Replace remaining MI uses of random(9)

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
please review the attached patch to remove the two remaining MI uses of
random(9). Neither should be in a performance critical code path and
both may actually benefit from the better distribution. The vfs_bio.c
change is a bit more involved as I am trying to replace a 32bit division
with a long multiplication, which should be cheaper pretty much
anywhere.

Joerg

Index: ufs/ffs/ffs_alloc.c
===================================================================
RCS file: /home/joerg/repo/netbsd/src/sys/ufs/ffs/ffs_alloc.c,v
retrieving revision 1.146
diff -u -p -r1.146 ffs_alloc.c
--- ufs/ffs/ffs_alloc.c 25 Jul 2014 08:24:31 -0000      1.146
+++ ufs/ffs/ffs_alloc.c 8 Sep 2014 13:03:50 -0000
@@ -90,6 +90,7 @@ __KERNEL_RCSID(0, "$NetBSD: ffs_alloc.c,
 #include <sys/syslog.h>
 #include <sys/vnode.h>
 #include <sys/wapbl.h>
+#include <sys/cprng.h>
 
 #include <miscfs/specfs/specdev.h>
 #include <ufs/ufs/quota.h>
@@ -697,7 +698,7 @@ ffs_dirpref(struct inode *pip)
         * Force allocation in another cg if creating a first level dir.
         */
        if (ITOV(pip)->v_vflag & VV_ROOT) {
-               prefcg = random() % fs->fs_ncg;
+               prefcg = cprng_fast32() % fs->fs_ncg;
                mincg = prefcg;
                minndir = fs->fs_ipg;
                for (cg = prefcg; cg < fs->fs_ncg; cg++)
Index: kern/vfs_bio.c
===================================================================
RCS file: /home/joerg/repo/netbsd/src/sys/kern/vfs_bio.c,v
retrieving revision 1.251
diff -u -p -r1.251 vfs_bio.c
--- kern/vfs_bio.c      5 Sep 2014 05:57:21 -0000       1.251
+++ kern/vfs_bio.c      8 Sep 2014 18:00:47 -0000
@@ -143,6 +143,7 @@ __KERNEL_RCSID(0, "$NetBSD: vfs_bio.c,v 
 #include <sys/cpu.h>
 #include <sys/wapbl.h>
 #include <sys/bitops.h>
+#include <sys/cprng.h>
 
 #include <uvm/uvm.h>   /* extern struct uvm uvm */
 
@@ -532,7 +533,7 @@ bufinit2(void)
 static int
 buf_lotsfree(void)
 {
-       int guess, thresh;
+       u_long guess;
 
        /* Always allocate if less than the low water mark. */
        if (bufmem < bufmem_lowater)
@@ -548,16 +549,13 @@ buf_lotsfree(void)
 
        /*
         * The probabily of getting a new allocation is inversely
-        * proportional to the current size of the cache, using
-        * a granularity of 16 steps.
+        * proportional to the current size of the cache above the low
+        * water mark. Divide into 16 steps to avoid overflows.
         */
-       guess = random() & 0x0000000fL;
+       guess = cprng_fast32() % 16;
 
-       /* Don't use "16 * bufmem" here to avoid a 32-bit overflow. */
-       thresh = (bufmem - bufmem_lowater) /
-           ((bufmem_hiwater - bufmem_lowater) / 16);
-
-       if (guess >= thresh)
+       if ((bufmem_hiwater - bufmem_lowater) / 16 * guess >=
+           (bufmem - bufmem_lowater))
                return 1;
 
        /* Otherwise don't allocate. */