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;
