linux-kernel - Re: [Letux-kernel] BUG: drivers/pinctrl/core: races in pinctrl

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Message-Id: <404C63D9-169E-4051-88E0-2856F3602A72@goldelico.com>
Date:   Mon, 18 Jun 2018 20:30:17 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Christ van Willegen <cvwillegen@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        kernel@...a-handheld.com,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>
Subject: Re: [Letux-kernel] BUG: drivers/pinctrl/core: races in pinctrl_groups and deferred probing

Hi Tony,

> Am 18.06.2018 um 20:17 schrieb Tony Lindgren <tony@...mide.com>:
> 
> * H. Nikolaus Schaller <hns@...delico.com> [180618 16:46]:
> 
>> 
>> I can also demonstrate that the duplication has gone:
> 
> OK good to hear.
> 
>> And I was no longer able to reproduce the strcmp(NULL) issue. So it is either better hidden
>> or gone.
> 
> It should not be possible with checks preventing registering
> a group or function with no name. I'll try to repost the whole
> series tomorrow with that added.

Fine.

> 
>> So code just needs group cleanup on failed probing and fixing the mutex around pinctrl_generic_add_group().
>> 
>> I think we need the mutex because a race still can happen when create_pinctrl() is calling pcs_dt_node_to_map()
>> and pinctrl_generic_add_group() w/o being locked on pinctrl_maps_mutex.
>> 
>> The race I suspect is that two drivers are trying to insert the same name and may come
>> both to the conclusion that it does not yet exist. And both insert into the radix tree.
>> 
>> The window of risk is small though... It is in pinctrl_generic_add_group() between calling
>> pinctrl_generic_group_name_to_selector() and radix_tree_insert() so we probably won't
>> see it in real hardware tests.
> 
> Hmm but that race should be already fixed with mutex held
> by the pin controller drivers with these fixes? Or am I
> missing something still?

Hm. Maybe we refer to a different mutex?

I had seen the call sequence

create_pinctrl()-> pinctrl_dt_to_map() -> pcs_dt_node_to_map() -> pinctrl_generic_add_group()

w/o any lock inside.

There is a mutex_lock(&pinctrl_maps_mutex); in create_pinctrl(), but locked after that.

Or is there a lock outside of create_pinctrl()?

If I look into the stack dumps, call nesting is

driver_probe_device() -> pinctrl_bind_pins() -> devm_pinctrl_get() -> create_pinctrl()

They all do no locking.

Maybe I am missing something.

BR,
Nikolaus