qemu-thread used to abstract pthread primitives into futex for the
QemuEvent implementation of POSIX systems other than Linux. However,
this abstraction has one key difference: unlike futex, pthread
primitives require an explicit destruction, and it must be ordered after
wait and wake operations.

It would be easier to perform destruction if a wait operation ensures
the corresponding wake operation finishes as POSIX semaphore does, but
that requires to protect state accesses in qemu_event_set() and
qemu_event_wait() with a mutex. On the other hand, real futex does not
need such a protection but needs complex barrier and atomic operations
to ensure ordering between the two functions.

Add special implementations of qemu_event_set() and qemu_event_wait()
using pthread primitives. qemu_event_wait() will ensure qemu_event_set()
finishes, and these functions will avoid complex barrier and atomic
operations to ensure ordering between them.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Tested-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Phil Dennis-Jordan <phil@philjordan.eu>
---
 util/qemu-thread-posix.c | 50 +++++++++++++++++++++++-------------------------
 1 file changed, 24 insertions(+), 26 deletions(-)

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 21ed12ddb56b..dc13b61f1bd8 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -319,36 +319,21 @@ void qemu_sem_wait(QemuSemaphore *sem)
 
 #ifdef CONFIG_LINUX
 #include "qemu/futex.h"
-#else
-static inline void qemu_futex_wake(QemuEvent *ev, int n)
-{
-    assert(ev->initialized);
-    pthread_mutex_lock(&ev->lock);
-    if (n == 1) {
-        pthread_cond_signal(&ev->cond);
-    } else {
-        pthread_cond_broadcast(&ev->cond);
-    }
-    pthread_mutex_unlock(&ev->lock);
-}
-
-static inline void qemu_futex_wait(QemuEvent *ev, unsigned val)
-{
-    assert(ev->initialized);
-    pthread_mutex_lock(&ev->lock);
-    if (ev->value == val) {
-        pthread_cond_wait(&ev->cond, &ev->lock);
-    }
-    pthread_mutex_unlock(&ev->lock);
-}
 #endif
 
 /* Valid transitions:
  * - free->set, when setting the event
- * - busy->set, when setting the event, followed by qemu_futex_wake_all
+ * - busy->set, when setting the event
  * - set->free, when resetting the event
  * - free->busy, when waiting
  *
+ * With futex, the waking and blocking operations follow busy->set and
+ * free->busy, respectively.
+ *
+ * Without futex, busy->set and free->busy never happen. Instead, the waking
+ * operation follows free->set and the blocking operation will happen when
+ * waiting if the event is not set.
+ *
  * set->busy does not happen (it can be observed from the outside but
  * it really is set->free->busy).
  *
@@ -386,6 +371,7 @@ void qemu_event_set(QemuEvent *ev)
 {
     assert(ev->initialized);
 
+#ifdef CONFIG_LINUX
     /*
      * Pairs with both qemu_event_reset() and qemu_event_wait().
      *
@@ -399,6 +385,12 @@ void qemu_event_set(QemuEvent *ev)
             qemu_futex_wake_all(ev);
         }
     }
+#else
+    pthread_mutex_lock(&ev->lock);
+    qatomic_store_release(&ev->value, EV_SET);
+    pthread_cond_broadcast(&ev->cond);
+    pthread_mutex_unlock(&ev->lock);
+#endif
 }
 
 void qemu_event_reset(QemuEvent *ev)
@@ -420,17 +412,16 @@ void qemu_event_reset(QemuEvent *ev)
 
 void qemu_event_wait(QemuEvent *ev)
 {
-    unsigned value;
-
     assert(ev->initialized);
 
+#ifdef CONFIG_LINUX
     while (true) {
         /*
          * qemu_event_wait must synchronize with qemu_event_set even if it does
          * not go down the slow path, so this load-acquire is needed that
          * synchronizes with the first memory barrier in qemu_event_set().
          */
-        value = qatomic_load_acquire(&ev->value);
+        unsigned value = qatomic_load_acquire(&ev->value);
         if (value == EV_SET) {
             break;
         }
@@ -454,6 +445,13 @@ void qemu_event_wait(QemuEvent *ev)
 
         qemu_futex_wait(ev, EV_BUSY);
     }
+#else
+    pthread_mutex_lock(&ev->lock);
+    while (qatomic_read(&ev->value) != EV_SET) {
+        pthread_cond_wait(&ev->cond, &ev->lock);
+    }
+    pthread_mutex_unlock(&ev->lock);
+#endif
 }
 
 static __thread NotifierList thread_exit;

-- 
2.49.0