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[Qemu-block] [RFC PATCH] virtio-blk: schedule virtio_notify_config to ru
From: |
Sergio Lopez |
Subject: |
[Qemu-block] [RFC PATCH] virtio-blk: schedule virtio_notify_config to run on main context |
Date: |
Thu, 12 Sep 2019 20:19:25 +0200 |
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 <address@hidden>
---
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
- [Qemu-block] [RFC PATCH] virtio-blk: schedule virtio_notify_config to run on main context,
Sergio Lopez <=