On Wed, Dec 06, 2017 at 02:45:41PM +0000, Stefan Hajnoczi wrote:
> 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
>
Thanks, applied to my block-next tree:
https://github.com/stefanha/qemu/commits/block-next
Stefan