linux-kernel - Re: [PATCH 2/2] sched/nohz: fix overflow error in scheduler_tick_max

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Date:	Mon, 06 Jan 2014 10:27:32 -0800
From:	Kevin Hilman <khilman@...aro.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	linaro-kernel@...ts.linaro.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/2] sched/nohz: fix overflow error in scheduler_tick_max_deferment()

Frederic Weisbecker <fweisbec@...il.com> writes:

> On Tue, Dec 17, 2013 at 01:23:08PM -0800, Kevin Hilman wrote:
>> The conversion of the max deferment from usecs to nsecs can easily
>> overflow on platforms where a long is 32-bits.  To fix, cast the usecs
>> value to u64 before multiplying by NSECS_PER_USEC.
>> 
>> This was discovered on 32-bit ARM platform when extending the max
>> deferment value.
>> 
>> Cc: Frederic Weisbecker <fweisbec@...il.com>
>> Signed-off-by: Kevin Hilman <khilman@...aro.org>
>> ---
>>  kernel/sched/core.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 4b1fe3e69fe4..3d7c80e1c4d9 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2203,7 +2203,7 @@ u64 scheduler_tick_max_deferment(void)
>>  	if (time_before_eq(next, now))
>>  		return 0;
>>  
>> -	return jiffies_to_usecs(next - now) * NSEC_PER_USEC;
>> +	return (u64)jiffies_to_usecs(next - now) * NSEC_PER_USEC;
>
> Just to be sure I understand the issue. The problem is that jiffies_to_usecs()
> return an unsigned int which is then multiplied by NSEC_PER_USEC. If the result
> of the mul is too big to be stored in an unsigned int, we overflow and may loose
> some high part of the result. Right?

Correct.

Kevin
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