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

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Message-ID: <ZtisrIt5MdJhuwD_@google.com>
Date: Wed, 4 Sep 2024 11:53:32 -0700
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Javier Carrasco <javier.carrasco.cruz@...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 Wed, Sep 04, 2024 at 08:41:30PM +0200, Javier Carrasco wrote:
> 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.

Simply less indentation ;) and in this driver uniformity with for example
iqs269_ati_target_set() where critical section does indeed extend to the
whole function.

Not super strong arguments either.

-- 
Dmitry