linux-kernel - RE: [PATCH 3/3] Thermal: do thermal zone update after a cooling device registered

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Date:	Mon, 12 Oct 2015 09:23:28 +0000
From:	"Chen, Yu C" <yu.c.chen@...el.com>
To:	Javi Merino <javi.merino@....com>
CC:	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"edubezval@...il.com" <edubezval@...il.com>,
	"Zhang, Rui" <rui.zhang@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"Pandruvada, Srinivas" <srinivas.pandruvada@...el.com>
Subject: RE: [PATCH 3/3] Thermal: do thermal zone update after a cooling
 device registered

Hi, Javi
Sorry for my late response,

> -----Original Message-----
> From: Javi Merino [mailto:javi.merino@....com]
> Sent: Wednesday, September 30, 2015 12:02 AM
> To: Chen, Yu C
> Cc: linux-pm@...r.kernel.org; edubezval@...il.com; Zhang, Rui; linux-
> kernel@...r.kernel.org; stable@...r.kernel.org
> Subject: Re: [PATCH 3/3] Thermal: do thermal zone update after a cooling
> device registered
> 
> Hi Yu,
> 
> On Mon, Sep 28, 2015 at 06:52:00PM +0100, Chen, Yu C wrote:
> > Hi, Javi,
> >
> > > -----Original Message-----
> > > From: Javi Merino [mailto:javi.merino@....com]
> > > Sent: Monday, September 28, 2015 10:29 PM
> > > To: Chen, Yu C
> > > Cc: linux-pm@...r.kernel.org; edubezval@...il.com; Zhang, Rui;
> > > linux- kernel@...r.kernel.org; stable@...r.kernel.org
> > > 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 <rui.zhang@...el.com>
> > > >
> > > >
> > >
> > > 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?
> 
RCU would need extra spinlocks to protect the list, and need to sync_rcu after we delete
one instance from thermal_instance list,  I think it is too complicated for me to rewrite: (
How about using thermal_list_lock instead of cdev ->lock?
This guy should be big enough to protect the device.thermal_instance list.

> 
> > > 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);
> 
Sure, ok
> 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.
> 
OK


Best Regards,
Yu