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, 8 insertions, 2 deletions

diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
index 2a66fc1467..78fb79acf8 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -132,7 +132,7 @@ void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue)
     qemu_co_queue_do_restart(queue, false);
 }
 
-bool qemu_co_enter_next(CoQueue *queue)
+bool qemu_co_enter_next_impl(CoQueue *queue, QemuLockable *lock)
 {
     Coroutine *next;
 
@@ -142,7 +142,13 @@ bool qemu_co_enter_next(CoQueue *queue)
     }
 
     QSIMPLEQ_REMOVE_HEAD(&queue->entries, co_queue_next);
-    qemu_coroutine_enter(next);
+    if (lock) {
+        qemu_lockable_unlock(lock);
+    }
+    aio_co_wake(next);
+    if (lock) {
+        qemu_lockable_lock(lock);
+    }
     return true;
 }