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Date:   Fri, 19 Jun 2020 20:58:29 +0200
From:   Pali =?utf-8?B?Um9ow6Fy?= <pali@kernel.org>
To:     Sebastian Reichel <sre@kernel.org>
Cc:     "Andrew F. Davis" <afd@ti.com>,
        Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Anton Vorontsov <cbouatmailru@gmail.com>,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: Re: [RFC] power: supply: bq27xxx_battery: Fix polling interval after
 re-bind
Message-ID: <20200619185829.jd4ognwwjlsrjhcb@pali>
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On Friday 19 June 2020 19:55:21 Sebastian Reichel wrote:
> Hi,
> 
> On Wed, May 27, 2020 at 09:42:54AM +0200, Pali Rohár wrote:
> > On Tuesday 26 May 2020 21:16:28 Andrew F. Davis wrote:
> > > On 5/25/20 7:32 AM, Krzysztof Kozlowski wrote:
> > > > This reverts commit 8cfaaa811894a3ae2d7360a15a6cfccff3ebc7db.
> > > > 
> > > > If device was unbound and bound, the polling interval would be set to 0.
> > > > This is both unexpected and messes up with other bq27xxx devices (if
> > > > more than one battery device is used).
> > > > 
> > > > This reset of polling interval was added in commit 8cfaaa811894
> > > > ("bq27x00_battery: Fix OOPS caused by unregistring bq27x00 driver")
> > > > stating that power_supply_unregister() calls get_property().  However in
> > > > Linux kernel v3.1 and newer, such call trace does not exist.
> > > > Unregistering power supply does not call get_property() on unregistered
> > > > power supply.
> > > > 
> > > > Fixes: 8cfaaa811894 ("bq27x00_battery: Fix OOPS caused by unregistring bq27x00 driver")
> > > > Cc: <stable@vger.kernel.org>
> > > > Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
> > > > 
> > > > ---
> > > > 
> > > > I really could not identify the issue being fixed in offending commit
> > > > 8cfaaa811894 ("bq27x00_battery: Fix OOPS caused by unregistring bq27x00
> > > > driver"), therefore maybe I missed here something important.
> > > > 
> > > > Please share your thoughts on this.
> > > 
> > > I'm having a hard time finding the OOPS also. Maybe there is a window
> > > where the poll function is running or about to run where
> > > cancel_delayed_work_sync() is called and cancels the work, only to have
> > > an interrupt or late get_property call in to the poll function and
> > > re-schedule it.
> > > 
> > > What we really need is to do is look at how we are handling the polling
> > > function. It gets called from the workqueue, from a threaded interrupt
> > > context, and from a power supply framework callback, possibly all at the
> > > same time. Sometimes its protected by a lock, sometimes not. Updating
> > > the device's cached data should always be locked.
> > > 
> > > What's more is the poll function is self-arming, so if we call
> > > cancel_delayed_work_sync() (remove it from the work queue then then wait
> > > for it to finish if running), are we sure it wont have just re-arm itself?
> > > 
> > > We should make the only way we call the poll function be through the
> > > work queue, (plus make sure all accesses to the cache are locked).
> > > 
> > > Andrew
> > 
> > I do not remember details too. It is long time ago.
> > 
> > CCing Ivaylo Dimitrov as he may remember something...

Hello!

I did some archeology and found some more details about this problem.

rmmoding bq module without that patch at that time sometimes caused oops
and reboot of N900 device. backtrace of crash contained:

=======================================================================
[80084.031646] Backtrace:
[80084.031677] [<c0158bbc>] (kobject_uevent_env+0x0/0x3a0) from [<c0158f70>] (kobject_uevent+0x14/0x18)
[80084.031768] [<c0158f5c>] (kobject_uevent+0x0/0x18) from [<bf2b8128>] (power_supply_changed_work+0x44/0x50 [power_supply])
[80084.031890] [<bf2b80e4>] (power_supply_changed_work+0x0/0x50 [power_supply]) from [<c0069a64>] (run_workqueue+0xd4/0x198)
[80084.031982]  r5:cf028000 r4:c40859a8
[80084.032012] [<c0069990>] (run_workqueue+0x0/0x198) from [<c006a804>] (worker_thread+0xf0/0x104)
[80084.032104]  r9:00000000 r8:00000000 r7:cf000780 r6:cf028000 r5:cf01ab40
[80084.032165] r4:cf029fb8
[80084.032196] [<c006a714>] (worker_thread+0x0/0x104) from [<c006da88>] (kthread+0x54/0x80)
[80084.032287]  r7:00000000 r6:00000000 r5:c006a714 r4:cf000780
[80084.032318] [<c006da34>] (kthread+0x0/0x80) from [<c005a948>] (do_exit+0x0/0x7bc)
[80084.032409]  r5:00000000 r4:00000000
[80084.032440] Code: e3530000 1afffff9 e3e05015 ea0000c6 (e59a802c)
=======================================================================

I dig more in my disk storage and private archives and found following
email from Ivaylo which describe that problem and is also source of that
patch. I hope that Ivo would not be against putting copy of it here :-)

=======================================================================
Date: Mon, 31 Oct 2011 16:00:33 +0200 (EET)
Message-ID: <1251363478.202876.1320069633552.JavaMail.apache@mail82.abv.bg>

Hi,

The bug in bq27x00_battery is in function bq27x00_battery_poll. What happens is that after all pending poll work requests are canceled with cancel_delayed_work_sync(&di->work); in function bq27x00_powersupply_unregister  there is a call to power_supply_unregister(&di->bat); which in turn calls bq27x00_battery_get_property. And bq27x00_battery_get_property calls
bq27x00_battery_poll which leads to another delayed poll work queued. So after the timer expires kernel tries to execute a function in an already unloaded module, so OOPS ;)

The fix would be to set poll_interval = 0; in bq27x00_powersupply_unregister so the function will look like:

static  void  bq27x00_powersupply_unregister(struct  bq27x00_device_info  *di)

{

        poll_interval = 0;

        cancel_delayed_work_sync(&di->work);

        power_supply_unregister(&di->bat);

        mutex_destroy(&di->lock);

}

thus no new delayed work to be scheduled in function bq27x00_battery_poll.

Hope the above helps.
=======================================================================

> Applying this revert introduces at least a race condition when
> userspace reads sysfs files while kernel removes the driver.
> 
> So looking at the entrypoints for schedules:
> 
> bq27xxx_battery_i2c_probe:
>   Not relevant, probe is done when the battery is being removed.
> 
> poll_interval_param_set:
>   Can be avoided by unregistering from the list earlier. This
>   is the right thing to do considering the battery is added to
>   the list as last step in the probe routine, it should be removed
>   first during teardown.
> 
> bq27xxx_external_power_changed:
>   This can happen at any time while the power-supply device is
>   registered, because of the code in get_property.
> 
> bq27xxx_battery_poll:
>   This can happen at any time while the power-supply device is
>   registered.
> 
> As far as I can see the only thing in the delayed work needing
> the power-supply device is power_supply_changed(). If we add a
> check, that di->bat is not NULL, we should be able to reorder
> teardown like this:
> 
> 1. remove from list
> 2. unregister power-supply device and set to di->bat to NULL
> 3. cancel delayed work
> 4. destroy mutex
> 
> Also I agree with Andrew, that the locking looks fishy. I think
> the lock needs to be moved, so that the call to
> bq27xx_battery_update(di) in bq27xxx_battery_poll is protected.
> 
> -- Sebastian

And... I found another discussion about crash in bq27x00 battery driver:
https://lkml.org/lkml/2015/5/18/364

> > > > ---
> > > >  drivers/power/supply/bq27xxx_battery.c | 8 --------
> > > >  1 file changed, 8 deletions(-)
> > > > 
> > > > diff --git a/drivers/power/supply/bq27xxx_battery.c b/drivers/power/supply/bq27xxx_battery.c
> > > > index 942c92127b6d..4c94ee72de95 100644
> > > > --- a/drivers/power/supply/bq27xxx_battery.c
> > > > +++ b/drivers/power/supply/bq27xxx_battery.c
> > > > @@ -1905,14 +1905,6 @@ EXPORT_SYMBOL_GPL(bq27xxx_battery_setup);
> > > >  
> > > >  void bq27xxx_battery_teardown(struct bq27xxx_device_info *di)
> > > >  {
> > > > -	/*
> > > > -	 * power_supply_unregister call bq27xxx_battery_get_property which
> > > > -	 * call bq27xxx_battery_poll.
> > > > -	 * Make sure that bq27xxx_battery_poll will not call
> > > > -	 * schedule_delayed_work again after unregister (which cause OOPS).
> > > > -	 */
> > > > -	poll_interval = 0;
> > > > -
> > > >  	cancel_delayed_work_sync(&di->work);
> > > >  
> > > >  	power_supply_unregister(di->bat);
> > > >