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

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Message-ID: <alpine.DEB.2.20.1711011748390.1942@nanos>
Date:   Wed, 1 Nov 2017 17:56:45 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Prarit Bhargava <prarit@...hat.com>
cc:     linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Piotr Luc <piotr.luc@...el.com>,
        Kan Liang <kan.liang@...el.com>, Borislav Petkov <bp@...e.de>,
        Stephane Eranian <eranian@...gle.com>,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Mathias Krause <minipli@...glemail.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 2/3 v4] x86/topology: Avoid wasting 128k for package id
 array

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