linux-kernel - Re: [PATCH] ledtrig-cpu: use spin

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Date:	Mon, 22 Oct 2012 16:12:13 -0700
From:	Bryan Wu <cooloney@...il.com>
To:	Nathan Lynch <ntl@...ox.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Miles Lane <miles.lane@...il.com>,
	akpm <akpm@...ux-foundation.org>,
	Linux LED Subsystem <linux-leds@...r.kernel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ledtrig-cpu: use spin_lock to replace mutex lock

On Mon, Oct 22, 2012 at 3:28 PM, Nathan Lynch <ntl@...ox.com> wrote:
> Hi Bryan,
>
> On Thu, 2012-10-18 at 11:18 -0700, Bryan Wu wrote:
>> @@ -117,14 +117,14 @@ static int __init ledtrig_cpu_init(void)
>>       for_each_possible_cpu(cpu) {
>>               struct led_trigger_cpu *trig = &per_cpu(cpu_trig, cpu);
>>
>> -             mutex_init(&trig->lock);
>> +             spin_lock_init(&trig->lock);
>>
>>               snprintf(trig->name, MAX_NAME_LEN, "cpu%d", cpu);
>>
>> -             mutex_lock(&trig->lock);
>> +             spin_lock(&trig->lock);
>>               led_trigger_register_simple(trig->name, &trig->_trig);
>>               trig->lock_is_inited = 1;
>> -             mutex_unlock(&trig->lock);
>> +             spin_unlock(&trig->lock);
>
> I wouldn't know how to fix the original problem, but I don't think this
> patch is okay -- led_trigger_register_simple() does things that
> potentially sleep (GFP_KERNEL allocation, down_write), so it's not safe
> to call while holding a spinlock.
>

Looks like we got a hard issue, since led_trigger_register() families
and even led_trigger_event() might use rwsem for locking, these are
all potentially sleep, which are not easy to modified for atomic
context.

Any hints are welcome.

Thanks,
-Bryan
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