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

[Juan Quintela <quintela@redhat.com>]

"Denis V. Lunev" <den@openvz.org> 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 <den@openvz.org>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Paolo Bonzini <pbonzini@redhat.com>
> CC: Juan Quintela <quintela@redhat.com>
> CC: Amit Shah <amit.shah@redhat.com>
> ---
>  block/snapshot.c   | 5 +++++
>  migration/savevm.c | 7 +++++++
>  2 files changed, 12 insertions(+)



Reviewed-by: Juan Quintela <quintela@redhat.com>

But once there, I can't understand why migration have to know about
aio_contexts at all.

I *think* that it would be a good idea to "hide" the
adi_context_acquire(aio_context) inside qemu_fopen_bdrv() (yes, it is
still in migration/*, but you get the idea).  But don't propose it,
because we don't have qemu_fclose_bdrv().  Yes we could add an
aio_context inside QemuFile, and release it on qemu_fclose(), but I
guess this needs more thought yet.

BTW, once that I got your attention, why is this needed on hmp_savevm()
but it is not needed on load_vmstate()?  We are also using
qemu_fopen_bdrv()?  Because we are only reading from there?  Just curios
the reason or if we are missing something there.

Thanks, Juan.


>
> 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);
> +
>      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();
>      }