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    SubjectRe: [PATCH 01/18] led-triggers: create a trigger for CPU activity
    On Fri, 30 Mar 2012 19:58:16 +0800
    Bryan Wu <bryan.wu@canonical.com> wrote:

    > Attempting to consolidate the ARM LED code, this removes the
    > custom RealView LED trigger code to turn LEDs on and off in
    > response to CPU activity and replace it with a standard trigger.
    >
    > (bryan.wu@canonical.com:
    > It moves arch/arm/kernel/leds.c syscore stubs into this trigger.
    > It also provides ledtrig_cpu trigger event stub in <linux/leds.h>.
    > Although it was inspired by ARM work, it can be used in other arch.)
    >
    >
    > ...
    >
    > +#include <linux/percpu.h>
    > +#include <linux/syscore_ops.h>
    > +#include "leds.h"
    > +
    > +#define MAX_NAME_LEN 8

    The kernel already has at least eight different definitions of
    MAX_NAME_LEN. I guess a ninth won't hurt ;)

    >
    > ...
    >
    > +static void __init ledtrig_cpu_register(void)
    > +{
    > + int cpuid = smp_processor_id();
    > + struct led_trigger *trig;
    > + char *name = __get_cpu_var(trig_name);
    > +
    > + snprintf(name, MAX_NAME_LEN, "cpu%d", cpuid);
    > + led_trigger_register_simple(name, &trig);
    > +
    > + pr_info("LED trigger %s indicate activity on CPU %d\n",
    > + trig->name, cpuid);
    > +
    > + __get_cpu_var(cpu_trig) = trig;
    > +}
    > +
    > +static void __exit ledtrig_cpu_unregister(void)
    > +{
    > + struct led_trigger *trig = __get_cpu_var(cpu_trig);
    > + char *name = __get_cpu_var(trig_name);
    > +
    > + led_trigger_unregister_simple(trig);
    > + __get_cpu_var(cpu_trig) = NULL;
    > + memset(name, 0, MAX_NAME_LEN);
    > +}
    > +
    > +static int __init ledtrig_cpu_init(void)
    > +{
    > + int cpu;
    > +
    > + for_each_possible_cpu(cpu)
    > + ledtrig_cpu_register();
    > +
    > + register_syscore_ops(&ledtrig_cpu_syscore_ops);
    > +
    > + return 0;
    > +}
    > +module_init(ledtrig_cpu_init);

    This all looks horridly broken. We call ledtrig_cpu_register() once
    for each CPU, but we call it on the same CPU each time, and it uses
    smp_processor_id() which a) is wrong and b) will emit a runtime
    preemption-is-enabled warning.

    I'm assuming this wasn't tested on SMP. It needs to be, please.

    Also, the code uses for_each_possible_cpu, ignoring CPU hotplug.
    That's probably justifiable for this small storage size and not-hotpath
    code, but the decision should be given prominence and justified in the
    changelog or, better, in code comments please.

    >
    > ...
    >

    The patchset is basically an ARM thing. Is there some ARM tree via
    which we can get it merged?


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