Re: [RFC PATCH-tip v4 01/10] locking/osq: Make lock/unlock proper acquire/release barrier

From: Waiman Long
Date: Thu Oct 06 2016 - 15:30:59 EST

On 10/06/2016 01:47 AM, Davidlohr Bueso wrote:
On Wed, 05 Oct 2016, Waiman Long wrote:

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.

+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?


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.

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