linux-kernel - Re: [PATCH v2 2/8] iio: gts: Simplify using _

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Message-ID: <964035d9-cccd-4e12-af71-00ca39cc3596@gmail.com>
Date: Mon, 25 Nov 2024 11:16:22 +0200
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
 Lars-Peter Clausen <lars@...afoo.de>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/8] iio: gts: Simplify using __free

Hi Jonathan,

Thanks once again for the review :)

On 23/11/2024 18:37, Jonathan Cameron wrote:
> On Thu, 21 Nov 2024 10:20:07 +0200
> Matti Vaittinen <mazziesaccount@...il.com> wrote:
> 
>> The error path in the gain_to_scaletables() uses goto for unwinding an
>> allocation on failure. This can be slightly simplified by using the
>> automated free when exiting the scope.
>>
>> Use __free(kfree) and drop the goto based error handling.
>>
>> Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>
>>
>> ---
>>
>> Revision history:
>>    v1 => v2:
>>    - patch number changed because a change was added to the series.
>>    - rebased on iio/testing to avoid conflicts with queued fixes
>> ---
>>   drivers/iio/industrialio-gts-helper.c | 19 ++++++++-----------
>>   1 file changed, 8 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/iio/industrialio-gts-helper.c b/drivers/iio/industrialio-gts-helper.c
>> index 291c0fc332c9..602d3d338e66 100644
>> --- a/drivers/iio/industrialio-gts-helper.c
>> +++ b/drivers/iio/industrialio-gts-helper.c
>> @@ -4,6 +4,7 @@
>>    * Copyright (c) 2023 Matti Vaittinen <mazziesaccount@...il.com>
>>    */
>>   
>> +#include <linux/cleanup.h>
>>   #include <linux/device.h>
>>   #include <linux/errno.h>
>>   #include <linux/export.h>
>> @@ -167,8 +168,8 @@ static int iio_gts_gain_cmp(const void *a, const void *b)
>>   
>>   static int gain_to_scaletables(struct iio_gts *gts, int **gains, int **scales)
>>   {
>> -	int i, j, new_idx, time_idx, ret = 0;
>> -	int *all_gains;
>> +	int ret, i, j, new_idx, time_idx;
>> +	int *all_gains __free(kfree) = NULL;
> See the docs in cleanup.h (added recently).
> 
> Constructor and destructor should go together.   Dan wrote good docs on this
> (which are now in cleanup.h) so I'll not go into why!

I went through the cleanup.h, and noticed the nice explanation for the 
pitfall where we have multiple "scoped operations" with specific 
ordering required. I didn't see other reasoning beyond that - I do hope 
I didn't miss anything.

I find introducing variables mid-function very confusing. Only exception 
for this has been introducing temporary variables at the start of a 
block, to reduce the scope. I would still like to avoid this when it 
isn't absolutely necessary, as it bleeds my eyes :)

I really don't see why we would have other cleanups which required 
specific ordering with the allocated "all_gains".

Anyways, if you think we really have a problem here, would it then 
suffice if I moved the:

         gain_bytes = array_size(gts->num_hwgain, sizeof(int));
         all_gains = kcalloc(gts->num_itime, gain_bytes, GFP_KERNEL);
         if (!all_gains)
                 return -ENOMEM;

to the beginning of the function, and the "int *all_gains __free(kfree) 
= NULL;" as last variable declaration?

(This is not optimal as we will then do the allocation even if 
converting gains to scales failed - but I don't think this is a real 
problem as this should never happen after the driver is proven working 
for the first time).

> Upshot is this goes where you do the kcalloc, not up here.

*whining* "but, but, but ... it is ugly..." :)

Yours,
	-- Matti