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

From: Davidlohr Bueso
Date: Thu Oct 06 2016 - 01:48:16 EST

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?

Thanks,
Davidlohr


diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h
index 703ea5c30a33..183ee51e6e54 100644
--- a/include/linux/osq_lock.h
+++ b/include/linux/osq_lock.h
@@ -29,9 +29,20 @@ static inline void osq_lock_init(struct optimistic_spin_queue *lock)
atomic_set(&lock->tail, OSQ_UNLOCKED_VAL);
}

+/*
+ * Versions of osq_lock/unlock that do not imply or guarantee (load)-ACQUIRE
+ * (store)-RELEASE barrier semantics.
+ *
+ * Note that a failed call to either osq_lock() or osq_lock_relaxed() does
+ * not imply barriers... we are next to block.
+ */
+extern bool osq_lock_relaxed(struct optimistic_spin_queue *lock);
+extern void osq_unlock_relaxed(struct optimistic_spin_queue *lock);
+
extern bool osq_lock(struct optimistic_spin_queue *lock);
extern void osq_unlock(struct optimistic_spin_queue *lock);

+
static inline bool osq_is_locked(struct optimistic_spin_queue *lock)
{
return atomic_read(&lock->tail) != OSQ_UNLOCKED_VAL;
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index a70b90db3909..b1bf1e057565 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -316,7 +316,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
* acquire the mutex all at once, the spinners need to take a
* MCS (queued) lock first before spinning on the owner field.
*/
- if (!osq_lock(&lock->osq))
+ if (!osq_lock_relaxed(&lock->osq))
goto done;

while (true) {
@@ -358,7 +358,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
}

mutex_set_owner(lock);
- osq_unlock(&lock->osq);
+ osq_unlock_relaxed(&lock->osq);
return true;
}

@@ -380,7 +380,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
cpu_relax_lowlatency();
}

- osq_unlock(&lock->osq);
+ osq_unlock_relaxed(&lock->osq);
done:
/*
* If we fell out of the spin path because of need_resched(),
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 05a37857ab55..8c3d57698702 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -28,6 +28,17 @@ static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
return per_cpu_ptr(&osq_node, cpu_nr);
}

+static inline void set_node_locked_release(struct optimistic_spin_node *node)
+{
+ smp_store_release(&node->locked, 1);
+}
+
+static inline void set_node_locked_relaxed(struct optimistic_spin_node *node)
+{
+ WRITE_ONCE(node->locked, 1);
+
+}
+
/*
* Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
* Can return NULL in case we were the last queued and we updated @lock instead.
@@ -81,130 +92,140 @@ osq_wait_next(struct optimistic_spin_queue *lock,
return next;
}

-bool osq_lock(struct optimistic_spin_queue *lock)
-{
- struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
- struct optimistic_spin_node *prev, *next;
- int curr = encode_cpu(smp_processor_id());
- int old;
-
- node->locked = 0;
- node->next = NULL;
- node->cpu = curr;
-
- /*
- * We need both ACQUIRE (pairs with corresponding RELEASE in
- * unlock() uncontended, or fastpath) and RELEASE (to publish
- * the node fields we just initialised) semantics when updating
- * the lock tail.
- */
- old = atomic_xchg(&lock->tail, curr);
- if (old == OSQ_UNLOCKED_VAL)
- return true;
-
- prev = decode_cpu(old);
- node->prev = prev;
- WRITE_ONCE(prev->next, node);
-
- /*
- * Normally @prev is untouchable after the above store; because at that
- * moment unlock can proceed and wipe the node element from stack.
- *
- * However, since our nodes are static per-cpu storage, we're
- * guaranteed their existence -- this allows us to apply
- * cmpxchg in an attempt to undo our queueing.
- */
-
- while (!READ_ONCE(node->locked)) {
- /*
- * If we need to reschedule bail... so we can block.
- */
- if (need_resched())
- goto unqueue;
-
- cpu_relax_lowlatency();
- }
- return true;
-
-unqueue:
- /*
- * Step - A -- stabilize @prev
- *
- * Undo our @prev->next assignment; this will make @prev's
- * unlock()/unqueue() wait for a next pointer since @lock points to us
- * (or later).
- */
-
- for (;;) {
- if (prev->next == node &&
- cmpxchg(&prev->next, node, NULL) == node)
- break;
-
- /*
- * We can only fail the cmpxchg() racing against an unlock(),
- * in which case we should observe @node->locked becomming
- * true.
- */
- if (smp_load_acquire(&node->locked))
- return true;
-
- cpu_relax_lowlatency();
-
- /*
- * Or we race against a concurrent unqueue()'s step-B, in which
- * case its step-C will write us a new @node->prev pointer.
- */
- prev = READ_ONCE(node->prev);
- }
-
- /*
- * Step - B -- stabilize @next
- *
- * Similar to unlock(), wait for @node->next or move @lock from @node
- * back to @prev.
- */
-
- next = osq_wait_next(lock, node, prev);
- if (!next)
- return false;
-
- /*
- * Step - C -- unlink
- *
- * @prev is stable because its still waiting for a new @prev->next
- * pointer, @next is stable because our @node->next pointer is NULL and
- * it will wait in Step-A.
- */
-
- WRITE_ONCE(next->prev, prev);
- WRITE_ONCE(prev->next, next);
-
- return false;
+#define OSQ_LOCK(EXT, FENCECB) \
+bool osq_lock##EXT(struct optimistic_spin_queue *lock) \
+{ \
+ struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); \
+ struct optimistic_spin_node *prev, *next; \
+ int old, curr = encode_cpu(smp_processor_id()); \
+ \
+ node->locked = 0; \
+ node->next = NULL; \
+ node->cpu = curr; \
+ \
+ /* \
+ * We need both ACQUIRE (pairs with corresponding RELEASE in \
+ * unlock() uncontended, or fastpath) and RELEASE (to publish \
+ * the node fields we just initialised) semantics when updating \
+ * the lock tail. \
+ */ \
+ old = atomic_xchg(&lock->tail, curr); \
+ if (old == OSQ_UNLOCKED_VAL) \
+ return true; \
+ \
+ prev = decode_cpu(old); \
+ node->prev = prev; \
+ WRITE_ONCE(prev->next, node); \
+ \
+ /* \
+ * Normally @prev is untouchable after the above store; because \
+ * at that moment unlock can proceed and wipe the node element \
+ * from stack. \
+ * \
+ * However, since our nodes are static per-cpu storage, we're \
+ * guaranteed their existence -- this allows us to apply \
+ * cmpxchg in an attempt to undo our queueing. \
+ */ \
+ while (!READ_ONCE(node->locked)) { \
+ /* \
+ * If we need to reschedule bail... so we can block. \
+ */ \
+ if (need_resched()) \
+ goto unqueue; \
+ \
+ cpu_relax_lowlatency(); \
+ } \
+ FENCECB; \
+ return true; \
+ \
+unqueue: \
+ /* \
+ * Step - A -- stabilize @prev \
+ * \
+ * Undo our @prev->next assignment; this will make @prev's \
+ * unlock()/unqueue() wait for a next pointer since @lock \
+ * points to us (or later). \
+ */ \
+ for (;;) { \
+ /* \
+ * Failed calls to osq_lock() do not guarantee \
+ * barriers, thus always rely on RELAXED semantics. \
+ */ \
+ if (prev->next == node && \
+ cmpxchg_relaxed(&prev->next, node, NULL) == node) \
+ break; \
+ \
+ /* \
+ * We can only fail the cmpxchg() racing against an \
+ * unlock(), in which case we should observe \
+ * @node->locked becoming true. \
+ */ \
+ if (READ_ONCE(node->locked)) { \
+ FENCECB; \
+ return true; \
+ } \
+ \
+ cpu_relax_lowlatency(); \
+ \
+ /* \
+ * Or we race against a concurrent unqueue()'s step-B, \
+ * in which case its step-C will write us a new \
+ * @node->prev pointer. \
+ */ \
+ prev = READ_ONCE(node->prev); \
+ } \
+ \
+ /* \
+ * Step - B -- stabilize @next \
+ * \
+ * Similar to unlock(), wait for @node->next or move @lock \
+ * from @node back to @prev. \
+ */ \
+ \
+ next = osq_wait_next(lock, node, prev); \
+ if (!next) \
+ return false; \
+ \
+ /* \
+ * Step - C -- unlink \
+ * \
+ * @prev is stable because its still waiting for a new \
+ * @prev->next pointer, @next is stable because our \
+ * @node->next pointer is NULL and it will wait in Step-A. \
+ */ \
+ \
+ WRITE_ONCE(next->prev, prev); \
+ WRITE_ONCE(prev->next, next); \
+ \
+ return false; \
}

-void osq_unlock(struct optimistic_spin_queue *lock)
-{
- struct optimistic_spin_node *node, *next;
- int curr = encode_cpu(smp_processor_id());
-
- /*
- * Fast path for the uncontended case.
- */
- if (likely(atomic_cmpxchg_release(&lock->tail, curr,
- OSQ_UNLOCKED_VAL) == curr))
- return;
-
- /*
- * Second most likely case.
- */
- node = this_cpu_ptr(&osq_node);
- next = xchg(&node->next, NULL);
- if (next) {
- WRITE_ONCE(next->locked, 1);
- return;
- }
-
- next = osq_wait_next(lock, node, NULL);
- if (next)
- WRITE_ONCE(next->locked, 1);
+OSQ_LOCK(, smp_acquire__after_ctrl_dep())
+OSQ_LOCK(_relaxed, )
+
+#define OSQ_UNLOCK(EXT, FENCE) \
+void osq_unlock##EXT(struct optimistic_spin_queue *lock) \
+{ \
+ struct optimistic_spin_node *node, *next; \
+ int curr = encode_cpu(smp_processor_id()); \
+ \
+ /* Fast path for the uncontended case. */ \
+ if (likely(atomic_cmpxchg_##FENCE(&lock->tail, curr, \
+ OSQ_UNLOCKED_VAL) == curr)) \
+ return; \
+ \
+ /* Second most likely case. */ \
+ node = this_cpu_ptr(&osq_node); \
+ next = xchg(&node->next, NULL); \
+ if (next) \
+ goto done_setlocked; \
+ \
+ next = osq_wait_next(lock, node, NULL); \
+ if (!next) \
+ return; \
+done_setlocked: \
+ set_node_locked_##FENCE(next); \
}
+
+OSQ_UNLOCK(, release)
+OSQ_UNLOCK(_relaxed, relaxed)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 2337b4bb2366..88e95b114392 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -389,7 +389,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
if (!rwsem_can_spin_on_owner(sem))
goto done;

- if (!osq_lock(&sem->osq))
+ if (!osq_lock_relaxed(&sem->osq))
goto done;

/*
@@ -425,7 +425,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
*/
cpu_relax_lowlatency();
}
- osq_unlock(&sem->osq);
+ osq_unlock_relaxed(&sem->osq);
done:
preempt_enable();
return taken;