linux-kernel - Re: [PATCH] sched: fair: avoid integer overflow

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Date:	Mon, 11 Nov 2013 11:46:59 -0800
From:	bsegall@...gle.com
To:	Michal Nazarewicz <mpn@...gle.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Michal Nazarewicz <mina86@...a86.com>
Subject: Re: [PATCH] sched: fair: avoid integer overflow

Michal Nazarewicz <mpn@...gle.com> writes:

> From: Michal Nazarewicz <mina86@...a86.com>
>
> sa->runnable_avg_sum is of type u32 but after shifting it by NICE_0_SHIFT
> bits it is promoted to u64.  This of course makes no sense, since the
> result will never be more then 32-bit long.  Casting sa->runnable_avg_sum
> to u64 before it is shifted, fixes this problem.
>
> Signed-off-by: Michal Nazarewicz <mina86@...a86.com>
Reviewed-by: Ben Segall <bsegall@...gle.com>

> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index df77c60..50f1e170 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2153,7 +2153,7 @@ static inline void __update_tg_runnable_avg(struct sched_avg *sa,
>  	long contrib;
>  
>  	/* The fraction of a cpu used by this cfs_rq */
> -	contrib = div_u64(sa->runnable_avg_sum << NICE_0_SHIFT,
> +	contrib = div_u64((u64)sa->runnable_avg_sum << NICE_0_SHIFT,
>  			  sa->runnable_avg_period + 1);
>  	contrib -= cfs_rq->tg_runnable_contrib;
--
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