Re: [Qemu-devel] [PATCH 4/5] migration: add missed aio_context_acquire into hmp_savevm/hmp_delvm

[Paolo Bonzini]

On 27/10/2015 15:09, Denis V. Lunev wrote:
> aio_context should be locked in the similar way as was done in QMP
> snapshot creation in the other case there are a lot of possible
> troubles if native AIO mode is enabled for disk.
> 
> - the command can hang (HMP thread) with missed wakeup (the operation is
>   actually complete)
>     io_submit
>     ioq_submit
>     laio_submit
>     raw_aio_submit
>     raw_aio_readv
>     bdrv_co_io_em
>     bdrv_co_readv_em
>     bdrv_aligned_preadv
>     bdrv_co_do_preadv
>     bdrv_co_do_readv
>     bdrv_co_readv
>     qcow2_co_readv
>     bdrv_aligned_preadv
>     bdrv_co_do_pwritev
>     bdrv_rw_co_entry
> 
> - QEMU can assert in coroutine re-enter
>     __GI_abort
>     qemu_coroutine_enter
>     bdrv_co_io_em_complete
>     qemu_laio_process_completion
>     qemu_laio_completion_bh
>     aio_bh_poll
>     aio_dispatch
>     aio_poll
>     iothread_run
> 
> AioContext lock is reqursive. Thus nested locking should not be a problem.
> 
> Signed-off-by: Denis V. Lunev <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> CC: Paolo Bonzini <address@hidden>
> CC: Juan Quintela <address@hidden>
> CC: Amit Shah <address@hidden>
> ---
>  block/snapshot.c   | 5 +++++
>  migration/savevm.c | 7 +++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/block/snapshot.c b/block/snapshot.c
> index 89500f2..f6fa17a 100644
> --- a/block/snapshot.c
> +++ b/block/snapshot.c
> @@ -259,6 +259,9 @@ void bdrv_snapshot_delete_by_id_or_name(BlockDriverState 
> *bs,
>  {
>      int ret;
>      Error *local_err = NULL;
> +    AioContext *aio_context = bdrv_get_aio_context(bs);
> +
> +    aio_context_acquire(aio_context);
>  
>      ret = bdrv_snapshot_delete(bs, id_or_name, NULL, &local_err);
>      if (ret == -ENOENT || ret == -EINVAL) {
> @@ -267,6 +270,8 @@ void bdrv_snapshot_delete_by_id_or_name(BlockDriverState 
> *bs,
>          ret = bdrv_snapshot_delete(bs, NULL, id_or_name, &local_err);
>      }
>  
> +    aio_context_release(aio_context);

Why here and not in hmp_delvm, for consistency?

The call from hmp_savevm is already protected.

Thanks for fixing the bug!

Paolo

>      if (ret < 0) {
>          error_propagate(errp, local_err);
>      }
> diff --git a/migration/savevm.c b/migration/savevm.c
> index dbcc39a..83d2efa 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1289,6 +1289,7 @@ void hmp_savevm(Monitor *mon, const QDict *qdict)
>      struct tm tm;
>      const char *name = qdict_get_try_str(qdict, "name");
>      Error *local_err = NULL;
> +    AioContext *aio_context;
>  
>      /* Verify if there is a device that doesn't support snapshots and is 
> writable */
>      bs = NULL;
> @@ -1320,6 +1321,9 @@ void hmp_savevm(Monitor *mon, const QDict *qdict)
>      }
>      vm_stop(RUN_STATE_SAVE_VM);
>  
> +    aio_context = bdrv_get_aio_context(bs);
> +    aio_context_acquire(aio_context);
> +
>      memset(sn, 0, sizeof(*sn));
>  
>      /* fill auxiliary fields */
> @@ -1378,6 +1382,8 @@ void hmp_savevm(Monitor *mon, const QDict *qdict)
>      }
>  
>   the_end:
> +    aio_context_release(aio_context);
> +
>      if (saved_vm_running) {
>          vm_start();
>      }
> 
>