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

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Message-ID: <20140105130612.GB17617@localhost.localdomain>
Date:	Sun, 5 Jan 2014 14:06:16 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Kevin Hilman <khilman@...aro.org>
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()

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?

>  }
>  
>  static __init int sched_nohz_full_init_debug(void)
> -- 
> 1.8.3
> 
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