Public bug reported:
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 <[email protected]>
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:
--- new/linux-4.4.0/drivers/scsi/virtio_scsi.c 2018-06-05
10:03:29.083428545 -0700
+++ old/linux-4.4.0/drivers/scsi/virtio_scsi.c 2018-06-04
10:23:07.000000000 -0700
@@ -641,10 +641,9 @@
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);
- atomic_inc(&tgt->reqs);
- } else
+ else
req_vq = virtscsi_pick_vq(vscsi, tgt);
return virtscsi_queuecommand(vscsi, req_vq, sc);
Signed-off-by: Felipe Franciosi <[email protected]>
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
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
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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
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