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

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Date:	Thu, 15 Oct 2015 15:05:18 +0100
From:	Javi Merino <javi.merino@....com>
To:	"Chen, Yu C" <yu.c.chen@...el.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

On Wed, Oct 14, 2015 at 07:23:55PM +0000, Chen, Yu C wrote:
> > -----Original Message-----
> > From: Javi Merino [mailto:javi.merino@....com]
> > Sent: Thursday, October 15, 2015 1:08 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; Pandruvada, Srinivas
> > Subject: Re: [PATCH 3/3] Thermal: do thermal zone update after a cooling
> > device registered
> > 
> > On Mon, Oct 12, 2015 at 09:23:28AM +0000, Chen, Yu C wrote:
> > > 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.
> > 
> > thermal_list_lock protects thermal_tz_list and thermal_cdev_list, but it
> > doesn't protect the thermal_instances list.  For example,
> > thermal_zone_bind_cooling_device() adds a cooling device to the
> > cdev->thermal_instances list without taking thermal_tz_list.
> > 
> Before thermal_zone_bind_cooling_device is invoked,
> the thermal_list_lock will be firstly gripped:
> 
> static void bind_cdev(struct thermal_cooling_device *cdev)
> {
> mutex_lock(&thermal_list_lock);
> either tz->ops->bind    :   thermal_zone_bind_cooling_device
> or __bind()  :   thermal_zone_bind_cooling_device
> mutex_unlock(&thermal_list_lock);
> }
> 
> And it is the same as in  passive_store.
> So when code is trying to add/delete thermal_instance of cdev,
> he has already hold thermal_list_lock IMO. Or do I miss anything?

thermal_zone_bind_cooling_device() is exported, so you can't really
rely on the static thermal_list_lock being acquired in every single
call.

thermal_list_lock and protects the lists thermal_tz_list and
thermal_cdev_list.  Making it implicitly protect the cooling device's
and thermal zone device's instances list because no sensible code
would call thermal_zone_bind_cooling_device() outside of a bind
function is just asking for trouble.

Locking is hard to understand and easy to get wrong so let's keep it
simple.

Cheers,
Javi
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