On 10/06/2016 01:47 AM, Davidlohr Bueso wrote:

On Wed, 05 Oct 2016, Waiman Long wrote:

quoted

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 05a3785..1e6823a 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c

@@ -12,6 +12,23 @@
 */

static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, 
osq_node);

+enum mbtype {
+    acquire,
+    release,
+    relaxed,
+};

No, please.

quoted

+
+static __always_inline int
+_atomic_cmpxchg_(const enum mbtype barrier, atomic_t *v, int old, 
int new)
+{
+    if (barrier == acquire)
+        return atomic_cmpxchg_acquire(v, old, new);
+    else if (barrier == release)
+        return atomic_cmpxchg_release(v, old, new);
+    else
+        return atomic_cmpxchg_relaxed(v, old, new);
+}

Things like the above are icky. How about something like below? I'm not
crazy about it, but there are other similar macros, ie lockref. We still
provide the osq_lock/unlock to imply acquire/release and the new _relaxed
flavor, as I agree that should be the correct naming

While I have not touched osq_wait_next(), the following are impacted:

- node->locked is now completely without ordering for _relaxed() 
(currently
its under smp_load_acquire, which does not match and the race is harmless
to begin with as we just iterate again. For the acquire flavor, it is 
always
formed with ctr dep + smp_rmb().

- If osq_lock() fails we never guarantee any ordering.

What do you think?

Thanks,
Davidlohr

Yes, I am OK with your change. However, I need some additional changes 
in osq_wait_next() as well. Either it is changed to use the release 
variants of atomic_cmpxchg and xchg or using macro like what you did 
with osq_lock and osq_unlock. The release variant is needed in the 
osq_lock(). As osq_wait_next() is only invoked in the failure path of 
osq_lock(), the barrier type doesn't really matter.

Cheers,
Longman