Re: [v2] powerpc: spinlock: Fix spin_unlock_wait()

On Mon, Jun 06, 2016 at 02:52:05PM +1000, Michael Ellerman wrote:

On Fri, 2016-03-06 at 03:49:48 UTC, Boqun Feng wrote:

quoted

There is an ordering issue with spin_unlock_wait() on powerpc, because
the spin_lock primitive is an ACQUIRE and an ACQUIRE is only ordering
the load part of the operation with memory operations following it.

...

quoted

diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
index 523673d7583c..2ed893662866 100644
--- a/arch/powerpc/include/asm/spinlock.h
+++ b/arch/powerpc/include/asm/spinlock.h

@@ -162,12 +181,23 @@ static inline void arch_spin_unlock(arch_spinlock_t *lock)
 	lock->slock = 0;
 }
 
-#ifdef CONFIG_PPC64
-extern void arch_spin_unlock_wait(arch_spinlock_t *lock);
-#else
-#define arch_spin_unlock_wait(lock) \
-	do { while (arch_spin_is_locked(lock)) cpu_relax(); } while (0)
-#endif
+static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
+{
+	smp_mb();
+
+	if (!arch_spin_is_locked_sync(lock))
+		goto out;
+
+	while (!arch_spin_value_unlocked(*lock)) {
+		HMT_low();
+		if (SHARED_PROCESSOR)
+			__spin_yield(lock);
+	}
+	HMT_medium();
+
+out:
+	smp_mb();
+}

I think this would actually be easier to follow if it was all just in one routine:

static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
{
	arch_spinlock_t lock_val;

	smp_mb();

	/*
	 * Atomically load and store back the lock value (unchanged). This
	 * ensures that our observation of the lock value is ordered with
	 * respect to other lock operations.
	 */
	__asm__ __volatile__(
"1:	" PPC_LWARX(%0, 0, %2, 1) "\n"
"	stwcx. %0, 0, %2\n"
"	bne- 1b\n"
	: "=&r" (lock_val), "+m" (*lock)
	: "r" (lock)
	: "cr0", "xer");

	if (arch_spin_value_unlocked(lock_val))
		goto out;

	while (!arch_spin_value_unlocked(*lock)) {
		HMT_low();
		if (SHARED_PROCESSOR)
			__spin_yield(lock);
	}
	HMT_medium();

out:
	smp_mb();
}


Thoughts?

Make sense. I admit that I sort of overdesigned by introducing
arch_spin_is_locked_sync().

This version is better, thank you!

Regards,
Boqun

cheers