linux-kernel - Re: [RFC PATCH] perf/x86/intel/rapl: avoid access unallocate memory

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Date:   Thu, 3 Nov 2016 18:47:53 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Charles (Chas) Williams" <ciwillia@...cade.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        "M. Vefa Bicakci" <m.v.b@...box.com>
Subject: Re: [RFC PATCH] perf/x86/intel/rapl: avoid access unallocate memory

On 2016-11-02 18:47:49 [-0400], Charles (Chas) Williams wrote:
> I don't this this is a race.  Here is some debugging from the two CPU VM
> (2 sockets, 1 core per socket).  In identify_cpu() we have:
> 
>         /* The boot/hotplug time assigment got cleared, restore it */
>         c->logical_proc_id = topology_phys_to_logical_pkg(c->phys_proc_id);
> 
> The values just after this:
> 
> 	[    0.228306] identify_cpu: c ffff88023fd0a040  logical_proc_id 65535  c->phys_proc_id 2
> 
> So what's interesting here, is the phys_proc_id of 2 for CPU1:
> 
>         int topology_phys_to_logical_pkg(unsigned int phys_pkg)
>         {
>                 if (phys_pkg >= max_physical_pkg_id)
>                         return -1;
>                 return physical_to_logical_pkg[phys_pkg];
>         }
> 
> And we happen to know the max_physical_pkg_id is 2 in this case.
> So apparently, topology_phys_to_logical_pkg() returns -1 and it gets
> assigned to the logical_proc_id.
> 
> I don't know why the CPU's phys_proc_id is 2.

This is the physical ID. You have two logical IDs (on your two sockets
machine). What is max_physical_pkg_id? In order to get that -1 you would
have to max_physical_pkg_id of 1 but code does
	max_physical_pkg_id = DIV_ROUND_UP(MAX_LOCAL_APIC, ncpus);

and I would be a little surprised if this is 1.

Sebastian