Re: Re: [PATCH linux-next v2] sched/core: Add WARN() to check overflow in migrate_disable()

[Peilin He]

> How about we do something like this?
>
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7a5ea8fc8abb..06a559532ed6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2237,6 +2237,12 @@ void migrate_disable(void)
>  
>      if (p->migration_disabled) {
>          p->migration_disabled++;
> +#ifdef CONFIG_DEBUG_PREEMPT
> +        /*
> +         * Warn about overflow half-way through the range.
> +         */
> +        WARN_ON_ONCE((s16)p->migration_disabled < 0);
> +#endif
>          return;
>      }

Agreed, converting p->migration_disabled to a signed number 
will detect overflow occurrences earlier than our current method.

> @@ -2254,14 +2260,20 @@ void migrate_enable(void)
>          .flags     = SCA_MIGRATE_ENABLE,
>      };
>  
> +#ifdef CONFIG_DEBUG_PREEMPT
> +    /*
> +     * Check both overflow from migrate_disable() and superfluous
> +     * migrate_enable().
> +     */
> +    if (WARN_ON_ONCE((s16)p->migration_disabled <= 0))
> +        return;
> +#endif
> +
>      if (p->migration_disabled > 1) {
>          p->migration_disabled--;
>          return;
>      }
>  
> -    if (WARN_ON_ONCE(!p->migration_disabled))
> -        return;
> -
>      /*
>       * Ensure stop_task runs either before or after this, and that
>       * __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule().

Agreed, similarly checking for overflow in p->migration_disabled 
within migrate_enable is fine, but placing WARN_ON_ONCE inside 
the CONFIG_DEBUG_PREEMPT macro may cause return logic deviations. 
It is recommended to support WARN_ON_ONCE as a standard feature.
The fix will be reflected in version v3.