coroutine-lock: make qemu_co_enter_next thread-safe

qemu_co_queue_next does not need to release and re-acquire the mutex,
because the queued coroutine does not run immediately.  However, this
does not hold for qemu_co_enter_next.  Now that qemu_co_queue_wait
can synchronize (via QemuLockable) with code that is not running in
coroutine context, it's important that code using qemu_co_enter_next
can easily use a standardized locking idiom.

First of all, qemu_co_enter_next must use aio_co_wake to restart the
coroutine.  Second, the function gains a second argument, a QemuLockable*,
and the comments of qemu_co_queue_next and qemu_co_queue_restart_all
are adjusted to clarify the difference.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20180203153935.8056-5-pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

1 files changed, 13 insertions, 6 deletions

diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
index 1e5f0957e6..6f8a487041 100644
--- a/include/qemu/coroutine.h
+++ b/include/qemu/coroutine.h
@@ -188,21 +188,28 @@ void qemu_co_queue_init(CoQueue *queue);
 void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock);
 
 /**
- * Restarts the next coroutine in the CoQueue and removes it from the queue.
- *
- * Returns true if a coroutine was restarted, false if the queue is empty.
+ * Removes the next coroutine from the CoQueue, and wake it up.
+ * Returns true if a coroutine was removed, false if the queue is empty.
  */
 bool coroutine_fn qemu_co_queue_next(CoQueue *queue);
 
 /**
- * Restarts all coroutines in the CoQueue and leaves the queue empty.
+ * Empties the CoQueue; all coroutines are woken up.
  */
 void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue);
 
 /**
- * Enter the next coroutine in the queue
+ * Removes the next coroutine from the CoQueue, and wake it up.  Unlike
+ * qemu_co_queue_next, this function releases the lock during aio_co_wake
+ * because it is meant to be used outside coroutine context; in that case, the
+ * coroutine is entered immediately, before qemu_co_enter_next returns.
+ *
+ * If used in coroutine context, qemu_co_enter_next is equivalent to
+ * qemu_co_queue_next.
  */
-bool qemu_co_enter_next(CoQueue *queue);
+#define qemu_co_enter_next(queue, lock) \
+    qemu_co_enter_next_impl(queue, QEMU_MAKE_LOCKABLE(lock))
+bool qemu_co_enter_next_impl(CoQueue *queue, QemuLockable *lock);
 
 /**
  * Checks if the CoQueue is empty.