linux-kernel - [PATCH] clocksource_cyc2ns: avoid overflowing 64 bits

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Date:   Wed, 16 Nov 2016 11:57:52 -0500
From:   Chris Metcalf <cmetcalf@...lanox.com>
To:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Salman Qazi <sqazi@...gle.com>, Paul Turner <pjt@...gle.com>,
        Tony Lindgren <tony@...mide.com>,
        Steven Miao <realmz6@...il.com>, linux-kernel@...r.kernel.org
Cc:     Chris Metcalf <cmetcalf@...lanox.com>
Subject: [PATCH] clocksource_cyc2ns: avoid overflowing 64 bits

For large values of "mult" and long uptimes, the intermediate
result of "cycles * mult" can overflow 64 bits.  For example,
the tile platform uses this helper function; for a 1.2 GHz clock,
we have mult = 853, and after 208.5 days, we overflow 64 bits.

The fix is basically the same as the fix for arch/x86 __cycles_2_ns()
in commit 4cecf6d401a0 ("sched, x86: Avoid unnecessary overflow in
sched_clock"), using the new mult_frac() helper.

In addition to tile, arm/plat-omap and blackfin also use this helper
function, so will presumably hit similar issues.

Signed-off-by: Chris Metcalf <cmetcalf@...lanox.com>
---
By the way, this is the bug that I was looking for when I tripped over
the missing bugfix for timekeeping_delta_to_ns() a couple of days ago :-)

 include/linux/clocksource.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 08398182f56e..b2a022acf232 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -175,7 +175,7 @@ static inline u32 clocksource_hz2mult(u32 hz, u32 shift_constant)
  */
 static inline s64 clocksource_cyc2ns(cycle_t cycles, u32 mult, u32 shift)
 {
-	return ((u64) cycles * mult) >> shift;
+	return mult_frac(cycles, mult, 1ULL << shift);
 }

-- 
2.7.2