From c8299cb605b27dd5a49f7a69e48fd23e5a206298 Mon Sep 17 00:00:00 2001 From: Michal Nazarewicz Date: Mon, 9 Nov 2015 14:58:10 -0800 Subject: kernel.h: make abs() work with 64-bit types For 64-bit arguments, the abs macro casts it to an int which leads to lost precision and may cause incorrect results. To deal with 64-bit types abs64 macro has been introduced but still there are places where abs macro is used incorrectly. To deal with the problem, expand abs macro such that it operates on s64 type when dealing with 64-bit types while still returning long when dealing with smaller types. This fixes one known bug (per John): The internal clocksteering done for fine-grained error correction uses a : logarithmic approximation, so any time adjtimex() adjusts the clock : steering, timekeeping_freqadjust() quickly approximates the correct clock : frequency over a series of ticks. : : Unfortunately, the logic in timekeeping_freqadjust(), introduced in commit : dc491596f639438 (Rework frequency adjustments to work better w/ nohz), : used the abs() function with a s64 error value to calculate the size of : the approximated adjustment to be made. : : Per include/linux/kernel.h: "abs() should not be used for 64-bit types : (s64, u64, long long) - use abs64()". : : Thus on 32-bit platforms, this resulted in the clocksteering to take a : quite dampended random walk trying to converge on the proper frequency, : which caused the adjustments to be made much slower then intended (most : easily observed when large adjustments are made). Signed-off-by: Michal Nazarewicz Reported-by: John Stultz Tested-by: John Stultz Cc: Ingo Molnar Cc: Steven Rostedt Cc: Peter Zijlstra Cc: Masami Hiramatsu Cc: Peter Zijlstra Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/kernel.h | 45 ++++++++++++++++++++++++--------------------- 1 file changed, 24 insertions(+), 21 deletions(-) diff --git a/include/linux/kernel.h b/include/linux/kernel.h index 2c13f747ac2e..05ce782d53ab 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -200,28 +200,31 @@ extern int _cond_resched(void); #define might_sleep_if(cond) do { if (cond) might_sleep(); } while (0) -/* - * abs() handles unsigned and signed longs, ints, shorts and chars. For all - * input types abs() returns a signed long. - * abs() should not be used for 64-bit types (s64, u64, long long) - use abs64() - * for those. +/** + * abs - return absolute value of an argument + * @x: the value. If it is unsigned type, it is converted to signed type first + * (s64, long or int depending on its size). + * + * Return: an absolute value of x. If x is 64-bit, macro's return type is s64, + * otherwise it is signed long. */ -#define abs(x) ({ \ - long ret; \ - if (sizeof(x) == sizeof(long)) { \ - long __x = (x); \ - ret = (__x < 0) ? -__x : __x; \ - } else { \ - int __x = (x); \ - ret = (__x < 0) ? -__x : __x; \ - } \ - ret; \ - }) - -#define abs64(x) ({ \ - s64 __x = (x); \ - (__x < 0) ? -__x : __x; \ - }) +#define abs(x) __builtin_choose_expr(sizeof(x) == sizeof(s64), ({ \ + s64 __x = (x); \ + (__x < 0) ? -__x : __x; \ + }), ({ \ + long ret; \ + if (sizeof(x) == sizeof(long)) { \ + long __x = (x); \ + ret = (__x < 0) ? -__x : __x; \ + } else { \ + int __x = (x); \ + ret = (__x < 0) ? -__x : __x; \ + } \ + ret; \ + })) + +/* Deprecated, use abs instead. */ +#define abs64(x) abs((s64)(x)) /** * reciprocal_scale - "scale" a value into range [0, ep_ro) -- cgit v1.2.3