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Re: [Qemu-devel] drive_del vs. device_del: what should come first?
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From: |
Heinz Graalfs |
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Subject: |
Re: [Qemu-devel] drive_del vs. device_del: what should come first? |
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Date: |
Fri, 11 Apr 2014 14:47:20 +0200 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 |
Hello Markus,
I finally managed to reproduce the problem, at least once ...
The scenario was:
dd if=/dev/vdx1 of=partitionone
followed by a virsh detach... (with the device_del() under the cover)
during active dd processing
dmesg shows:
[79026.220718] User process fault: interruption code 0x40009 in
qemu-system-s390x[80000000+43c000]
[79026.220737] CPU: 3 PID: 20117 Comm: qemu-system-s39 Not tainted
3.10.33+ #4
[79026.220742] task: 0000000085930000 ti: 00000000e0b74000 task.ti:
00000000e0b74000
[79026.220746] User PSW : 0705000180000000 0000000080269722 (0x80269722)
[79026.220774] R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:1 AS:0 CC:0
PM:0 EA:3
User GPRS: 0000000000000000 0000000000000000 0000000000000000
0000000000000000
[79026.220827] 0000000000000000 0000000000000000
00000000803761dc 00000000809001d0
[79026.220831] 000003fffd0391a8 00000000808ff560
00000000808eb020 000003ffffc03838
[79026.220834] 0000000000000000 0000000080375e88
00000000802696f6 000003ffffc03838
[79026.220847] User Code: 0000000080269716: b9040034 lgr %r3,%r4
000000008026971a: a7290000 lghi %r2,0
#000000008026971e: b9870021 dlgr %r2,%r1
>0000000080269722: b9040012 lgr %r1,%r2
0000000080269726: 5010b0a0 st %r1,160(%r11)
000000008026972a: 5820b0a0 l %r2,160(%r11)
000000008026972e: e310b0a80004 lg %r1,168(%r11)
0000000080269734: 58101000 l %r1,0(%r1)
[79026.220875] Last Breaking-Event-Address:
[79026.220878] [<000000008026904c>] 0x8026904c
with
PSW addr: 0000000080269722 pointing into virtio_queue_empty
...
000000008026a67c t virtio_notify_vector
00000000802694a4 T virtio_queue_empty
000000008026b440 T virtio_queue_get_addr
...
and coming from:
R14 00000000802696f6 virtqueue_fill
...
0000000080269f74 T virtqueue_avail_bytes
0000000080269540 T virtqueue_fill
000000008026979c T virtqueue_flush
0000000080269c2c T virtqueue_get_avail_bytes
...
see below the complete core_backtrace ...
On 02/04/14 19:40, Markus Armbruster wrote:
Heinz Graalfs <address@hidden> writes:
On 01/04/14 17:48, Markus Armbruster wrote:
Heinz Graalfs <address@hidden> writes:
Hi Kevin,
doing a
virsh detach-device ...
ends up in the following QEMU monitor commands:
1. device_del ...
2. drive_del ...
qmp_device_del() performs the device unplug path.
In case of a block device do_drive_del() tries to
prevent further IO against the host device.
However, bdrv_find() during drive_del() results in
an error, because the device is already gone. Due to
this error all the bdrv_xxx calls to quiesce the block
driver as well as all other processing is skipped.
Is the sequence that libvirt triggers OK?
Shouldn't drive_del be executed first?
No.
OK, I see. The drive is deleted implicitly (release_drive()).
Doing a device_del() requires another drive_add() AND device_add().
Yes. The automatic drive delete on unplug is a wart that will not be
preserved in the new interfaces we're building now.
(Doing just a device_add() complains about the missing drive.
A subsequent info qtree lets QEMU abort.)
Really? Got a reproducer for me?
drive_del is nasty. Its purpose is to revoke access to an image even
when the guest refuses to cooperate. To the guest, this looks like
hardware failure.
Deleting a device during active IO is nasty and it should look like a
hardware failure. I would expect lots of errors.
If you drive_del before device_del, even a perfectly well-behaved guest
guest is exposed to a terminally broken device between drive_del and
completion of unplug.
The early drive_del() would mean that no further IO would be
possible.
A polite way to describe the effect of drive_del on the guest.
drive_del makes all attempts at actual I/O error out without any
forewarning whatsoever. If you do that to your guest, and something
breaks, you get to keep the pieces. Even if you "only" do it for a
short period of time.
Always try a device_del first, and only if that does not complete within
reasonable time, and you absolutely must revoke access to the image,
then whack it over the head with drive_del.
What is this reasonable time?
Depends on how heavily loaded host and guest are. Suggest to measure
with a suitably thrashing guest, then shift left to taste.
On 390 we experience problems (QEMU abort) when asynch block IO
completes and the virtqueues are already gone. I suppose the
bdrv_drain_all() in bdrv_close() is a little late. I don't see such
problems with an early bdrv_drain_all() (drive_del) and an unplug
(device_del) afterwards.
Sounds like a bug. Got a reproducer?
here is the complete core_backtrace:
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x269722 virtqueue_fill
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x269924 virtqueue_push
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x23a400
virtio_blk_req_complete /test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x23a6a4 virtio_blk_rw_complete
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x34686 bdrv_co_em_bh
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0xcd40 aio_bh_poll
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0xc7f0 aio_poll
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x2ac9c bdrv_drain_all
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x2a718 bdrv_close
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x2b97e bdrv_delete
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x36db4 bdrv_unref
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0xa02ea drive_uninit
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0xa0448 drive_put_ref
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x9fae6 blockdev_auto_del
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0xd0bf0 release_drive
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x18c158
object_property_del_all /test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x18c4e4 object_finalize
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x18d6d2 object_unref
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x18c382 object_unparent
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x25ca14
virtio_ccw_busdev_unplug /test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0xd6c88 qdev_unplug
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x15c5f0 qmp_device_del
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x182b92
qmp_marshal_input_device_del
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x28fa36 qmp_call_cmd
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x28fd2c handle_qmp_command
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x2fff3c
json_message_process_token
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x32adfe json_lexer_feed_char
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x32b08c json_lexer_feed
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x3000ee
json_message_parser_feed /test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x28fe4c monitor_control_read
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x15cd66 qemu_chr_be_write
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x162bfe tcp_chr_read
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
b8d9d8c328fccab2bc69bfa6d17bcb2f46dfb163 0x4ff58 g_main_context_dispatch
/lib64/libglib-2.0.so.0 -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x10b654 glib_pollfds_poll
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x10b7e0 os_host_main_loop_wait
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x10b934 main_loop_wait
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x1e6a7e main_loop
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
7d40b30ece2f256dfd8600446a9da7b0a1962730 0x1ef642 main
/test/qemu/build/s390x-softmmu/qemu-system-s390x -
I suppose that asynchronous block IO processing completes after the
guest already did the virtqueue cleanup.
Both, machine check injection and device release processing are
triggered in the unplug callback virtio_ccw_busdev_unplug().
I suppose the device release processing (with its wait for asynchronous
processing to complete) happens too late.
What do you think?
Thanks,
Heinz