Messages in this thread Patch in this message | | | From | Chris Metcalf <> | Subject | [PATCH] clocksource_cyc2ns: avoid overflowing 64 bits | Date | Wed, 16 Nov 2016 11:57:52 -0500 |
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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@mellanox.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
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