coroutine-lock: qemu_co_queue_restart_all is a coroutine-only qemu_co_enter_all

qemu_co_queue_restart_all is basically the same as qemu_co_enter_all
but without a QemuLockable argument.  That's perfectly fine, but only as
long as the function is marked coroutine_fn.  If used outside coroutine
context, qemu_co_queue_wait will attempt to take the lock and that
is just broken: if you are calling qemu_co_queue_restart_all outside
coroutine context, the lock is going to be a QemuMutex which cannot be
taken twice by the same thread.

The patch adds the marker to qemu_co_queue_restart_all and to its sole
non-coroutine_fn caller; it then reimplements the function in terms of
qemu_co_enter_all_impl, to remove duplicated code and to clarify that the
latter also works in coroutine context.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20220427130830.150180-4-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

block/io.c

@@ -751,7 +751,7 @@ void bdrv_drain_all(void)
  *
  * This function should be called when a tracked request is completing.
  */
-static void tracked_request_end(BdrvTrackedRequest *req)
+static void coroutine_fn tracked_request_end(BdrvTrackedRequest *req)
 {
     if (req->serialising) {
         qatomic_dec(&req->bs->serialising_in_flight);

include/qemu/coroutine.h

@@ -216,10 +216,11 @@ void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock);
 bool coroutine_fn qemu_co_queue_next(CoQueue *queue);
 
 /**
- * Empties the CoQueue; all coroutines are woken up.
- * OK to run from coroutine and non-coroutine context.
+ * Empties the CoQueue and queues the coroutine to run after
+ * the currently-running coroutine yields.
+ * Used from coroutine context, use qemu_co_enter_all outside.
  */
-void qemu_co_queue_restart_all(CoQueue *queue);
+void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue);
 
 /**
  * Removes the next coroutine from the CoQueue, and wake it up.  Unlike

util/qemu-coroutine-lock.c

@@ -67,21 +67,6 @@ void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock)
     }
 }
 
-void qemu_co_queue_restart_all(CoQueue *queue)
-{
-    Coroutine *next;
-
-    if (QSIMPLEQ_EMPTY(&queue->entries)) {
-        return false;
-    }
-
-    while ((next = QSIMPLEQ_FIRST(&queue->entries)) != NULL) {
-        QSIMPLEQ_REMOVE_HEAD(&queue->entries, co_queue_next);
-        aio_co_wake(next);
-    }
-    return true;
-}
-
 bool qemu_co_enter_next_impl(CoQueue *queue, QemuLockable *lock)
 {
     Coroutine *next;
@@ -115,6 +100,12 @@ void qemu_co_enter_all_impl(CoQueue *queue, QemuLockable *lock)
     }
 }
 
+void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue)
+{
+    /* No unlock/lock needed in coroutine context.  */
+    qemu_co_enter_all_impl(queue, NULL);
+}
+
 bool qemu_co_queue_empty(CoQueue *queue)
 {
     return QSIMPLEQ_FIRST(&queue->entries) == NULL;