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Date:   Tue, 1 Jun 2010 12:15:28 +0900
From:   Paul Mundt <lethal@linux-sh.org>
To:     Kevin Cernekee <cernekee@gmail.com>
Cc:     mingo@elte.hu, akpm@linux-foundation.org,
        simon.kagstrom@netinsight.net, David.Woodhouse@intel.com,
        rgetz@analog.com, linux-kernel@vger.kernel.org,
        linux-mips@linux-mips.org
Subject: Re: [PATCH v2] printk: fix delayed messages from CPU hotplug events
Message-ID: <20100601031528.GC15411@linux-sh.org>
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On Mon, May 31, 2010 at 04:57:15PM -0700, Kevin Cernekee wrote:
>  /**
> + * console_cpu_notify - print deferred console messages after CPU hotplug
> + *
> + * If printk() is called from a CPU that is not online yet, the messages
> + * will be spooled but will not show up on the console.  This function is
> + * called when a new CPU comes online and ensures that any such output
> + * gets printed.
> + */
> +static int __cpuinit console_cpu_notify(struct notifier_block *self,
> +	unsigned long action, void *hcpu)
> +{
> +	switch (action) {
> +	case CPU_ONLINE:
> +	case CPU_UP_CANCELED:
> +		if (try_acquire_console_sem() == 0)
> +			release_console_sem();
> +	}
> +	return NOTIFY_OK;
> +}
> +
> +static struct notifier_block __cpuinitdata console_nb = {
> +	.notifier_call		= console_cpu_notify,
> +};
> +
> +static int __init console_notifier_init(void)
> +{
> +	register_cpu_notifier(&console_nb);
> +	return 0;
> +}

If this is to be entirely restricted to CPU hotplug then you could use
the hotcpu notifier here instead of the open-coded cpu notifier directly,
the former wraps to the latter in the CPU hotplug case and is simply a
nop for the regular SMP case.