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* [PATCH v3] time: Make sure jiffies_to_msecs() preserves non-zero time periods
@ 2018-06-22 14:33 Geert Uytterhoeven
  2018-06-22 15:51 ` [tip:timers/urgent] " tip-bot for Geert Uytterhoeven
  0 siblings, 1 reply; 2+ messages in thread
From: Geert Uytterhoeven @ 2018-06-22 14:33 UTC (permalink / raw)
  To: John Stultz, Thomas Gleixner, Stephen Boyd
  Cc: Arnd Bergmann, linux-alpha, linux-mips, linux-kernel,
	Geert Uytterhoeven, stable

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).

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
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 @@
  */
 
 #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
 }
-- 
2.17.1


^ permalink raw reply	[flat|nested] 2+ messages in thread

* [tip:timers/urgent] time: Make sure jiffies_to_msecs() preserves non-zero time periods
  2018-06-22 14:33 [PATCH v3] time: Make sure jiffies_to_msecs() preserves non-zero time periods Geert Uytterhoeven
@ 2018-06-22 15:51 ` " tip-bot for Geert Uytterhoeven
  0 siblings, 0 replies; 2+ messages in thread
From: tip-bot for Geert Uytterhoeven @ 2018-06-22 15:51 UTC (permalink / raw)
  To: linux-tip-commits
  Cc: arnd, john.stultz, geert, sboyd, tglx, linux-kernel, hpa, mingo

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
 }

^ permalink raw reply	[flat|nested] 2+ messages in thread

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2018-06-22 14:33 [PATCH v3] time: Make sure jiffies_to_msecs() preserves non-zero time periods Geert Uytterhoeven
2018-06-22 15:51 ` [tip:timers/urgent] " tip-bot for Geert Uytterhoeven

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