linux-kernel - Re: [PATCH] pwm: Avoid double mutex lock on pwm

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Date:	Fri, 11 Dec 2015 11:27:10 +0530
From:	Anand Moon <linux.amoon@...il.com>
To:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	Thierry Reding <thierry.reding@...il.com>,
	Javier Martinez Canillas <javier@....samsung.com>,
	linux-pwm@...r.kernel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Jonathan Richardson <jonathar@...adcom.com>
Subject: Re: [PATCH] pwm: Avoid double mutex lock on pwm_enable

Hi Krzysztof

On 11 December 2015 at 09:53, Krzysztof Kozlowski
<k.kozlowski@...sung.com> wrote:
> On 11.12.2015 13:07, Anand Moon wrote:
>> Hi Krzysztof,
>>
>> On 22 November 2015 at 05:43, Krzysztof Kozlowski
>> <k.kozlowski@...sung.com> wrote:
>>> 2015-11-22 3:14 GMT+09:00 Anand Moon <linux.amoon@...il.com>:
>>>> Hi Krzysztof,
>
> [...]
>
>>> Yes, now you pasted the same warning I did...
>>>
>>> This is still the same issue. I already wrote it:
>>>> 1. BUG: sleeping function called from invalid context
>>>> 2. DEBUG_LOCKS_WARN_ON(in_interrupt())
>>>
>>> We can repeat it many times but that won't change anything...
>>>
>>> Best regards,
>>> Krzysztof
>>
>> Would you consider below changes to fix the above issue.
>> I have tested this change by enabling CONFIG_DEBUG_ATOMIC_SLEEP
>> And I don't observed issue.
>>
>> 1. BUG: sleeping function called from invalid context
>> 2. DEBUG_LOCKS_WARN_ON(in_interrupt())
>>
>> Please share your thought on this changes.
>>
>> root@...oidxu4:/usr/src/odroidxu3-4.y-devel# git diff drivers/pwm/core.c
>> diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
>> index d24ca5f..f3f6cf9 100644
>> --- a/drivers/pwm/core.c
>> +++ b/drivers/pwm/core.c
>> @@ -506,6 +506,9 @@ int pwm_enable(struct pwm_device *pwm)
>>         if (!pwm)
>>                 return -EINVAL;
>>
>> +       if (!mutex_is_locked(&pwm->lock))
>> +               return -EINVAL;
>> +
>>         mutex_lock(&pwm->lock);
>>
>>         if (!test_and_set_bit(PWMF_ENABLED, &pwm->flags)) {
>
> First of all, Thierry suggested way of fixing this:
> "Any objections to simply removing it and make all users use a workqueue
> or some such if they need to control a PWM as a result of an interrupt
> trigger?"

You are correct, current design need to be changes.

> what is wrong with his approach?
>

pwm->lock is locked but it never get unlocked, although the code looks correct.

> Second, you are writing something that looks like mutex-try-lock...
> which will fail the pwm_enable(). IMHO this *hides* the real issue and
> does not solve anything (except hiding also the warning).

Thanks for the inputs. I have realized my mistake.

>
> Best regards,
> Krzysztof

-Anand Moon
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