Re: [Qemu-devel] [PATCH for-2.9-rc5 v3] block: Drain BH in bdrv_drained_begin

[Fam Zheng <famz@redhat.com>]

On Tue, 04/18 14:36, Paolo Bonzini wrote:
> 
> 
> On 18/04/2017 12:39, Fam Zheng wrote:
> > +    QLIST_FOREACH_SAFE(child, &bs->children, next, tmp) {
> > +        BlockDriverState *bs = child->bs;
> > +        assert(bs->refcnt > 0);
> > +        bdrv_ref(bs);
> > +        waited |= bdrv_drain_recurse(bs);
> > +        bdrv_unref(bs);
> >      }
> 
> I think this accesses global state that is not protected by the
> AioContext lock?

Good catch! If called from IOThread, this bdrv_unref is simply wrong, although
in practice it cannot delete bs because of the reference held by the owning
device.

It may be better to wrap the bdrv_ref/bdrv_unref calls with

     if (qemu_get_current_aio_context() == qemu_get_aio_context())

because only the main loop needs it.

Will make this hunk a separate patch in v4.

Fam