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

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Date:   Thu, 21 Dec 2017 17:39:15 -0800
From:   Stephen Boyd <sboyd@...eaurora.org>
To:     David Lechner <david@...hnology.com>
Cc:     Michael Turquette <mturquette@...libre.com>,
        linux-clk@...r.kernel.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

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);


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