From mboxrd@z Thu Jan 1 00:00:00 1970 From: Geert Uytterhoeven Subject: [PATCH v3] time: Make sure jiffies_to_msecs() preserves non-zero time periods Date: Fri, 22 Jun 2018 16:33:57 +0200 Message-ID: <20180622143357.7495-1-geert@linux-m68k.org> Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Return-path: Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="iso-8859-1" To: John Stultz , Thomas Gleixner , Stephen Boyd Cc: Arnd Bergmann , linux-alpha@vger.kernel.org, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org, Geert Uytterhoeven , stable@vger.kernel.org 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 =C2=B5s if HZ > 1000000, and such large values of HZ are already rejected at build time, twice: - include/linux/jiffies.h does #error if HZ >=3D 12288, - kernel/time/time.c has BUILD_BUG_ON(HZ > USEC_PER_SEC). Signed-off-by: Geert Uytterhoeven Reviewed-by: Arnd Bergmann Cc: stable@vger.kernel.org --- Broken since forever. v3: - Add Reviewed-by, - Cc stable, - Explain better why jiffies_to_usecs() does not need a fix, v2: - Add examples of affected systems, - Use DIV_ROUND_UP(). --- 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 6fa99213fc720e4b..2b41e8e2d31db26f 100644 --- a/kernel/time/time.c +++ b/kernel/time/time.c @@ -28,6 +28,7 @@ */ =20 #include +#include #include #include #include @@ -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 =3D=3D 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 } --=20 2.17.1