Date:   Fri, 1 Dec 2023 18:06:42 +0800
From:   "Aiqun(Maria) Yu" <quic_aiquny@...cinc.com>
To:     Linus Walleij <linus.walleij@...aro.org>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "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 12/1/2023 4:10 PM, Linus Walleij wrote:
> Hi Nathan, Nick,
> 
> (just picking some LLVM compiler people I know of... and trust)
> 
> Context is this patch:
> https://lore.kernel.org/linux-gpio/20231115102824.23727-1-quic_aiquny@quicinc.com/
> 
> On Thu, Nov 30, 2023 at 6:37 AM Aiqun(Maria) Yu <quic_aiquny@...cinc.com> wrote:
>> 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.
> 
> Luckily I am the pin control maintainer :D
> And I also ha my morning coffee and looked over the patch again.
> 
> So tilting the compiler to generate code that is less prone to race
> conditions with READ_ONCE() isn't really the solution is it? We need
> to introduce a proper lock that stops this from happening if it is
> a problem people are facing.
> 
> Can you try to make a patch that removes READ_ONCE()
> and introduce a lock instead?
> 
> Racing is rarely an issue in pin control for reasons explained
> in another context here:
> https://lore.kernel.org/linux-gpio/CACRpkdZ0cnJpYuzU=47-oW-7N_YGMo2vXpKOeXeNi5PhPY7QMA@mail.gmail.com/
> 
> ...but if people still manage to run into it, we better have a lock
> there. Just make sure it is not just an issue with outoftree code,
> but a real problem?
> 
> The change that changes the code to use the old_state variable
> should stay in, it makes the code more readable, it's just the
> READ_ONCE() macro which is dubious.
Thx for confirm. I am preparing the new change now. :)

READ_ONCE can only avoid the possible infinite loop and not crash the 
whole kernel, while the lock is needed to protect the multi parallel 
call of pinctrl_commit_state api have a consistent atomic hardware 
result as well.
> 
> Yours,
> Linus Walleij

-- 
Thx and BRs,
Aiqun(Maria) Yu

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