On 12/12/2017 10:14 PM, David Lechner wrote:
On 12/12/2017 05:43 PM, David Lechner wrote:
If clk_enable() is called in reentrant way and spin_trylock_irqsave() is
not working as expected, it is possible to get a negative enable_refcnt
which results in a missed call to spin_unlock_irqrestore().
It works like this:
1. clk_enable() is called.
2. clk_enable_unlock() calls spin_trylock_irqsave() and sets
enable_refcnt = 1.
3. Another clk_enable() is called before the first has returned
(reentrant), but somehow spin_trylock_irqsave() is returning true.
(I'm not sure how/why this is happening yet, but it is happening
to me
with arch/arm/mach-davinci clocks that I am working on).
I think I have figured out that since CONFIG_SMP=n and
CONFIG_DEBUG_SPINLOCK=n on my kernel that
#define arch_spin_trylock(lock)({ barrier(); (void)(lock); 1; })
in include/linux/spinlock_up.h is causing the problem.
So, basically, reentrancy of clk_enable() is broken for non-SMP systems,
but I'm not sure I know how to fix it.
Here is what I came up with for a fix. If it looks reasonable, I will
resend as a proper patch.
diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index bb1b1f9..53fad5f 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -136,12 +136,23 @@ static void clk_prepare_unlock(void)
mutex_unlock(&prepare_lock);
}
+#ifdef CONFIG_SMP
+#define NO_SMP 0
+#else
+#define NO_SMP 1
+#endif
+
static unsigned long clk_enable_lock(void)
__acquires(enable_lock)
{
- unsigned long flags;
+ unsigned long flags = 0;
- if (!spin_trylock_irqsave(&enable_lock, flags)) {
+ /*
+ * spin_trylock_irqsave() always returns true on non-SMP system
(unless
+ * spin lock debugging is enabled) so don't call
spin_trylock_irqsave()
+ * unless we are on an SMP system.
+ */
+ if (NO_SMP || !spin_trylock_irqsave(&enable_lock, flags)) {
if (enable_owner == current) {
enable_refcnt++;
__acquire(enable_lock);
