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    Subject[tip:timers/urgent] time: Make sure jiffies_to_msecs() preserves non-zero time periods
    Commit-ID:  abcbcb80cd09cd40f2089d912764e315459b71f7
    Gitweb: https://git.kernel.org/tip/abcbcb80cd09cd40f2089d912764e315459b71f7
    Author: Geert Uytterhoeven <geert@linux-m68k.org>
    AuthorDate: Fri, 22 Jun 2018 16:33:57 +0200
    Committer: Thomas Gleixner <tglx@linutronix.de>
    CommitDate: Fri, 22 Jun 2018 17:48:36 +0200

    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.

    Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Reviewed-by: Arnd Bergmann <arnd@arndb.de>
    Cc: John Stultz <john.stultz@linaro.org>
    Cc: Stephen Boyd <sboyd@kernel.org>
    Cc: linux-alpha@vger.kernel.org
    Cc: linux-mips@linux-mips.org
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/20180622143357.7495-1-geert@linux-m68k.org

    ---
    kernel/time/time.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    diff --git a/kernel/time/time.c b/kernel/time/time.c
    index 6fa99213fc72..2b41e8e2d31d 100644
    --- a/kernel/time/time.c
    +++ b/kernel/time/time.c
    @@ -28,6 +28,7 @@
    */

    #include <linux/export.h>
    +#include <linux/kernel.h>
    #include <linux/timex.h>
    #include <linux/capability.h>
    #include <linux/timekeeper_internal.h>
    @@ -314,9 +315,10 @@ unsigned int jiffies_to_msecs(const unsigned long j)
    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
    }
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