linux-kernel - Re: [PATCH] clk: fix spin_lock/unlock imbalance on bad clk

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Date:   Wed, 20 Dec 2017 11:24:18 -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

Quoting David Lechner (2017-12-20 10:53:27)
> 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.

Several WARN_ON_ONCE? Have you narrowed down exactly which conditions in
the lock/unlock path are firing?

Also, I wasn't referring to the lock/unlock imbalance in my email above.
I was referring to the enable count mismatch. Have you fixed that bug? I
assume it's an incorrect clk consumer driver causing it.

Regards,
Mike

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