div64_u64(): improve precision on 32bit platforms

The current implementation of div64_u64 for 32bit systems returns an
approximately correct result when the divisor exceeds 32bits.  Since doing
64bit division using 32bit hardware is a long since solved problem we just
use one of the existing proven methods.

Additionally, add a div64_s64 function to correctly handle doing signed
64bit division.

Addresses https://bugzilla.redhat.com/show_bug.cgi?id=616105

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Ben Woodard <bwoodard@llnl.gov>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Mark Grondona <mgrondona@llnl.gov>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 42 insertions, 10 deletions

diff --git a/lib/div64.c b/lib/div64.c
index a111eb8de9cf..5b4919191778 100644
--- a/lib/div64.c
+++ b/lib/div64.c
@@ -77,26 +77,58 @@ s64 div_s64_rem(s64 dividend, s32 divisor, s32 *remainder)
 EXPORT_SYMBOL(div_s64_rem);
 #endif
 
-/* 64bit divisor, dividend and result. dynamic precision */
+/**
+ * div64_u64 - unsigned 64bit divide with 64bit divisor
+ * @dividend:   64bit dividend
+ * @divisor:    64bit divisor
+ *
+ * This implementation is a modified version of the algorithm proposed
+ * by the book 'Hacker's Delight'.  The original source and full proof
+ * can be found here and is available for use without restriction.
+ *
+ * 'http://www.hackersdelight.org/HDcode/newCode/divDouble.c'
+ */
 #ifndef div64_u64
 u64 div64_u64(u64 dividend, u64 divisor)
 {
-        u32 high, d;
+        u32 high = divisor >> 32;
+        u64 quot;
 
-        high = divisor >> 32;
-        if (high) {
-                unsigned int shift = fls(high);
+        if (high == 0) {
+                quot = div_u64(dividend, divisor);
+        } else {
+                int n = 1 + fls(high);
+                quot = div_u64(dividend >> n, divisor >> n);
 
-                d = divisor >> shift;
-                dividend >>= shift;
-        } else
-                d = divisor;
+                if (quot != 0)
+                        quot--;
+                if ((dividend - quot * divisor) >= divisor)
+                        quot++;
+        }
 
-        return div_u64(dividend, d);
+        return quot;
 }
 EXPORT_SYMBOL(div64_u64);
 #endif
 
+/**
+ * div64_s64 - signed 64bit divide with 64bit divisor
+ * @dividend:   64bit dividend
+ * @divisor:    64bit divisor
+ */
+#ifndef div64_s64
+s64 div64_s64(s64 dividend, s64 divisor)
+{
+        s64 quot, t;
+
+        quot = div64_u64(abs64(dividend), abs64(divisor));
+        t = (dividend ^ divisor) >> 63;
+
+        return (quot ^ t) - t;
+}
+EXPORT_SYMBOL(div64_s64);
+#endif
+
 #endif /* BITS_PER_LONG == 32 */
 
 /*