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

[Paolo Bonzini <pbonzini@redhat.com>]

On 06/12/2017 15:45, 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
> 

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>