linux-kernel - Re: [PATCH v2] sched/fair: Fix integer underflow

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Message-ID: <d629b646-d04b-4a26-86a2-34300dcd3e9f@arm.com>
Date: Tue, 25 Mar 2025 16:10:30 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: linux-kernel@...r.kernel.org
Cc: stable@...r.kernel.org, Vincent Guittot <vincent.guittot@...aro.org>,
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
 Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v2] sched/fair: Fix integer underflow

Hello Vincent,

This patch should still be relevant, would it be possible to pick it ?
Or maybe something is missing ?

Regards,
Pierre

On 10/1/24 15:46, Pierre Gondois wrote:
> (struct sg_lb_stats).idle_cpus is of type 'unsigned int'.
> (local->idle_cpus - busiest->idle_cpus) can underflow to UINT_MAX
> for instance, and max_t(long, 0, UINT_MAX) will output UINT_MAX.
> 
> Use lsub_positive() instead of max_t().
> 
> Fixes: 16b0a7a1a0af ("sched/fair: Ensure tasks spreading in LLC during LB")
> cc: stable@...r.kernel.org
> Signed-off-by: Pierre Gondois <pierre.gondois@....com>
> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>   kernel/sched/fair.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9057584ec06d..6d9124499f52 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10775,8 +10775,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>   			 * idle CPUs.
>   			 */
>   			env->migration_type = migrate_task;
> -			env->imbalance = max_t(long, 0,
> -					       (local->idle_cpus - busiest->idle_cpus));
> +			env->imbalance = local->idle_cpus;
> +			lsub_positive(&env->imbalance, busiest->idle_cpus);
>   		}
>   
>   #ifdef CONFIG_NUMA