linux-kernel - Re: [PATCH 2/2] hwmon: (dell-smm) Unify i8k_ioctl() and i8k_ioctl

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Message-ID: <b73d5a00-ecce-0517-4fb5-4af451e1facb@roeck-us.net>
Date:   Mon, 22 Nov 2021 11:28:30 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Pali Rohár <pali@...nel.org>,
        Armin Wolf <W_Armin@....de>
Cc:     jdelvare@...e.com, linux-hwmon@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] hwmon: (dell-smm) Unify i8k_ioctl() and
 i8k_ioctl_unlocked()

On 11/22/21 11:10 AM, Pali Rohár wrote:
> On Monday 22 November 2021 19:50:14 Armin Wolf wrote:
>> Am 22.11.21 um 18:55 schrieb Guenter Roeck:
>>> On 11/22/21 8:01 AM, Pali Rohár wrote:
>>>> On Saturday 20 November 2021 18:03:19 Armin Wolf wrote:
>>>>> The only purpose of i8k_ioctl() is to call i8k_ioctl_unlocked()
>>>>> with i8k_mutex held. Judging from the hwmon code, this mutex
>>>>> only needs to be held when setting the fan speed/mode.
>>>>
>>>> Really? I think that there is no difference between setting and getting
>>>> fan speed/mode. At least I do not see why 'set' needs mutex and 'get' do
>>>> not need it. Some more explanation is needed...
>>>>
>>> I8K_SET_FAN sets the fan speed and returns the current status. Without
>>> locking, the returned status may not match or be associated with the
>>> previous
>>> set operation.
>>>
>>> Maybe that doesn't matter, and the synchronization is not needed. If so,
>>> you can probably remove the locking entirely.
>>>
>>> Guenter
>>
>> That is the reason i kept the locking code. Since i do not want to break
>> the ioctl interfacein any way, removing the locking code seems too risky
>> to me.
> 
> I see. That is a good point.
> 
> But there is same race condition also when at the same time going to
> change speed via ioctl and also via hwmon sysfs.
> 

I thought the sysfs code does not change the fan speed and report the
fan status in the same request. Did I miss something ?

Guenter