lib/div64.c: off by one in shift

fls counts bits starting from 1 to 32 (returns 0 for zero argument).  If
we add 1 we shift right one bit more and loose precision from divisor,
what cause function incorect results with some numbers.

Corrected code was tested in user-space, see bugzilla:
   https://bugzilla.kernel.org/show_bug.cgi?id=202391

Link: http://lkml.kernel.org/r/1548686944-11891-1-git-send-email-sgruszka@redhat.com
Fixes: 658716d19f8f ("div64_u64(): improve precision on 32bit platforms")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Reported-by: Siarhei Volkau <lis8215@gmail.com>
Tested-by: Siarhei Volkau <lis8215@gmail.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/div64.c b/lib/div64.c
index 01c8602bb6ff..ee146bb4c558 100644
--- a/lib/div64.c
+++ b/lib/div64.c
@@ -109,7 +109,7 @@ u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
                 quot = div_u64_rem(dividend, divisor, &rem32);
                 *remainder = rem32;
         } else {
-                int n = 1 + fls(high);
+                int n = fls(high);
                 quot = div_u64(dividend >> n, divisor >> n);
 
                 if (quot != 0)
@@ -147,7 +147,7 @@ u64 div64_u64(u64 dividend, u64 divisor)
         if (high == 0) {
                 quot = div_u64(dividend, divisor);
         } else {
-                int n = 1 + fls(high);
+                int n = fls(high);
                 quot = div_u64(dividend >> n, divisor >> n);
 
                 if (quot != 0)