group.of.nepali.translators team mailing list archive

[Joseph Salisbury <joseph.salisbury@xxxxxxxxxxxxx>]

I built a test kernel with the patch you posted in the description.  The test kernel can be downloaded from:
http://kernel.ubuntu.com/~jsalisbury/lp1775235

Can you test this kernel and see if it resolves this bug?

Note about installing test kernels:
• If the test kernel is prior to 4.15(Bionic) you need to install the linux-image and linux-image-extra .deb packages.
• If the test kernel is 4.15(Bionic) or newer, you need to install the linux-modules, linux-modules-extra and linux-image-unsigned .deb packages.

Thanks in advance!

** Changed in: linux (Ubuntu Xenial)
       Status: Confirmed => In Progress

** Changed in: linux (Ubuntu)
       Status: Confirmed => In Progress

** Changed in: linux (Ubuntu Xenial)
     Assignee: (unassigned) => Joseph Salisbury (jsalisbury)

** Changed in: linux (Ubuntu)
       Status: In Progress => Invalid

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https://bugs.launchpad.net/bugs/1775235

Title:
  Ubuntu 16.04 (4.4.0-127) hangs on boot with virtio-scsi MQ enabled

Status in linux package in Ubuntu:
  Invalid
Status in linux source package in Xenial:
  In Progress

Bug description:
  We noticed that Ubuntu 16.04 guests running on Nutanix AHV stopped
  booting after they were upgraded to the latest kernel (4.4.0-127).
  Only guests with scsi mq enabled suffered from this problem. AHV is
  one of the few hypervisor products to offer multiqueue for virtio-scsi
  devices.

  Upon further investigation, we could see that the kernel would hang
  during the scanning of scsi targets. More specifically, immediately
  after coming across a target without any luns present. That's the
  first time the kernel destroys a target (given it doesn't have luns).
  This could be confirmed with gdb (attached to qemu's gdbserver):

  #0  0xffffffffc0045039 in ?? ()
  #1  0xffff88022c753c98 in ?? ()
  #2  0xffffffff815d1de6 in scsi_target_destroy (starget=0xffff88022ad62400)
      at /build/linux-E14mqW/linux-4.4.0/drivers/scsi/scsi_scan.c:322

  This shows the guest vCPU stuck on virtio-scsi's implementation of
  target_destroy. Despite lacking symbols, we managed to examine the
  virtio_scsi_target_state to see that the 'reqs' counter was invalid:

  (gdb) p *(struct virtio_scsi_target_state  *)starget->hostdata
  $6 = {tgt_seq = {sequence = 0}, reqs = {counter = -1}, req_vq = 0xffff88022cbdd9e8}
  (gdb)

  This drew our attention to the following patch which is exclusive to the Ubuntu kernel:
  commit f1f609d8015e1d34d39458924dcd9524fccd4307
  Author: Jay Vosburgh <jay.vosburgh@xxxxxxxxxxxxx>
  Date:   Thu Apr 19 21:40:00 2018 +0200

  In a nutshell, the patch spins on the target's 'reqs' counter waiting for the target to quiesce:
  --- a/drivers/scsi/virtio_scsi.c
  +++ b/drivers/scsi/virtio_scsi.c
  @@ -785,6 +785,10 @@ static int virtscsi_target_alloc(struct scsi_target *starget)
   static void virtscsi_target_destroy(struct scsi_target *starget)
   {
          struct virtio_scsi_target_state *tgt = starget->hostdata;
  +
  +       /* we can race with concurrent virtscsi_complete_cmd */
  +       while (atomic_read(&tgt->reqs))
  +               cpu_relax();
          kfree(tgt);
   }

  Personally, I think this is a catastrophic way of waiting for a target
  to quiesce since virtscsi_target_destroy() is called with IRQs
  disabled from scsi_scan.c:scsi_target_destroy(). Devices which take a
  long time to quiesce during a target_destroy() could hog the CPU for
  relatively long periods of time.

  Nevertheless, further study revealed that virtio-scsi itself is broken
  in a way that it doesn't increment the 'reqs' counter when submitting
  requests on MQ in certain conditions. That caused the counter to go to
  -1 (on the completion of the first request) and the CPU to hang
  indefinitely.

  The following patch fixes the issue:

  --- old/linux-4.4.0/drivers/scsi/virtio_scsi.c        2018-06-04 10:23:07.000000000 -0700
  +++ new/linux-4.4.0/drivers/scsi/virtio_scsi.c        2018-06-05 10:03:29.083428545 -0700
  @@ -641,9 +641,10 @@
                                  scsi_target(sc->device)->hostdata;
          struct virtio_scsi_vq *req_vq;

  -       if (shost_use_blk_mq(sh))
  +       if (shost_use_blk_mq(sh)) {
                  req_vq = virtscsi_pick_vq_mq(vscsi, sc);
  -       else
  +               atomic_inc(&tgt->reqs);
  +       } else
                  req_vq = virtscsi_pick_vq(vscsi, tgt);

          return virtscsi_queuecommand(vscsi, req_vq, sc);

  Signed-off-by: Felipe Franciosi <felipe@xxxxxxxxxxx>

  Please consider this a urgent fix as all of our customers which use
  Ubuntu 16.04 and have MQ enabled for better performance will be
  affected by your latest update. Our workaround is to recommend that
  they disable SCSI MQ while you work on the issue.

  Best regards,
  Felipe

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