Re: [PATCH v2] power_supply: fix return value of get_property

[Rhyland Klein]

On 6/16/2016 4:43 AM, Krzysztof Kozlowski wrote:
> On 06/16/2016 12:13 AM, Rhyland Klein wrote:
>> power_supply_get_property() should ideally return -EAGAIN if it is
>> called while the power_supply is being registered. There was no way
>> previously to determine if use_cnt == 0 meant that the power_supply
>> wasn't fully registered yet, or if it had already been unregistered.
>>
>> Add a new boolean to the power_supply struct to simply show if
>> registration is completed. Lastly, modify the check in
>> power_supply_show_property() to also ignore -EAGAIN when so it
>> doesn't complain about not returning the property.
>>
>> Signed-off-by: Rhyland Klein <rklein@xxxxxxxxxx>
>> ---
>> v2:
>>  - Modify power_supply_show_property() to not complain if it
>>    sees a return value of -EAGAIN after calling
>>    power_supply_get_property().
>>
>>  drivers/power/power_supply_core.c  | 6 +++++-
>>  drivers/power/power_supply_sysfs.c | 2 +-
>>  include/linux/power_supply.h       | 1 +
>>  3 files changed, 7 insertions(+), 2 deletions(-)
> 
> I don't like it for two reasons:
> 1. There is still a short window when the information will be
> inaccurate. See comment below.
> 
> 2. Although the code is not very complicated but it adds another field
> and some checks just for differentiating EAGAIN/ENODEV. It is
> unnecessary complexity.
> 
>>
>> diff --git a/drivers/power/power_supply_core.c b/drivers/power/power_supply_core.c
>> index b13cd074c52a..a39a47672979 100644
>> --- a/drivers/power/power_supply_core.c
>> +++ b/drivers/power/power_supply_core.c
>> @@ -491,8 +491,11 @@ int power_supply_get_property(struct power_supply *psy,
>>  			    enum power_supply_property psp,
>>  			    union power_supply_propval *val)
>>  {
>> -	if (atomic_read(&psy->use_cnt) <= 0)
>> +	if (atomic_read(&psy->use_cnt) <= 0) {
>> +		if (!psy->initialized)
>> +			return -EAGAIN;
>>  		return -ENODEV;
>> +	}
>>  
>>  	return psy->desc->get_property(psy, psp, val);
>>  }
>> @@ -775,6 +778,7 @@ __power_supply_register(struct device *parent,
>>  	if (rc)
>>  		goto create_triggers_failed;
>>  
>> +	psy->initialized = true;
> 
> If someone calls power_supply_get_property() here, then ENODEV will be
> returned which is wrong.
> 
> The problem is not solved entirely... I am not convinced that introduced
> complexity is worth fixing it.
> 

Right now, without this patch, this causes the thermal function
"update_temperature" in:

thermal_zone_device_register->
  thermal_zone_device_update->
    update_temperature
      (->thermal_zone_get_temp() from the original stack)

to print a warning as it sees ret != -EAGAIN. This causes a warning
"failed to read out thermal zone". I think the idea there is if anything
other than "try again" it complains. While this doesn't cause functional
problems, I do think avoid the warning is worth while.

I think that there is an onus on the power_supply code to be accurate in
its return codes, and EAGAIN is the correct one we should be returning.
I don't see how someone would call power_supply_get_property, but I
agree there is the possibility that if someone did call there, that it
could return the wrong value.

We could wrap the setting of initialized and use_cnt inside a mutex,
which should prevent anyone calling anything in between if we wanted to
completely plug that hole. I am not fond of the idea of adding a struct
member for such a small, specific case, but as we found before, I don't
think there is another way to differentiate otherwise.

-rhyland


-- 
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