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

[David Lechner]

On 12/21/2017 07:39 PM, Stephen Boyd wrote:
> On 12/20, Stephen Boyd wrote:
> > On 12/20, David Lechner wrote:
> > > On 12/20/2017 02:33 PM, David Lechner wrote:
> > >
> > >
> > > So, the question I have is: what is the actual "correct" behavior of
> > > spin_trylock_irqsave()? Is it really supposed to always return true
> > > when CONFIG_DEBUG_SPINLOCK=n and CONFIG_SMP=n or is this a bug?
> >
> > Thanks for doing the analysis in this thread.
> >
> > When CONFIG_DEBUG_SPINLOCK=n and CONFIG_SMP=n, spinlocks are
> > compiler barriers, that's it. So even if it is a bug to always
> > return true, I fail to see how we can detect that a spinlock is
> > already held in this configuration and return true or false.
> >
> > I suppose the best option is to make clk_enable_lock() and
> > clk_enable_unlock() into nops or pure owner/refcount/barrier
> > updates when CONFIG_SMP=n. We pretty much just need the barrier
> > semantics when there's only a single CPU.
>
> How about this patch? It should make the trylock go away on UP
> configs and then we keep everything else for refcount and
> ownership. We would test enable_owner outside of any
> irqs/preemption disabled section though. That needs a think.
>
> ---8<----
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index 3526bc068f30..b6f61367aa8d 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -143,7 +143,8 @@ static unsigned long clk_enable_lock(void)
>   {
>   	unsigned long flags;
>   
> -	if (!spin_trylock_irqsave(&enable_lock, flags)) {
> +	if (!IS_ENABLED(CONFIG_SMP) ||
> +	    !spin_trylock_irqsave(&enable_lock, flags)) {
>   		if (enable_owner == current) {
>   			enable_refcnt++;
>   			__acquire(enable_lock);


After sleeping on it, this is what I came up with. This keeps 
enable_owner and enable_refcnt protected and basically does the same 
thing that spin_lock_irqsave()/spin_unlock_irqrestore() would do on a UP 
system anyway, just more explicitly.

---

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index bb1b1f9..adbace3 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -136,6 +136,8 @@ static void clk_prepare_unlock(void)
 	mutex_unlock(&prepare_lock);
 }

+#ifdef CONFIG_SMP
+
 static unsigned long clk_enable_lock(void)
 	__acquires(enable_lock)
 {
@@ -170,6 +172,43 @@ static void clk_enable_unlock(unsigned long flags)
 	spin_unlock_irqrestore(&enable_lock, flags);
 }

+#else
+
+static unsigned long clk_enable_lock(void)
+	__acquires(enable_lock)
+{
+	unsigned long flags;
+
+	__acquire(enable_lock);
+	local_irq_save(flags);
+	preempt_disable();
+
+	if (enable_refcnt++ == 0) {
+		WARN_ON_ONCE(enable_owner != NULL);
+		enable_owner = current;
+	} else {
+		WARN_ON_ONCE(enable_owner != current);
+	}
+
+	return flags;
+}
+
+static void clk_enable_unlock(unsigned long flags)
+	__releases(enable_lock)
+{
+	WARN_ON_ONCE(enable_owner != current);
+	WARN_ON_ONCE(enable_refcnt == 0);
+
+	if (--enable_refcnt == 0)
+		enable_owner = NULL;
+
+	__release(enable_lock);
+	local_irq_restore(flags);
+	preempt_enable();
+}
+
+#endif
+
 static bool clk_core_is_prepared(struct clk_core *core)
 {
 	bool ret = false;