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LVM2 lib/metadata/metadata.c ./configure.in li ...
- From: zkabelac at sourceware dot org
- To: lvm-devel at redhat dot com, lvm2-cvs at sourceware dot org
- Date: 13 Oct 2010 12:18:54 -0000
- Subject: LVM2 lib/metadata/metadata.c ./configure.in li ...
CVSROOT: /cvs/lvm2
Module name: LVM2
Changes by: zkabelac@sourceware.org 2010-10-13 12:18:54
Modified files:
lib/metadata : metadata.c
. : configure.in
lib/misc : configure.h.in
Log message:
Don't use floor() in _bitset_with_random_bits
Use _even_rand() function instead of floor() in _bitset_with_random_bits().
floor() function is missing in dietlibc (on architectures other than x86).
Moreover using floor() to clip rand results does not assure even result
distribution. _even_rand() uses integer arithmetic only and is designed to
return evenly distributed results.
> Looks OK to me. It took a while to decipher what is the exact meaning of
> the loop in _even_rand (to a non-pseudorandomness-expert) but I am
> fairly comfortable with it now. If I understand this correctly, it
> rejects numbers that come from an "incomplete" slice of the RAND_MAX
> space (considering the number space [0, RAND_MAX] is divided into some
> "max"-sized slices and at most a single smaller slice, between [n*max,
> RAND_MAX] for suitable n -- numbers from this last slice are discarded
> because they could distort the distribution in favour of smaller
> numbers).
Signed-off-by: Przemyslaw Iskra <sparky <at> pld-linux.org>
Reviewed-by: Petr Rockai <prockai <at> redhat.com>
Patches:
http://sourceware.org/cgi-bin/cvsweb.cgi/LVM2/lib/metadata/metadata.c.diff?cvsroot=lvm2&r1=1.404&r2=1.405
http://sourceware.org/cgi-bin/cvsweb.cgi/LVM2/configure.in.diff?cvsroot=lvm2&r1=1.155&r2=1.156
http://sourceware.org/cgi-bin/cvsweb.cgi/LVM2/lib/misc/configure.h.in.diff?cvsroot=lvm2&r1=1.28&r2=1.29
--- LVM2/lib/metadata/metadata.c 2010/10/05 17:34:06 1.404
+++ LVM2/lib/metadata/metadata.c 2010/10/13 12:18:53 1.405
@@ -1015,6 +1015,29 @@
return (uint64_t) size / extent_size;
}
+/*
+ * Return random integer in [0,max) interval
+ *
+ * The loop rejects numbers that come from an "incomplete" slice of the
+ * RAND_MAX space (considering the number space [0, RAND_MAX] is divided
+ * into some "max"-sized slices and at most a single smaller slice,
+ * between [n*max, RAND_MAX] for suitable n -- numbers from this last slice
+ * are discarded because they could distort the distribution in favour of
+ * smaller numbers.
+ */
+static unsigned _even_rand( unsigned *seed, unsigned max )
+{
+ unsigned r, ret;
+
+ /* make sure distribution is even */
+ do {
+ r = (unsigned) rand_r( seed );
+ ret = r % max;
+ } while ( r - ret > RAND_MAX - max );
+
+ return ret;
+}
+
static dm_bitset_t _bitset_with_random_bits(struct dm_pool *mem, uint32_t num_bits,
uint32_t num_set_bits, unsigned *seed)
{
@@ -1037,7 +1060,7 @@
/* Perform loop num_set_bits times, selecting one bit each time */
while (i++ < num_bits) {
/* Select a random bit between 0 and (i-1) inclusive. */
- bit_selected = (unsigned) floor(i * (rand_r(seed) / (RAND_MAX + 1.0)));
+ bit_selected = _even_rand(seed, i);
/*
* If the bit was already set, set the new bit that became
--- LVM2/configure.in 2010/08/23 11:37:02 1.155
+++ LVM2/configure.in 2010/10/13 12:18:53 1.156
@@ -125,8 +125,7 @@
################################################################################
dnl -- Check for functions
-AC_SEARCH_LIBS([floor], [m], , [AC_MSG_ERROR(bailing out)])
-AC_CHECK_FUNCS([floor ftruncate gethostname getpagesize \
+AC_CHECK_FUNCS([ftruncate gethostname getpagesize \
gettimeofday memset mkdir mkfifo rmdir munmap nl_langinfo setenv setlocale \
strcasecmp strchr strcspn strspn strdup strncasecmp strerror strrchr \
strstr strtol strtoul uname], , [AC_MSG_ERROR(bailing out)])
--- LVM2/lib/misc/configure.h.in 2010/08/21 00:16:37 1.28
+++ LVM2/lib/misc/configure.h.in 2010/10/13 12:18:53 1.29
@@ -111,9 +111,6 @@
/* Define to 1 if you have the <fcntl.h> header file. */
#undef HAVE_FCNTL_H
-/* Define to 1 if you have the `floor' function. */
-#undef HAVE_FLOOR
-
/* Define to 1 if you have the `fork' function. */
#undef HAVE_FORK