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

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Date:	Fri, 11 Dec 2015 13:23:42 +0900
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Anand Moon <linux.amoon@...il.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

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?"
what is wrong with his approach?

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).

Best regards,
Krzysztof
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