Commit c8299cb6 authored Nov 09, 2015 by Michal Nazarewicz Committed by Linus Torvalds Nov 09, 2015

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
: dc491596 (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 <mina86@mina86.com>
Reported-by: John Stultz <john.stultz@linaro.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 9c2d5eeb

include/linux/kernel.h

+24 −21

Original line number	Diff line number	Diff line
		@@ -200,13 +200,18 @@ 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) ({ \
		#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); \
		@@ -216,12 +221,10 @@ extern int _cond_resched(void);
		ret = (__x < 0) ? -__x : __x; \
		} \
		ret; \
		})
		}))

		#define abs64(x) ({ \
		s64 __x = (x); \
		(__x < 0) ? -__x : __x; \
		})
		/* Deprecated, use abs instead. */
		#define abs64(x) abs((s64)(x))

		/**
		* reciprocal_scale - "scale" a value into range [0, ep_ro)

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