Hi Yu,
On Mon, Sep 28, 2015 at 06:52:00PM +0100, Chen, Yu C wrote:
> Hi, Javi,
>
> > -----Original Message-----
> > From: Javi Merino [mailto:[email protected]]
> > Sent: Monday, September 28, 2015 10:29 PM
> > To: Chen, Yu C
> > Cc: [email protected]; [email protected]; Zhang, Rui; linux-
> > [email protected]; [email protected]
> > Subject: Re: [PATCH 3/3] Thermal: do thermal zone update after a cooling
> > device registered
> >
> > On Sun, Sep 27, 2015 at 06:48:44AM +0100, Chen Yu wrote:
> > > From: Zhang Rui <[email protected]>
> > >
> > >
> >
> > I think you need to hold cdev->lock here, to make sure that no thermal zone
> > is added or removed from cdev->thermal_instances while you are looping.
> >
> Ah right, will add. If I add the cdev ->lock here, will there be a AB-BA lock
> with
> thermal_zone_unbind_cooling_device?
You're right, it could lead to a deadlock. The locks can't be
swapped because that won't work in step_wise.
The best way that I can think of accessing thermal_instances
atomically is by making it RCU protected instead of with mutexes.
What do you think?
> > Why list_for_each_entry_safe() ? You are not going to remove any entry, so
> > you can just use list_for_each_entry()
> >
> >
> > Why is this so complicated? Can't you just do:
> >
> > list_for_each_entry(pos, &cdev->thermal_instances, cdev_node)
> > thermal_zone_device_update(pos->tz);
> >
>
> This is an optimization here:
> Ignore thermal instance that refers to the same thermal zone in this loop,
> this works because bind_cdev() always binds the cooling device to one
> thermal zone first, and then binds to the next thermal zone.
It has taken me a while to understand this optimization. Please
document both "if"s in the code. For the first "if" maybe you can use
list_is_last() to make it easier to understand that you're looking for
the last element in the list:
if (list_is_last(&pos->cdev_node, &cdev->thermal_instances)) {
thermal_zone_device_update(pos->tz);
For the second "if" you can say that you only need to run
thermal_zone_device_update() once per thermal zone, even though
multiple thermal instances may refer to the same thermal zone.
Cheers,
Javi
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