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Date: Thu, 22 Oct 2015 07:05:51 -0400
From: Jeff Cody <jcody@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v6 09/12] block: Add "drained begin/end"
 for internal snapshot
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To: Kevin Wolf <kwolf@redhat.com>
Cc: stefanha@redhat.com, pbonzini@redhat.com, Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org, qemu-block@nongnu.org

On Thu, Oct 22, 2015 at 12:18:11PM +0200, Kevin Wolf wrote:
> Am 22.10.2015 um 08:32 hat Fam Zheng geschrieben:
> > 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 | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/blockdev.c b/blockdev.c
> > index 52f44b2..adc0e69 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,
> > @@ -1414,6 +1415,9 @@ static void internal_snapshot_prepare(BlkTransactionState *common,
> >      }
> >      bs = blk_bs(blk);
> >  
> > +    state->bs = bs;
> > +    bdrv_drained_begin(bs);
> > +
> >      if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_INTERNAL_SNAPSHOT, errp)) {
> >          return;
> >      }
> 
> More context:
> 
>     /* AioContext is released in .clean() */
>     state->aio_context = blk_get_aio_context(blk);
>     aio_context_acquire(state->aio_context);
> 
>     if (!blk_is_available(blk)) {
>         error_setg(errp, QERR_DEVICE_HAS_NO_MEDIUM, device);
>         return;
>     }
>     bs = blk_bs(blk);
> 
>     state->bs = bs;
>     bdrv_drained_begin(bs);
> 
> If we error out because of !blk_is_available(blk), we will still call
> bdrv_drained_end() in .clean even though bdrv_drained_begin() hasn't
> been called yet.
>

Right, if the bdrv_drained_begin() is moved down as it is in this
patch, then the state->aio_context assignment needs moved down as
well.