Re: [Qemu-devel] [PATCH 3/7] nbd: use BDS refcount

[Paolo Bonzini <pbonzini@redhat.com>]

Il 03/07/2013 08:30, Fam Zheng ha scritto:
>> > The close notifier runs when the user invokes a drive_del or eject
>> > command from the monitor.  The drive_get_ref/drive_put_ref delays the
>> > bdrv_delete until after nbd.c has cleaned up all the connections.
> But drive_put_ref is called by close notifier.

Not necessarily.  nbd_export_close calls nbd_client_close, which shuts
down the socket.  However, if requests are being processed, they will
complete after nbd_export_close returns.  Completing the requests leads
to the following call chain:

   nbd_request_put (from nbd_trip)
   calls nbd_client_put
   calls nbd_export_put
   calls exp->close (if refcount goes to 0)
   calls drive_put_ref

Completion will happen as soon as the main loop runs again, because
after shutdown() the reads and writes will fail.  Still, it is
asynchronous, hence the call to drive_put_ref is also asynchronous.

> I think it can be
> omitted, registering a close notifier is enough, and close the export
> when drive_del calls it. It doesn't make more sense w/ drive_get_ref,
> does it?

I think that would cause a dangling pointer if NBD requests are being
processed at the time drive_del runs.

Paolo