From: Dima Stepanov <dimastep@yandex-team.ru>
To: Jason Wang <jasowang@redhat.com>
Cc: fam@euphon.net, kwolf@redhat.com, stefanha@redhat.com,
qemu-block@nongnu.org, mst@redhat.com, qemu-devel@nongnu.org,
dgilbert@redhat.com, arei.gonglei@huawei.com, fengli@smartx.com,
yc-core@yandex-team.ru, pbonzini@redhat.com,
marcandre.lureau@redhat.com, raphael.norwitz@nutanix.com,
mreitz@redhat.com
Subject: Re: [PATCH v2 5/5] vhost: add device started check in migration set log
Date: Tue, 12 May 2020 12:35:30 +0300 [thread overview]
Message-ID: <20200512093521.GA5363@dimastep-nix> (raw)
In-Reply-To: <fd260f1f-75f3-46ac-8ca5-bbb7e41e712f@redhat.com>
On Tue, May 12, 2020 at 11:32:50AM +0800, Jason Wang wrote:
>
> On 2020/5/11 下午5:25, Dima Stepanov wrote:
> >On Mon, May 11, 2020 at 11:15:53AM +0800, Jason Wang wrote:
> >>On 2020/4/30 下午9:36, Dima Stepanov wrote:
> >>>If vhost-user daemon is used as a backend for the vhost device, then we
> >>>should consider a possibility of disconnect at any moment. If such
> >>>disconnect happened in the vhost_migration_log() routine the vhost
> >>>device structure will be clean up.
> >>>At the start of the vhost_migration_log() function there is a check:
> >>> if (!dev->started) {
> >>> dev->log_enabled = enable;
> >>> return 0;
> >>> }
> >>>To be consistent with this check add the same check after calling the
> >>>vhost_dev_set_log() routine. This in general help not to break a
> >>>migration due the assert() message. But it looks like that this code
> >>>should be revised to handle these errors more carefully.
> >>>
> >>>In case of vhost-user device backend the fail paths should consider the
> >>>state of the device. In this case we should skip some function calls
> >>>during rollback on the error paths, so not to get the NULL dereference
> >>>errors.
> >>>
> >>>Signed-off-by: Dima Stepanov <dimastep@yandex-team.ru>
> >>>---
> >>> hw/virtio/vhost.c | 39 +++++++++++++++++++++++++++++++++++----
> >>> 1 file changed, 35 insertions(+), 4 deletions(-)
> >>>
> >>>diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >>>index 3ee50c4..d5ab96d 100644
> >>>--- a/hw/virtio/vhost.c
> >>>+++ b/hw/virtio/vhost.c
> >>>@@ -787,6 +787,17 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
> >>> static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
> >>> {
> >>> int r, i, idx;
> >>>+
> >>>+ if (!dev->started) {
> >>>+ /*
> >>>+ * If vhost-user daemon is used as a backend for the
> >>>+ * device and the connection is broken, then the vhost_dev
> >>>+ * structure will be reset all its values to 0.
> >>>+ * Add additional check for the device state.
> >>>+ */
> >>>+ return -1;
> >>>+ }
> >>>+
> >>> r = vhost_dev_set_features(dev, enable_log);
> >>> if (r < 0) {
> >>> goto err_features;
> >>>@@ -801,12 +812,19 @@ static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
> >>> }
> >>> return 0;
> >>> err_vq:
> >>>- for (; i >= 0; --i) {
> >>>+ /*
> >>>+ * Disconnect with the vhost-user daemon can lead to the
> >>>+ * vhost_dev_cleanup() call which will clean up vhost_dev
> >>>+ * structure.
> >>>+ */
> >>>+ for (; dev->started && (i >= 0); --i) {
> >>> idx = dev->vhost_ops->vhost_get_vq_index(
> >>
> >>Why need the check of dev->started here, can started be modified outside
> >>mainloop? If yes, I don't get the check of !dev->started in the beginning of
> >>this function.
> >>
> >No dev->started can't change outside the mainloop. The main problem is
> >only for the vhost_user_blk daemon. Consider the case when we
> >successfully pass the dev->started check at the beginning of the
> >function, but after it we hit the disconnect on the next call on the
> >second or third iteration:
> > r = vhost_virtqueue_set_addr(dev, dev->vqs + i, idx, enable_log);
> >The unix socket backend device will call the disconnect routine for this
> >device and reset the structure. So the structure will be reset (and
> >dev->started set to false) inside this set_addr() call.
>
>
> I still don't get here. I think the disconnect can not happen in the middle
> of vhost_dev_set_log() since both of them were running in mainloop. And even
> if it can, we probably need other synchronization mechanism other than
> simple check here.
Disconnect isn't happened in the separate thread it is happened in this
routine inside vhost_dev_set_log. When for instance vhost_user_write()
call failed:
vhost_user_set_log_base()
vhost_user_write()
vhost_user_blk_disconnect()
vhost_dev_cleanup()
vhost_user_backend_cleanup()
So the point is that if we somehow got a disconnect with the
vhost-user-blk daemon before the vhost_user_write() call then it will
continue clean up by running vhost_user_blk_disconnect() function. I
wrote a more detailed backtrace stack in the separate thread, which is
pretty similar to what we have here:
Re: [PATCH v2 4/5] vhost: check vring address before calling unmap
The places are different but the problem is pretty similar.
So if vhost-user commands handshake then everything is fine and
reconnect will work as expected. The only problem is how to handle
reconnect properly between vhost-user command send/receive.
As i wrote we have a test:
- run src VM with vhost-usr-blk daemon used
- run fio inside it
- perform reconnect every X seconds (just kill and restart daemon),
X is random
- run dst VM
- perform migration
- fio should complete in dst VM
And we cycle this test like forever.
So it fails once per ~25 iteration. By adding some delays inside qemu we
were able to make the race window larger.
>
>
> > So
> >we shouldn't call the clean up calls because this virtqueues were clean
> >up in the disconnect call. But we should protect these calls somehow, so
> >it will not hit SIGSEGV and we will be able to pass migration.
> >
> >Just to summarize it:
> >For the vhost-user-blk devices we ca hit clean up calls twice in case of
> >vhost disconnect:
> >1. The first time during the disconnect process. The clean up is called
> >inside it.
> >2. The second time during roll back clean up.
> >So if it is the case we should skip p2.
> >
> >>>dev, dev->vq_index + i);
> >>> vhost_virtqueue_set_addr(dev, dev->vqs + i, idx,
> >>> dev->log_enabled);
> >>> }
> >>>- vhost_dev_set_features(dev, dev->log_enabled);
> >>>+ if (dev->started) {
> >>>+ vhost_dev_set_features(dev, dev->log_enabled);
> >>>+ }
> >>> err_features:
> >>> return r;
> >>> }
> >>>@@ -832,7 +850,15 @@ static int vhost_migration_log(MemoryListener *listener, int enable)
> >>> } else {
> >>> vhost_dev_log_resize(dev, vhost_get_log_size(dev));
> >>> r = vhost_dev_set_log(dev, true);
> >>>- if (r < 0) {
> >>>+ /*
> >>>+ * The dev log resize can fail, because of disconnect
> >>>+ * with the vhost-user-blk daemon. Check the device
> >>>+ * state before calling the vhost_dev_set_log()
> >>>+ * function.
> >>>+ * Don't return error if device isn't started to be
> >>>+ * consistent with the check above.
> >>>+ */
> >>>+ if (dev->started && r < 0) {
> >>> return r;
> >>> }
> >>> }
> >>>@@ -1739,7 +1765,12 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
> >>> fail_log:
> >>> vhost_log_put(hdev, false);
> >>> fail_vq:
> >>>- while (--i >= 0) {
> >>>+ /*
> >>>+ * Disconnect with the vhost-user daemon can lead to the
> >>>+ * vhost_dev_cleanup() call which will clean up vhost_dev
> >>>+ * structure.
> >>>+ */
> >>>+ while ((--i >= 0) && (hdev->started)) {
> >>> vhost_virtqueue_stop(hdev,
> >>> vdev,
> >>> hdev->vqs + i,
> >>
> >>This should be a separate patch.
> >Do you mean i should split this patch to two patches?
>
>
> Yes.
>
> Thanks
Got it. Will do it in v3.
No other comments mixed in below.
Thanks.
>
>
> >
> >Thanks.
> >
> >>Thanks
> >>
>
next prev parent reply other threads:[~2020-05-12 9:36 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-30 13:36 [PATCH v2 0/5] vhost-user reconnect issues during vhost initialization Dima Stepanov
2020-04-30 13:36 ` [PATCH v2 1/5] char-socket: return -1 in case of disconnect during tcp_chr_write Dima Stepanov
2020-05-06 8:54 ` Li Feng
2020-05-06 9:46 ` Marc-André Lureau
2020-04-30 13:36 ` [PATCH v2 2/5] vhost: introduce wrappers to set guest notifiers for virtio device Dima Stepanov
2020-05-04 0:36 ` Raphael Norwitz
2020-05-06 8:54 ` Dima Stepanov
2020-05-11 3:03 ` Jason Wang
2020-05-11 8:55 ` Dima Stepanov
2020-04-30 13:36 ` [PATCH v2 3/5] vhost-user-blk: add mechanism to track the guest notifiers init state Dima Stepanov
2020-05-04 1:06 ` Raphael Norwitz
2020-05-06 8:51 ` Dima Stepanov
2020-04-30 13:36 ` [PATCH v2 4/5] vhost: check vring address before calling unmap Dima Stepanov
2020-05-04 1:13 ` Raphael Norwitz
2020-05-11 3:05 ` Jason Wang
2020-05-11 9:11 ` Dima Stepanov
2020-05-12 3:26 ` Jason Wang
2020-05-12 9:08 ` Dima Stepanov
2020-05-13 3:00 ` Jason Wang
2020-05-13 9:36 ` Dima Stepanov
2020-05-14 7:28 ` Jason Wang
2020-04-30 13:36 ` [PATCH v2 5/5] vhost: add device started check in migration set log Dima Stepanov
2020-05-06 22:08 ` Raphael Norwitz
2020-05-07 7:15 ` Michael S. Tsirkin
2020-05-07 15:35 ` Dima Stepanov
2020-05-11 0:03 ` Raphael Norwitz
2020-05-11 9:43 ` Dima Stepanov
2020-05-11 3:15 ` Jason Wang
2020-05-11 9:25 ` Dima Stepanov
2020-05-12 3:32 ` Jason Wang
2020-05-12 3:47 ` Li Feng
2020-05-12 9:23 ` Dima Stepanov
2020-05-12 9:35 ` Dima Stepanov [this message]
2020-05-13 3:20 ` Jason Wang
2020-05-13 9:39 ` Dima Stepanov
2020-05-13 4:15 ` Michael S. Tsirkin
2020-05-13 5:56 ` Jason Wang
2020-05-13 9:47 ` Dima Stepanov
2020-05-14 7:34 ` Jason Wang
2020-05-15 16:54 ` Dima Stepanov
2020-05-16 3:20 ` Li Feng
2020-05-18 2:52 ` Jason Wang
2020-05-18 9:33 ` Dima Stepanov
2020-05-18 9:27 ` Dima Stepanov
2020-05-18 2:50 ` Jason Wang
2020-05-18 9:41 ` Dima Stepanov
2020-05-18 9:53 ` Dr. David Alan Gilbert
2020-05-19 9:07 ` Dima Stepanov
2020-05-19 10:24 ` Dr. David Alan Gilbert
2020-05-19 9:59 ` Michael S. Tsirkin
2020-05-19 9:13 ` Dima Stepanov
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