lkml.org 
[lkml]   [2021]   [Nov]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/2] hwmon: (dell-smm) Unify i8k_ioctl() and i8k_ioctl_unlocked()
On Tuesday 23 November 2021 08:00:51 Guenter Roeck wrote:
> On Mon, Nov 22, 2021 at 08:43:10PM +0100, Pali Rohár wrote:
> > On Monday 22 November 2021 11:28:30 Guenter Roeck wrote:
> > > 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 ?
> >
> > No. I mean something different. Let me to write trace call:
> >
> > CPU 0: CPU 1:
> > 1. dell_smm_write()
> > 1. ioctl(I8K_SET_FAN)
> > 2. i8k_set_fan()
> > 2. i8k_set_fan()
> > 3. i8k_get_fan_status()
> >
> > So to ensure that i8k_get_fan_status() on CPU 0 returns value which
> > belongs to i8k_set_fan() from CPU 0 it is needed to still use mutex.
> >
> > Armin is right here and I think that patch is correct.
>
> Quoting your earlier reply:
>
> > 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...
>
> This was the reason for my comment. Your latest reply is leaving me a bit
> puzzled. If you are ok with the patch as-is, please provide a Reviewed-by:
> or Acked-by: tag.

Sorry for that. I should have explicitly wrote that I realized how it
works after Armin explained it.

\
 
 \ /
  Last update: 2021-11-23 17:03    [W:0.055 / U:0.108 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site