coroutine: add flag to re-queue at front of CoQueue

When a coroutine wakes up it may determine that it must re-queue.
Normally coroutines are pushed onto the back of the CoQueue, but for
fairness it may be necessary to push it onto the front of the CoQueue.

Add a flag to specify that the coroutine should be pushed onto the front
of the CoQueue. A later patch will use this to ensure fairness in the
bounce buffer CoQueue used by the blkio BlockDriver.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20221013185908.1297568-2-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

include/qemu/coroutine.h

@@ -198,14 +198,25 @@ typedef struct CoQueue {
  */
 void qemu_co_queue_init(CoQueue *queue);
 
+typedef enum {
+    /*
+     * Enqueue at front instead of back. Use this to re-queue a request when
+     * its wait condition is not satisfied after being woken up.
+     */
+    CO_QUEUE_WAIT_FRONT = 0x1,
+} CoQueueWaitFlags;
+
 /**
  * Adds the current coroutine to the CoQueue and transfers control to the
  * caller of the coroutine.  The mutex is unlocked during the wait and
  * locked again afterwards.
  */
 #define qemu_co_queue_wait(queue, lock) \
-    qemu_co_queue_wait_impl(queue, QEMU_MAKE_LOCKABLE(lock))
-void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock);
+    qemu_co_queue_wait_impl(queue, QEMU_MAKE_LOCKABLE(lock), 0)
+#define qemu_co_queue_wait_flags(queue, lock, flags) \
+    qemu_co_queue_wait_impl(queue, QEMU_MAKE_LOCKABLE(lock), (flags))
+void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock,
+                                          CoQueueWaitFlags flags);
 
 /**
  * Removes the next coroutine from the CoQueue, and queue it to run after

util/qemu-coroutine-lock.c

@@ -39,10 +39,15 @@ void qemu_co_queue_init(CoQueue *queue)
     QSIMPLEQ_INIT(&queue->entries);
 }
 
-void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock)
+void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock,
+                                          CoQueueWaitFlags flags)
 {
     Coroutine *self = qemu_coroutine_self();
-    QSIMPLEQ_INSERT_TAIL(&queue->entries, self, co_queue_next);
+    if (flags & CO_QUEUE_WAIT_FRONT) {
+        QSIMPLEQ_INSERT_HEAD(&queue->entries, self, co_queue_next);
+    } else {
+        QSIMPLEQ_INSERT_TAIL(&queue->entries, self, co_queue_next);
+    }
 
     if (lock) {
         qemu_lockable_unlock(lock);