Re: [Qemu-devel] [Qemu-block] [PATCH v5 09/12] block: Add "drained begin/end" for internal snapshot

[Jeff Cody <jcody@redhat.com>]

On Wed, Oct 21, 2015 at 10:06:46AM +0800, Fam Zheng wrote:
> This ensures the atomicity of the transaction by avoiding processing of
> external requests such as those from ioeventfd.
> 
> state->bs is assigned right after bdrv_drained_begin. Because it was
> used as the flag for deletion or not in abort, now we need a separate
> flag - InternalSnapshotState.created.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  blockdev.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/blockdev.c b/blockdev.c
> index 52f44b2..92c2d0d 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1370,6 +1370,7 @@ typedef struct InternalSnapshotState {
>      BlockDriverState *bs;
>      AioContext *aio_context;
>      QEMUSnapshotInfo sn;
> +    bool created;
>  } InternalSnapshotState;
>  
>  static void internal_snapshot_prepare(BlkTransactionState *common,
> @@ -1407,6 +1408,8 @@ static void internal_snapshot_prepare(BlkTransactionState *common,
>      /* AioContext is released in .clean() */
>      state->aio_context = blk_get_aio_context(blk);
>      aio_context_acquire(state->aio_context);
> +    state->bs = blk_bs(blk);
> +    bdrv_drained_begin(state->bs);
> 

Do you think it'd make sense to move the context acquisition & call to
bdrv_drained_begin() below the check for blk_is_available()?  That way
the assignment of state->bs doesn't happen until after the call to
blk_is_available().  It looks like state->created protects us either
way, though - but if blk_is_inserted() fails, then there is no need
for _abort or _clean to do anything, if we hold off on grabbing the
context.

I think I am fine with it as is though, too.

>      if (!blk_is_available(blk)) {
>          error_setg(errp, QERR_DEVICE_HAS_NO_MEDIUM, device);
> @@ -1465,7 +1468,7 @@ static void internal_snapshot_prepare(BlkTransactionState *common,
>      }
>  
>      /* 4. succeed, mark a snapshot is created */
> -    state->bs = bs;
> +    state->created = true;
>  }
>  
>  static void internal_snapshot_abort(BlkTransactionState *common)
> @@ -1476,7 +1479,7 @@ static void internal_snapshot_abort(BlkTransactionState *common)
>      QEMUSnapshotInfo *sn = &state->sn;
>      Error *local_error = NULL;
>  
> -    if (!bs) {
> +    if (!state->created) {
>          return;
>      }
>  
> @@ -1497,6 +1500,7 @@ static void internal_snapshot_clean(BlkTransactionState *common)
>                                               common, common);
>  
>      if (state->aio_context) {
> +        bdrv_drained_end(state->bs);
>          aio_context_release(state->aio_context);
>      }
>  }
> -- 
> 2.4.3
> 
>