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

From: Stephen Boyd
Date: Thu Dec 21 2017 - 20:39:41 EST

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.

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)) {
+ !spin_trylock_irqsave(&enable_lock, flags)) {
if (enable_owner == current) {

Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project