linux-kernel - Re: [PATCH 01/18] led-triggers: create a trigger for CPU activity

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Date:	Thu, 19 Apr 2012 23:59:13 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Bryan Wu <bryan.wu@...onical.com>
CC:	linux@....linux.org.uk, rpurdie@...ys.net,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linus.walleij@...aro.org, akpm@...ux-foundation.org,
	arnd.bergmann@...aro.org, nicolas.pitre@...aro.org,
	tim.gardner@...onical.com
Subject: Re: [PATCH 01/18] led-triggers: create a trigger for CPU activity

On 4/17/2012 3:53 AM, Bryan Wu wrote:
> +static int __init ledtrig_cpu_init(void)
> +{
> +	int cpu;
> +
> +	/* Supports up to 9999 cpu cores */
> +	BUILD_BUG_ON(CONFIG_NR_CPUS > 9999);
> +
> +	/*
> +	 * Registering CPU led trigger for each CPU cores here
> +	 * ignores CPU hotplug, but after this CPU hotplug works
> +	 * fine with this trigger.
> +	 */
> +	for_each_possible_cpu(cpu) {
> +		struct led_trigger *trig;
> +		char *name = per_cpu(trig_name, cpu);
> +		struct rw_semaphore *lock = &per_cpu(trig_lock, cpu);
> +
> +		init_rwsem(lock);

What is this lock protecting?

> +
> +		snprintf(name, MAX_NAME_LEN, "cpu%d", cpu);
> +
> +		down_write(lock);
> +		led_trigger_register_simple(name, &trig);
> +		per_cpu(cpu_trig, cpu) = trig;
> +		up_write(lock);

This is the only place it is locked and unlocked.

I think the previous comment was about how led_trigger_register_simple()
exposed a live trigger but it wasn't assigned to the per_cpu variable
yet. Can we not just reorder the assignment so we have

  struct led_trigger *trig = NULL;
  ...
  per_cpu(cpu_trig, cpu) = trig;
  led_trigger_register_simple();

?

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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