time: Make sure jiffies_to_msecs() preserves non-zero time periods

For the common cases where 1000 is a multiple of HZ, or HZ is a multiple of 1000, jiffies_to_msecs() never returns zero when passed a non-zero time period. However, if HZ > 1000 and not an integer multiple of 1000 (e.g. 1024 or 1200, as used on alpha and DECstation), jiffies_to_msecs() may return zero for small non-zero time periods. This may break code that relies on receiving back a non-zero value. jiffies_to_usecs() does not need such a fix: one jiffy can only be less than one µs if HZ > 1000000, and such large values of HZ are already rejected at build time, twice: - include/linux/jiffies.h does #error if HZ >= 12288, - kernel/time/time.c has BUILD_BUG_ON(HZ > USEC_PER_SEC). Broken since forever.

Geert Uytterhoeven committed 8 years ago

Commit abcbcb81 parent 0fe27951 file · +4 −2Browse files

Unified · Split

kernel/time/time.c+4−2

  */
 #include <linux/export.h>
+#include <linux/kernel.h>
 #include <linux/timex.h>
 #include <linux/capability.h>
 #include <linux/timekeeper_internal.h>
 	return (j + (HZ / MSEC_PER_SEC) - 1)/(HZ / MSEC_PER_SEC);
 #else
 # if BITS_PER_LONG == 32
-	return (HZ_TO_MSEC_MUL32 * j) >> HZ_TO_MSEC_SHR32;
+	return (HZ_TO_MSEC_MUL32 * j + (1ULL << HZ_TO_MSEC_SHR32) - 1) >>
+	       HZ_TO_MSEC_SHR32;
 # else
-	return (j * HZ_TO_MSEC_NUM) / HZ_TO_MSEC_DEN;
+	return DIV_ROUND_UP(j * HZ_TO_MSEC_NUM, HZ_TO_MSEC_DEN);
 # endif
 #endif
 }