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Date:   Tue, 19 Dec 2017 14:29:47 -0800
From:   Michael Turquette <mturquette@...libre.com>
To:     David Lechner <david@...hnology.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

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.

> +        * 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.

Best regards,
Mike

>                  if (enable_owner == current) {
>                          enable_refcnt++;
>                          __acquire(enable_lock);
> 

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