Re: [Qemu-block] [PATCH v5 11/12] qed: Implement .bdrv_drain

[Fam Zheng]

On Wed, 10/21 22:20, Jeff Cody wrote:
> On Wed, Oct 21, 2015 at 10:06:48AM +0800, Fam Zheng wrote:
> > The "need_check_timer" is used to clear the "NEED_CHECK" flag in the
> > image header after a grace period once metadata update has finished. In
> > compliance to the bdrv_drain semantics we should make sure it remains
> > deleted once .bdrv_drain is called.
> > 
> > Call the qed_need_check_timer_cb manually to update the header
> > immediately.
> > 
> > Signed-off-by: Fam Zheng <address@hidden>
> > ---
> >  block/qed.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/block/qed.c b/block/qed.c
> > index 5ea05d4..e9dcb4d 100644
> > --- a/block/qed.c
> > +++ b/block/qed.c
> > @@ -375,6 +375,12 @@ static void 
> > bdrv_qed_attach_aio_context(BlockDriverState *bs,
> >      }
> >  }
> >  
> > +static void bdrv_qed_drain(BlockDriverState *bs)
> > +{
> > +    qed_cancel_need_check_timer(bs->opaque);
> > +    qed_need_check_timer_cb(bs->opaque);
> > +}
> > +
> 
> Uh oh.
> 
> This causes a segfault sometimes, and other times an abort:
> 
> 
>    # ./qemu-img create -f qed test.qed 512M
>    Formatting 'test.qed', fmt=qed size=536870912 cluster_size=65536
> 
>    # ./qemu-io -c "read 0 512M" test.qed
>    read 536870912/536870912 bytes at offset 0
>    512 MiB, 1 ops; 0.0556 sec (8.988 GiB/sec and 17.9759 ops/sec)
>    Segmentation fault (core dumped)
> 
> 
> If I run the above qemu-io command with gdb, it will abort in
> qed_plug_allocating_write_reqs().
> 
> I'd hazard a guess (I have not verified) that it is due to the
> qed_header_write() call triggered by the aio flush callback function
> qed_clear_need_check().  The aio flush is done inside the
> qed_need_check_timer_cb() call.

Good catch, I think it's because of the second bdrv_drain in bdrv_close(),
when the first bdrv_aio_flush in qed_need_check_timer_cb hasn't finished.

We need a different bdrv_qed_drain implementation here.

Fam

> 
> 
> 
> >  static int bdrv_qed_open(BlockDriverState *bs, QDict *options, int flags,
> >                           Error **errp)
> >  {
> > @@ -1676,6 +1682,7 @@ static BlockDriver bdrv_qed = {
> >      .bdrv_check               = bdrv_qed_check,
> >      .bdrv_detach_aio_context  = bdrv_qed_detach_aio_context,
> >      .bdrv_attach_aio_context  = bdrv_qed_attach_aio_context,
> > +    .bdrv_drain               = bdrv_qed_drain,
> >  };
> >  
> >  static void bdrv_qed_init(void)
> > -- 
> > 2.4.3
> > 
> >