Re: [PATCH 2/3 v4] x86/topology: Avoid wasting 128k for package id array

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, 25 Oct 2017, Prarit Bhargava wrote:
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index b390ff76e58f..f4ab1edf4e24 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -124,8 +124,10 @@ struct cpuinfo_x86 {
>  	u16			booted_cores;
>  	/* Physical processor id: */
>  	u16			phys_proc_id;
> -	/* Logical processor id: */
> +	/* Logical processor (package) id: */
>  	u16			logical_proc_id;
> +	/* Physical package ID */
> +	u16			phys_pkg_id;

How is this new field used aside of being written to and how is it
different from phys_proc_id? AFAICT, it's the same as all callers to
topology_update_package_map() are handing in cpu_data->phys_proc_id.

> +/**
> + * topology_phys_to_logical_pkg - Map a physical package id to a logical
> + *
> + * Returns logical package id or -1 if not found
> + */
> +int topology_phys_to_logical_pkg(unsigned int phys_pkg)
> +{
> +	int log_pkg;
> +
> +	for (log_pkg = 0; log_pkg < logical_packages; log_pkg++)
> +		if (logical_to_physical_pkg_map[log_pkg] == phys_pkg)
> +			return log_pkg;
> +
> +	return -1;
> +}
> +EXPORT_SYMBOL(topology_phys_to_logical_pkg);

....

> +
> +	/* Allocate and copy a new array */
> +	ltp_pkg_map_new = kmalloc(logical_packages * sizeof(u16), GFP_KERNEL);
> +	BUG_ON(!ltp_pkg_map_new);
> +	if (logical_to_physical_pkg_map) {
> +		memcpy(ltp_pkg_map_new, logical_to_physical_pkg_map,
> +		       logical_packages * sizeof(u16));
> +		kfree(logical_to_physical_pkg_map);
>  	}
> -	physical_to_logical_pkg[pkg] = new;
> +	logical_to_physical_pkg_map = ltp_pkg_map_new;

This lacks serialization and is therefore broken against a concurrent
topology_phys_to_logical_pkg() call for obvious reasons. The current user
is probably safe, but this really needs to be fixed now.

Thanks,

	tglx