[PATCH] fix mult_frac() multiple argument evaluation bug

From: Alexey Dobriyan
Date: Fri May 19 2023 - 16:25:02 EST


mult_frac() evaluates _all_ arguments multiple times in the body.

Clarify comment while I'm at it.

Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
---

include/linux/math.h | 22 +++++++++++-----------
1 file changed, 11 insertions(+), 11 deletions(-)

--- a/include/linux/math.h
+++ b/include/linux/math.h
@@ -118,17 +118,17 @@ __STRUCT_FRACT(s32)
__STRUCT_FRACT(u32)
#undef __STRUCT_FRACT

-/*
- * Multiplies an integer by a fraction, while avoiding unnecessary
- * overflow or loss of precision.
- */
-#define mult_frac(x, numer, denom)( \
-{ \
- typeof(x) quot = (x) / (denom); \
- typeof(x) rem = (x) % (denom); \
- (quot * (numer)) + ((rem * (numer)) / (denom)); \
-} \
-)
+/* Calculate "x * n / d" without unnecessary overflow or loss of precision. */
+#define mult_frac(x, n, d) \
+({ \
+ typeof(x) x_ = (x); \
+ typeof(n) n_ = (n); \
+ typeof(d) d_ = (d); \
+ \
+ typeof(x) q = x_ / d_; \
+ typeof(x) r = x_ % d_; \
+ q * n_ + r * n_ / d_; \
+})

#define sector_div(a, b) do_div(a, b)