Re: [PATCH v10 10/19] qspinlock, x86: Allow unfair spinlock in a virtual guest

On 05/08/2014 03:12 PM, Peter Zijlstra wrote:

On Wed, May 07, 2014 at 11:01:38AM -0400, Waiman Long wrote:


No, we want the unfair thing for VIRT, not PARAVIRT.

Yes, you are right. I will change that to VIRT.

quoted

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 9e7659e..10e87e1 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c

@@ -227,6 +227,14 @@ static __always_inline int get_qlock(struct qspinlock *lock)
  {
  	struct __qspinlock *l = (void *)lock;

+#ifdef CONFIG_PARAVIRT_UNFAIR_LOCKS
+	if (static_key_false(&paravirt_unfairlocks_enabled))
+		/*
+		 * Need to use atomic operation to get the lock when
+		 * lock stealing can happen.
+		 */
+		return cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0;

That's missing {}.

It is a single statement which doesn't need braces according to kernel coding style. I could move the comments up a bit to make it easier to read.

quoted

+#endif

quoted

  	barrier();
  	ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
  	barrier();

But no, what you want is:

static __always_inline bool virt_lock(struct qspinlock *lock)
{
#ifdef CONFIG_VIRT_MUCK
	if (static_key_false(&virt_unfairlocks_enabled)) {
		while (!queue_spin_trylock(lock))
			cpu_relax();

		return true;
	}
#else
	return false;
}


void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
{
	if (virt_lock(lock))
		return;

	...
}

This is a possible way of doing it. I can do that in the patch series to simplify it. Hopefully that will speed up the review process and get it done quicker.

-Longman