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

Waiman Long
2016-08-18T21:11:03Z

The osq_lock() and osq_unlock() function may not provide the necessary
acquire and release barrier in some cases. This patch makes sure
that the proper barriers are provided when osq_lock() is successful
or when osq_unlock() is called.

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
---
kernel/locking/osq_lock.c | 24 ++++++++++++++++++------
1 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 05a3785..3da0b97 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -124,6 +124,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)

cpu_relax_lowlatency();
}
+ /*
+ * Add an acquire memory barrier for pairing with the release barrier
+ * in unlock.
+ */
+ smp_acquire__after_ctrl_dep();
return true;

unqueue:
@@ -198,13 +203,20 @@ void osq_unlock(struct optimistic_spin_queue *lock)
* Second most likely case.
*/
node = this_cpu_ptr(&osq_node);
- next = xchg(&node->next, NULL);
- if (next) {
- WRITE_ONCE(next->locked, 1);
+ next = xchg_relaxed(&node->next, NULL);
+ if (next)
+ goto unlock;
+
+ next = osq_wait_next(lock, node, NULL);
+ if (unlikely(!next)) {
+ /*
+ * In the unlikely event that the OSQ is empty, we need to
+ * provide a proper release barrier.
+ */
+ smp_mb();
return;
}

- next = osq_wait_next(lock, node, NULL);
- if (next)
- WRITE_ONCE(next->locked, 1);
+unlock:
+ smp_store_release(&next->locked, 1);
}
--
1.7.1