Re: [PATCH] clk: fix spin_lock/unlock imbalance on bad clk_enable() reentrancy

[David Lechner]

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
> > > >  ÂÂÂ enable_refcnt = 1.
> > > > 3. Another clk_enable() is called before the first has returned
> > > >  ÂÂÂ (reentrant), but somehow spin_trylock_irqsave() is returning true.
> > > >  ÂÂÂ (I'm not sure how/why this is happening yet, but it is happening
> > > > to me
> > > >  ÂÂÂ 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);