linux-kernel - Re: [PATCH 12/22] Input: iqs269a - use guard notation when acquiring mutex

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Message-ID: <818a1972-2862-460c-89b7-476ac0680db7@gmail.com>
Date: Wed, 4 Sep 2024 20:41:30 +0200
From: Javier Carrasco <javier.carrasco.cruz@...il.com>
To: Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc: linux-input@...r.kernel.org,
 Michael Hennerich <michael.hennerich@...log.com>,
 Ville Syrjala <syrjala@....fi>,
 Support Opensource <support.opensource@...semi.com>,
 Eddie James <eajames@...ux.ibm.com>, Andrey Moiseev <o2g.org.ru@...il.com>,
 Hans de Goede <hdegoede@...hat.com>, Jeff LaBundy <jeff@...undy.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 12/22] Input: iqs269a - use guard notation when acquiring
 mutex

On 04/09/2024 20:21, Dmitry Torokhov wrote:
> Hi Javier,
> 
> On Wed, Sep 04, 2024 at 03:53:40PM +0200, Javier Carrasco wrote:
>> On 04/09/2024 06:47, Dmitry Torokhov wrote:
>>> Using guard notation makes the code more compact and error handling
>>> more robust by ensuring that mutexes are released in all code paths
>>> when control leaves critical section.
>>>
>>> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
>>> ---
>>>  drivers/input/misc/iqs269a.c | 46 +++++++++++++-----------------------
>>>  1 file changed, 16 insertions(+), 30 deletions(-)
>>>
>>> diff --git a/drivers/input/misc/iqs269a.c b/drivers/input/misc/iqs269a.c
>>> index 843f8a3f3410..c34d847fa4af 100644
>>> --- a/drivers/input/misc/iqs269a.c
>>> +++ b/drivers/input/misc/iqs269a.c
>>
>> ...
>>
>>> @@ -453,9 +449,9 @@ static int iqs269_ati_base_get(struct iqs269_private *iqs269,
>>>  	if (ch_num >= IQS269_NUM_CH)
>>>  		return -EINVAL;
>>>  
>>> -	mutex_lock(&iqs269->lock);
>>> +	guard(mutex)(&iqs269->lock);
>>> +
>>>  	engine_b = be16_to_cpu(ch_reg[ch_num].engine_b);
>>
>> maybe scoped_guard() to keep the scope of the mutex as it used to be?
> 
> Thank you for looking over patches.
> 
> It is just a few computations extra, so I decided not to use
> scoped_guard(). Note that original code was forced to release mutex
> early to avoid having to unlock it in all switch branches.
> 
>>
>>> -	mutex_unlock(&iqs269->lock);
>>>  
>>>  	switch (engine_b & IQS269_CHx_ENG_B_ATI_BASE_MASK) {
>>>  	case IQS269_CHx_ENG_B_ATI_BASE_75:
>>> @@ -491,7 +487,7 @@ static int iqs269_ati_target_set(struct iqs269_private *iqs269,
>>>  	if (target > IQS269_CHx_ENG_B_ATI_TARGET_MAX)
>>>  		return -EINVAL;
>>>  
>>> -	mutex_lock(&iqs269->lock);
>>> +	guard(mutex)(&iqs269->lock);
>>>  
>>>  	engine_b = be16_to_cpu(ch_reg[ch_num].engine_b);
>>>  
>>> @@ -501,8 +497,6 @@ static int iqs269_ati_target_set(struct iqs269_private *iqs269,
>>>  	ch_reg[ch_num].engine_b = cpu_to_be16(engine_b);
>>>  	iqs269->ati_current = false;
>>>  
>>> -	mutex_unlock(&iqs269->lock);
>>> -
>>>  	return 0;
>>>  }
>>>  
>>> @@ -515,10 +509,9 @@ static int iqs269_ati_target_get(struct iqs269_private *iqs269,
>>>  	if (ch_num >= IQS269_NUM_CH)
>>>  		return -EINVAL;
>>>  
>>> -	mutex_lock(&iqs269->lock);
>>> -	engine_b = be16_to_cpu(ch_reg[ch_num].engine_b);
>>> -	mutex_unlock(&iqs269->lock);
>>> +	guard(mutex)(&iqs269->lock);
>>
>> same here?
>>
>>>  
>>> +	engine_b = be16_to_cpu(ch_reg[ch_num].engine_b);
>>>  	*target = (engine_b & IQS269_CHx_ENG_B_ATI_TARGET_MASK) * 32;
> 
> Same here, calculating the line above will take no time at all...
> 
> Thanks.
> 

As you pointed out, in reality the extra locked instructions will not
make any difference. But as the conversion added instructions to be
locked by the mutex without mentioning it, I thought it should be either
left as it used to be with scoped_guard(), or explicitly mentioned in
the description.

No strong feelings against it, but out of curiosity, why would you
rather use guard()? I think scoped_guard() is a better way to
self-document what has to be accessed via mutex, and what not.

Best regards,
Javier Carrasco