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Date:   Wed, 20 Dec 2017 12:53:27 -0600
From:   David Lechner <david@...hnology.com>
To:     Michael Turquette <mturquette@...libre.com>,
        linux-clk@...r.kernel.org
Cc:     Stephen Boyd <sboyd@...eaurora.org>, linux-kernel@...r.kernel.org,
        Jerome Brunet <jbrunet@...libre.com>
Subject: Re: [PATCH] clk: fix spin_lock/unlock imbalance on bad clk_enable()
 reentrancy

On 12/19/2017 04:29 PM, Michael Turquette wrote:
> Hi David,
> 
> Quoting David Lechner (2017-12-15 08:29:56)
>> On 12/12/2017 10:14 PM, David Lechner wrote:
>>> On 12/12/2017 05:43 PM, David Lechner wrote:
>>>> If clk_enable() is called in reentrant way and spin_trylock_irqsave() is
>>>> not working as expected, it is possible to get a negative enable_refcnt
>>>> which results in a missed call to spin_unlock_irqrestore().
>>>>
>>>> It works like this:
>>>>
>>>> 1. clk_enable() is called.
>>>> 2. clk_enable_unlock() calls spin_trylock_irqsave() and sets
>>>>      enable_refcnt = 1.
>>>> 3. Another clk_enable() is called before the first has returned
>>>>      (reentrant), but somehow spin_trylock_irqsave() is returning true.
>>>>      (I'm not sure how/why this is happening yet, but it is happening
>>>> to me
>>>>      with arch/arm/mach-davinci clocks that I am working on).
>>>
>>> I think I have figured out that since CONFIG_SMP=n and
>>> CONFIG_DEBUG_SPINLOCK=n on my kernel that
>>>
>>> #define arch_spin_trylock(lock)({ barrier(); (void)(lock); 1; })
>>>
>>> in include/linux/spinlock_up.h is causing the problem.
>>>
>>> So, basically, reentrancy of clk_enable() is broken for non-SMP systems,
>>> but I'm not sure I know how to fix it.
>>>
>>>
>>
>> Here is what I came up with for a fix. If it looks reasonable, I will
>> resend as a proper patch.
>>
>> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
>> index bb1b1f9..53fad5f 100644
>> --- a/drivers/clk/clk.c
>> +++ b/drivers/clk/clk.c
>> @@ -136,12 +136,23 @@ static void clk_prepare_unlock(void)
>>           mutex_unlock(&prepare_lock);
>>    }
>>
>> +#ifdef CONFIG_SMP
>> +#define NO_SMP 0
>> +#else
>> +#define NO_SMP 1
>> +#endif
>> +
>>    static unsigned long clk_enable_lock(void)
>>           __acquires(enable_lock)
>>    {
>> -       unsigned long flags;
>> +       unsigned long flags = 0;
>>
>> -       if (!spin_trylock_irqsave(&enable_lock, flags)) {
>> +       /*
>> +        * spin_trylock_irqsave() always returns true on non-SMP system
>> (unless
> 
> Ugh, wrapped lines in patch make me sad.

Sorry, I was being lazy. :-/

> 
>> +        * spin lock debugging is enabled) so don't call
>> spin_trylock_irqsave()
>> +        * unless we are on an SMP system.
>> +        */
>> +       if (NO_SMP || !spin_trylock_irqsave(&enable_lock, flags)) {
> 
> I'm not sure that this looks reasonable. The inverse logic (NO_SMP = 0
> being equivalent to SMP = 1) just makes things harder to read for no
> reason.
> 
> More to the point, did you fix your enable/disable call imbalance? If
> so, did you test that fix without this patch? I'd like to know if fixing
> the enable/disable imbalance is Good Enough. I'd prefer to take only
> that change and not this patch.

Without this patch, the imbalance in calls to spin lock/unlock are 
fixed, but I still get several WARN_ONCE_ON() because the reference 
count becomes negative, so I would not call it Good Enough.

> 
> Best regards,
> Mike
> 
>>                   if (enable_owner == current) {
>>                           enable_refcnt++;
>>                           __acquire(enable_lock);
>>

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