Date:   Thu, 30 Nov 2023 13:37:51 +0800
From:   "Aiqun(Maria) Yu" <quic_aiquny@...cinc.com>
To:     Linus Walleij <linus.walleij@...aro.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Charles Keepax <ckeepax@...nsource.cirrus.com>,
        Chester Lin <clin@...e.com>
Subject: Re: [GIT PULL] Pin control fixes for v6.7

On 11/29/2023 11:08 PM, Linus Walleij wrote:
> On Wed, Nov 29, 2023 at 3:56 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Wed, 29 Nov 2023 at 04:09, Linus Walleij <linus.walleij@...aro.org> wrote:
>>>
>>> The most interesting patch is the list iterator fix in the core by Maria
>>> Yu, it took a while for me to realize what was going on there.
>>
>> That commit message still doesn't explain what the problem was.
>>
>> Why is p->state volatile there? It seems to be a serious locking bug
>> if p->state can randomly change there, and the READ_ONCE() looks like
>> a "this hides the problem" rather than an actual real fix.
This is indeed an interesting issue. Thx for the comment, Linus.
**Let me explain how: "p->state becomes volatile in the list iterator", 
and "why READ_ONCE is suggested".

The current critical code is:
   list_for_each_entry(setting, &p->state->settings, node)

after elaborating the define list_for_each_entry, so above critical code 
will be:
   for (setting = list_head(&p->state->settings, typeof(*setting), node); \
&setting->node != (&p->state->settings); \
setting = list_next(setting , node))

The asm code(refer result from Clang version 10.0) can cleared explain 
the step of p->state reload actions:
loop:
ldr x22,[x22] ; x22=list_next(setting , node))
add x9,x8,#0x18 ; x9=&p->state->setting
cmp x22,x9 ; setting,x9
b.eq 0xFFFFFF9B24483530

ldr w9,[x22,#0x10] ; w9,[setting,#16]
cmp w9,#0x2 ; w9,#2
b.ne 0xFFFFFF9B24483504

mov x0,x22 ; x0,setting
bl 0xFFFFFF9B24486048 ; pinmux_disable_setting

ldr x8,[x19,#0x28] ; x19=p, x8=[p->state], *reload p->state*
b loop

The *reload p->state* inside the loop for checking is not needed and can 
cause possible infinite loop. So READ_ONCE is highly suggested even if 
p->state is not randomly changed. And then unnecessary "ldr 
x8,[x19,#0x28]" can be removed from above loop code.

**Comments about the locking bug:
currently pinctrl_select_state is an export symbol and don't have 
effective reentrance protect design. That's why current infinite loop 
issue was observed with customer's multi thread call with 
pinctrl_select_state without lock protection. pinctrl_select_state 
totally rely on driver module user side to ensure the reentrant state.

Usually the suggested usage from driver side who are using pinctrl would be:
LOCK;
pinctrl_select_state();
gpio pulling;
udelay();
check state;
other hardware behaviors;
UNLOCK;

So the locking bug fix I have told customer side to fix from their own 
driver part. Since usually not only a simple pinctrl_select_state call 
can finish the hardware state transaction.

I myself also is fine to have a small per pinctrl lock to only protect 
the current pinctrl_select_state->pinctrl_commit_state reentrance 
issues. Pls any pinctrl maintainer help to comment to suggest or not and 
I can prepare a change as well.
> 
> Thanks for looking into it Linus, Maria can you look closer at this and
> try to pinpoint exactly what happens? 
Sure, I am trying to explain from my end. If there is other thoughts pls 
feel free to chime in.
> 
> Is the bug never manifesting with GCC for example?
> 
> In the meantime I'll cook a fixes branch without this one commit.
> 
> Yours,
> Linus Walleij

-- 
Thx and BRs,
Aiqun(Maria) Yu

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