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

[David Lechner]

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