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);

I came up with the exact same patch earlier today, but did not have a 
chance to send it. I've tested it and it fixes the problem for me.

I'm afraid I don't know enough about how preemption works yet to be of 
much help to say what or if something else is needed to protect 
enable_owner/enable_refcnt.