[Qemu-devel] [RFC PATCH] virtio-blk: schedule virtio_notify_config to run on main context

[Sergio Lopez <slp@redhat.com>]

Another AioContext-related issue, and this is a tricky one.

Executing a QMP block_resize request for a virtio-blk device running
on an iothread may cause a deadlock involving the following mutexes:

 - main thead
  * Has acquired: qemu_mutex_global.
  * Is trying the acquire: iothread AioContext lock via
    AIO_WAIT_WHILE (after aio_poll).

 - iothread
  * Has acquired: AioContext lock.
  * Is trying to acquire: qemu_mutex_global (via
    virtio_notify_config->prepare_mmio_access).

With this change, virtio_blk_resize checks if it's being called from a
coroutine context running on a non-main thread, and if that's the
case, creates a new coroutine and schedules it to be run on the main
thread.

This works, but means the actual operation is done
asynchronously, perhaps opening a window in which a "device_del"
operation may fit and remove the VirtIODevice before
virtio_notify_config() is executed.

I *think* it shouldn't be possible, as BHs will be processed before
any new QMP/monitor command, but I'm open to a different approach.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1744955
Signed-off-by: Sergio Lopez <slp@redhat.com>
---
 hw/block/virtio-blk.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 18851601cb..c763d071f6 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -16,6 +16,7 @@
 #include "qemu/iov.h"
 #include "qemu/module.h"
 #include "qemu/error-report.h"
+#include "qemu/main-loop.h"
 #include "trace.h"
 #include "hw/block/block.h"
 #include "hw/qdev-properties.h"
@@ -1086,11 +1087,33 @@ static int virtio_blk_load_device(VirtIODevice *vdev, QEMUFile *f,
     return 0;
 }
 
+static void coroutine_fn virtio_resize_co_entry(void *opaque)
+{
+    VirtIODevice *vdev = opaque;
+
+    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
+    virtio_notify_config(vdev);
+    aio_wait_kick();
+}
+
 static void virtio_blk_resize(void *opaque)
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(opaque);
+    Coroutine *co;
 
-    virtio_notify_config(vdev);
+    if (qemu_in_coroutine() &&
+        qemu_get_current_aio_context() != qemu_get_aio_context()) {
+        /*
+         * virtio_notify_config() needs to acquire the global mutex,
+         * so calling it from a coroutine running on a non-main context
+         * may cause a deadlock. Instead, create a new coroutine and
+         * schedule it to be run on the main thread.
+         */
+        co = qemu_coroutine_create(virtio_resize_co_entry, vdev);
+        aio_co_schedule(qemu_get_aio_context(), co);
+    } else {
+        virtio_notify_config(vdev);
+    }
 }
 
 static const BlockDevOps virtio_block_ops = {
-- 
2.21.0