostulate.html
+ * https://en.wikipedia.org/wiki/Bertrand's_postulate
+ */
+ sz = 2 * x;
+ if (sz < x)
+ return false;
+
+ sz = round_up(sz, BITS_PER_LONG);
+ new = kmalloc(sizeof(*new) + bitmap_size(sz), GFP_KERNEL);
+ if (!new)
+ return false;
+
+ mutex_lock(&lock);
+ p = rcu_dereference_protected(primes, lockdep_is_held(&lock));
+ if (x < p->last) {
+ kfree(new);
+ goto unlock;
+ }
+
+ /* Where memory permits, track the primes using the
+ * Sieve of Eratosthenes. The sieve is to remove all multiples of known
+ * primes from the set, what remains in the set is therefore prime.
+ */
+ bitmap_fill(new->primes, sz);
+ bitmap_copy(new->primes, p->primes, p->sz);
+ for (y = 2UL; y < sz; y = find_next_bit(new->primes, sz, y + 1))
+ new->last = clear_multiples(y, new->primes, p->sz, sz);
+ new->sz = sz;
+
+ BUG_ON(new->last <= x);
+
+ rcu_assign_pointer(primes, new);
+ if (p != &small_primes)
+ kfree_rcu((struct primes *)p, rcu);
+
+unlock:
+ mutex_unlock(&lock);
+ return true;
+}
+
+static void free_primes(void)
+{
+ const struct primes *p;
+
+ mutex_lock(&lock);
+ p = rcu_dereference_protected(primes, lockdep_is_held(&lock));
+ if (p != &small_primes) {
+ rcu_assign_pointer(primes, &small_primes);
+ kfree_rcu((struct primes *)p, rcu);
+ }
+ mutex_unlock(&lock);
+}
+
+/**
+ * next_prime_number - return the next prime number
+ * @x: the starting point for searching to test
+ *
+ * A prime number is an integer greater than 1 that is only divisible by
+ * itself and 1. The set of prime numbers is computed using the Sieve of
+ * Eratoshenes (on finding a prime, all multiples of that prime are removed
+ * from the set) enabling a fast lookup of the next prime number larger than
+ * @x. If the sieve fails (memory limitation), the search falls back to using
+ * slow trial-divison, up to the value of ULONG_MAX (which is reported as the
+ * final prime as a sentinel).
+ *
+ * Returns: the next prime number larger than @x
+ */
+unsigned long next_prime_number(unsigned long x)
+{
+ const struct primes *p;
+
+ rcu_read_lock();
+ p = rcu_dereference(primes);
+ while (x >= p->last) {
+ rcu_read_unlock();
+
+ if (!expand_to_next_prime(x))
+ return slow_next_prime_number(x);
+
+ rcu_read_lock();
+ p = rcu_dereference(primes);
+ }
+ x = find_next_bit(p->primes, p->last, x + 1);
+ rcu_read_unlock();
+
+ return x;
+}
+EXPORT_SYMBOL(next_prime_number);
+
+/**
+ * is_prime_number - test whether the given number is prime
+ * @x: the number to test
+ *
+ * A prime number is an integer greater than 1 that is only divisible by
+ * itself and 1. Internally a cache of prime numbers is kept (to speed up
+ * searching for sequential primes, see next_prime_number()), but if the number
+ * falls outside of that cache, its primality is tested using trial-divison.
+ *
+ * Returns: true if @x is prime, false for composite numbers.
+ */
+bool is_prime_number(unsigned long x)
+{
+ const struct primes *p;
+ bool result;
+
+ rcu_read_lock();
+ p = rcu_dereference(primes);
+ while (x >= p->sz) {
+ rcu_read_unlock();
+
+ if (!expand_to_next_prime(x))
+ return slow_is_prime_number(x);
+
+ rcu_read_lock();
+ p = rcu_dereference(primes);
+ }
+ result = test_bit(x, p->primes);
+ rcu_read_unlock();
+
+ return result;
+}
+EXPORT_SYMBOL(is_prime_number);
+
+static void dump_primes(void)
+{
+ const struct primes *p;
+ char *buf;
+
+ buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
+
+ rcu_read_lock();
+ p = rcu_dereference(primes);
+
+ if (buf)
+ bitmap_print_to_pagebuf(true, buf, p->primes, p->sz);
+ pr_info("primes.{last=%lu, .sz=%lu, .primes[]=...x%lx} = %s",
+ p->last, p->sz, p->primes[BITS_TO_LONGS(p->sz) - 1], buf);
+
+ rcu_read_unlock();
+
+ kfree(buf);
+}
+
+static int selftest(unsigned long max)
+{
+ unsigned long x, last;
+
+ if (!max)
+ return 0;
+
+ for (last = 0, x = 2; x < max; x++) {
+ bool slow = slow_is_prime_number(x);
+ bool fast = is_prime_number(x);
+
+ if (slow != fast) {
+ pr_err("inconsistent result for is-prime(%lu): slow=%s, fast=%s!",
+ x, slow ? "yes" : "no", fast ? "yes" : "no");
+ goto err;
+ }
+
+ if (!slow)
+ continue;
+
+ if (next_prime_number(last) != x) {
+ pr_err("incorrect result for next-prime(%lu): expected %lu, got %lu",
+ last, x, next_prime_number(last));
+ goto err;
+ }
+ last = x;
+ }
+
+ pr_info("selftest(%lu) passed, last prime was %lu", x, last);
+ return 0;
+
+err:
+ dump_primes();
+ return -EINVAL;
+}
+
+static int __init primes_init(void)
+{
+ return selftest(selftest_max);
+}
+
+static void __exit primes_exit(void)
+{
+ free_primes();
+}
+
+module_init(primes_init);
+module_exit(primes_exit);
+
+module_param_named(selftest, selftest_max, ulong, 0400);
+
+MODULE_AUTHOR("Intel Corporation");
+MODULE_LICENSE("GPL");