Re: [PATCH v2 5/5] vhost: add device started check in migration set log

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, May 12, 2020 at 12:35:30PM +0300, Dima Stepanov wrote:
> 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.



So vhost net had this problem too.

commit e7c83a885f865128ae3cf1946f8cb538b63cbfba
Author: Marc-André Lureau <marcandre.lureau@redhat.com>
Date:   Mon Feb 27 14:49:56 2017 +0400

    vhost-user: delay vhost_user_stop
    
    Since commit b0a335e351103bf92f3f9d0bd5759311be8156ac, a socket write
    may trigger a disconnect events, calling vhost_user_stop() and clearing
    all the vhost_dev strutures holding data that vhost.c functions expect
    to remain valid. Delay the cleanup to keep the vhost_dev structure
    valid during the vhost.c functions.
    
    Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Message-id: 20170227104956.24729-1-marcandre.lureau@redhat.com
    Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

it now has this code to address this:


    case CHR_EVENT_CLOSED:
        /* a close event may happen during a read/write, but vhost
         * code assumes the vhost_dev remains setup, so delay the
         * stop & clear to idle.
         * FIXME: better handle failure in vhost code, remove bh
         */
        if (s->watch) {
            AioContext *ctx = qemu_get_current_aio_context();

            g_source_remove(s->watch);
            s->watch = 0;
            qemu_chr_fe_set_handlers(&s->chr, NULL, NULL, NULL, NULL,
                                     NULL, NULL, false);

            aio_bh_schedule_oneshot(ctx, chr_closed_bh, opaque);
        }
        break;

I think it's time we dropped the FIXME and moved the handling to common
code. Jason? Marc-André?





> 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
> > >>
> >