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Date:   Tue, 24 Jan 2017 20:54:12 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Yasuaki Ishimatsu <yasu.isimatu@...il.com>
cc:     linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] perf/x86/intel/rapl: Rename rapl_cpu_prepare() to
 rapl_cpu_starting()

On Tue, 24 Jan 2017, Yasuaki Ishimatsu wrote:
> rapl_cpu_prepare() must be called after logical package id of CPU
> is set by topology_update_package_map().
> 
> But when onlining hot-added CPU, rapl_cpu_prepare() is called before
> setting logical package id of the hot-added CPU. So cpu_to_rapl_pmu()
> in rapl_cpu_prepare() finds a rapl_pmu of wrong logical package id and
> rapl_cpu_prepare() initializes the wrong rapl_pmu.
> 
> After that logical package id of the hot-added CPU is set by
> topology_update_package_map(). But rapl_cpu_prepare() does
> not initialize pmu of the logical package id of the hot-added CPU.
> So when calling rapl_cpu_online(), cpu_to_rapl_pmu() returns NULL and
> the following NULL pointer dereference occurs.
> 
>   BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
>   IP: rapl_cpu_online+0x8d/0xb0
>   <snip>
>   Call Trace:
>    ? rapl_cpu_offline+0xc0/0xc0
>    cpuhp_invoke_callback+0x8d/0x3f0
>    cpuhp_up_callbacks+0x37/0xb0
>    cpuhp_thread_fun+0xc9/0xe0
>    smpboot_thread_fn+0x110/0x160
>    kthread+0x101/0x140
>    ? sort_range+0x30/0x30
>    ? kthread_park+0x90/0x90
>    ret_from_fork+0x25/0x30
> 
> The patch renames rapl_cpu_prepare() to rapl_cpu_starting() and changes
> the position of cpuhp_state so that rapl_cpu_starting() is called
> after topology_update_package_map().

Does not work. You cannot call that callback in the starting context. It
does allocations. This needs be fixed in a different way. I'll have a look
tomorrow.

Thanks,

	tglx

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