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Re: [Qemu-devel] [RFC PATCH] virtio-blk: schedule virtio_notify_config t
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [RFC PATCH] virtio-blk: schedule virtio_notify_config to run on main context |
Date: |
Thu, 12 Sep 2019 15:51:53 -0400 |
On Thu, Sep 12, 2019 at 08:19:25PM +0200, Sergio Lopez wrote:
> 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).
Hmm is this really the only case iothread takes qemu mutex?
If any such access can deadlock, don't we need a generic
solution? Maybe main thread can drop qemu mutex
before taking io thread AioContext lock?
> 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