block: Repair the throttling code.

The throttling code was segfaulting since commit
02ffb504485f0920cfc75a0982a602f824a9a4f4 because some qemu_co_queue_next caller
does not run in a coroutine.
qemu_co_queue_do_restart assume that the caller is a coroutinne.
As suggested by Stefan fix this by entering the coroutine directly.
Also make sure like suggested that qemu_co_queue_next() and
qemu_co_queue_restart_all() can be called only in coroutines.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -127,7 +127,7 @@ void bdrv_io_limits_disable(BlockDriverState *bs)
 {
     bs->io_limits_enabled = false;
 
-    while (qemu_co_queue_next(&bs->throttled_reqs));
+    do {} while (qemu_co_enter_next(&bs->throttled_reqs));
 
     if (bs->block_timer) {
         qemu_del_timer(bs->block_timer);
@@ -143,7 +143,7 @@ static void bdrv_block_timer(void *opaque)
 {
     BlockDriverState *bs = opaque;
 
-    qemu_co_queue_next(&bs->throttled_reqs);
+    qemu_co_enter_next(&bs->throttled_reqs);
 }
 
 void bdrv_io_limits_enable(BlockDriverState *bs)
@@ -1452,8 +1452,7 @@ void bdrv_drain_all(void)
          * a busy wait.
          */
         QTAILQ_FOREACH(bs, &bdrv_states, list) {
-            if (!qemu_co_queue_empty(&bs->throttled_reqs)) {
-                qemu_co_queue_restart_all(&bs->throttled_reqs);
+            while (qemu_co_enter_next(&bs->throttled_reqs)) {
                 busy = true;
             }
         }

include/block/coroutine.h

@@ -130,12 +130,17 @@ void coroutine_fn qemu_co_queue_wait_insert_head(CoQueue *queue);
  *
  * Returns true if a coroutine was restarted, false if the queue is empty.
  */
-bool qemu_co_queue_next(CoQueue *queue);
+bool coroutine_fn qemu_co_queue_next(CoQueue *queue);
 
 /**
  * Restarts all coroutines in the CoQueue and leaves the queue empty.
  */
-void qemu_co_queue_restart_all(CoQueue *queue);
+void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue);
+
+/**
+ * Enter the next coroutine in the queue
+ */
+bool qemu_co_enter_next(CoQueue *queue);
 
 /**
  * Checks if the CoQueue is empty.

qemu-coroutine-lock.c

@@ -88,16 +88,32 @@ static bool qemu_co_queue_do_restart(CoQueue *queue, bool single)
     return true;
 }
 
-bool qemu_co_queue_next(CoQueue *queue)
+bool coroutine_fn qemu_co_queue_next(CoQueue *queue)
 {
+    assert(qemu_in_coroutine());
     return qemu_co_queue_do_restart(queue, true);
 }
 
-void qemu_co_queue_restart_all(CoQueue *queue)
+void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue)
 {
+    assert(qemu_in_coroutine());
     qemu_co_queue_do_restart(queue, false);
 }
 
+bool qemu_co_enter_next(CoQueue *queue)
+{
+    Coroutine *next;
+
+    next = QTAILQ_FIRST(&queue->entries);
+    if (!next) {
+        return false;
+    }
+
+    QTAILQ_REMOVE(&queue->entries, next, co_queue_next);
+    qemu_coroutine_enter(next, NULL);
+    return true;
+}
+
 bool qemu_co_queue_empty(CoQueue *queue)
 {
     return (QTAILQ_FIRST(&queue->entries) == NULL);