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

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Date:   Tue, 19 Jun 2018 06:51:31 +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 19.06.2018 um 06:34 schrieb Tony Lindgren <tony@...mide.com>:
> 
> * H. Nikolaus Schaller <hns@...delico.com> [180618 18:33]:
>>>> 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?
> 
> Yes I think that's the case, you're talking about a different
> mutex here :)
> 
>> 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.
> 
> Can you please post a patch for that as you already have it
> debugged? That's easier to understand than reading a verbal
> patch :)

I have no patch for it and all tests were without, but I can suggest a change which IMHO
could solve it:

diff --git a/drivers/pinctrl/core.c b/drivers/pinctrl/core.c
index eb2b217f5e1e..7d125f9a7804 100644
--- a/drivers/pinctrl/core.c
+++ b/drivers/pinctrl/core.c
@@ -1037,15 +1037,16 @@ static struct pinctrl *create_pinctrl(struct device *dev,
        INIT_LIST_HEAD(&p->states);
        INIT_LIST_HEAD(&p->dt_maps);
 
+       mutex_lock(&pinctrl_maps_mutex);
        ret = pinctrl_dt_to_map(p, pctldev);
        if (ret < 0) {
                kfree(p);
+               mutex_unlock(&pinctrl_maps_mutex);
                return ERR_PTR(ret);
        }
 
        devname = dev_name(dev);
 
-       mutex_lock(&pinctrl_maps_mutex);
        /* Iterate over the pin control maps to locate the right ones */
        for_each_maps(maps_node, i, map) {
                /* Map must be for this device */

Description: we should also protect pinctrl_dt_to_map(), which calls pinctrl_generic_add_group()
and the calls inside pinctrl_generic_add_group() to pinctrl_generic_group_name_to_selector()
and radix_tree_insert() with a mutex.

But I haven't tested this case (because it is unlikely to happen and be testable).

BR,
Nikolaus