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

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Date:   Mon, 22 Nov 2021 20:10:08 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Armin Wolf <W_Armin@....de>
Cc:     Guenter Roeck <linux@...ck-us.net>, 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 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.