[Qemu-devel] [PATCH v2 0/9] blockdev: fix QMP 'transaction' with IOThreads

[Stefan Hajnoczi <stefanha@redhat.com>]

v2:
 * Use StrOrNull for x-blockdev-set-iothread iothread argument [eblake]

(This is for QEMU 2.12 because this bug is not -rc4 critical)

Previously AioContext was held across QMP 'transaction' in an attempt to
achieve bdrv_drained_begin/end() semantics.  Nowadays we have
bdrv_drained_begin/end() and the AioContext lock just protects state.
Therefore there is no reason to hold AioContext across
.prepare/.commit/.abort/.clean() anymore.

Besides being cleanup-worthy, holding AioContext also exposes a bug:
BDRV_POLL_WHILE() doesn't support recursive AioContext locking and will hang if
depth > 1.  This is easy to trigger by submitting a transaction with 2 actions
that touch two drives assigned to an IOThread.  The IOThread's AioContext will
be locked twice and BDRV_POLL_WHILE() will hang.

BDRV_POLL_WHILE() is best fixed by eliminating the AioContext lock entirely in
favor of fine-grained locking.  I discussed some fixes for BDRV_POLL_WHILE()
with Paolo but we came to the conclusion that it will just add complexity when
we really want to stop using AioContext locking.

Summary:

 * Patch 1 fixes missing AioContext lock protection

 * Patches 2-6 clean up excessive AioContext locked regions in QMP
   'transaction' to solve the hang

 * Patches 7-9 add a qemu-iotests test case and the necessary infrastructure

Stefan Hajnoczi (9):
  blockdev: hold AioContext for bdrv_unref() in
    external_snapshot_clean()
  block: don't keep AioContext acquired after
    external_snapshot_prepare()
  block: don't keep AioContext acquired after drive_backup_prepare()
  block: don't keep AioContext acquired after blockdev_backup_prepare()
  block: don't keep AioContext acquired after
    internal_snapshot_prepare()
  block: drop unused BlockDirtyBitmapState->aio_context field
  iothread: add iothread_by_id() API
  blockdev: add x-blockdev-set-iothread testing command
  qemu-iotests: add 202 external snapshots IOThread test

 qapi/block-core.json       |  36 +++++++
 include/sysemu/iothread.h  |   1 +
 blockdev.c                 | 258 +++++++++++++++++++++++++++++++++------------
 iothread.c                 |   7 ++
 tests/qemu-iotests/202     |  95 +++++++++++++++++
 tests/qemu-iotests/202.out |  11 ++
 tests/qemu-iotests/group   |   1 +
 7 files changed, 339 insertions(+), 70 deletions(-)
 create mode 100755 tests/qemu-iotests/202
 create mode 100644 tests/qemu-iotests/202.out

-- 
2.14.3