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Date: Sat, 21 Feb 2015 13:46:59 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Nicolas Pitre <nico@fluxnic.net>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, rjw@rjwysocki.net,
        tglx@linutronix.de, Preeti U Murthy <preeti@linux.vnet.ibm.com>
Subject: Re: [PATCH 32/35] clockevents: Fix cpu down race for hrtimer based
 broadcasting
Message-ID: <20150221124659.GG23367@worktop.ger.corp.intel.com>
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On Thu, Feb 19, 2015 at 12:51:52PM -0500, Nicolas Pitre wrote:
> 
> This breaks the b.L switcher disabling code which essentially does:
> 
> static void bL_switcher_restore_cpus(void)
> {
>         int i;
> 
>         for_each_cpu(i, &bL_switcher_removed_logical_cpus) {
>                 struct device *cpu_dev = get_cpu_device(i);
>                 int ret = device_online(cpu_dev);
>                 if (ret)
>                         dev_err(cpu_dev, "switcher: unable to restore CPU\n");
>         }
> }

Just so I understand, this device_{on,ofF}line() stuff is basically just
cpu_{up,down}() but obfuscated through the device model nonsense, right?

Also it seems bL_switcher_enable() relies on lock_device_hotplug() to
stabilize the online cpu mask; it does not, only the hotplug lock does.

I'm having a very hard time trying to follow wth this thing all is
doing; its using hotplug but its also doing magic with cpu_suspend().

/me confused..