Re: [Qemu-block] [PATCH v7 14/24] nbd: Switch from close to eject notifier

[Kevin Wolf]

Am 30.11.2015 um 18:22 hat Max Reitz geschrieben:
> On 30.11.2015 16:36, Kevin Wolf wrote:
> > Am 09.11.2015 um 23:39 hat Max Reitz geschrieben:
> >> The NBD code uses the BDS close notifier to determine when a medium is
> >> ejected. However, now it should use the BB's BDS removal notifier for
> >> that instead of the BDS's close notifier.
> >>
> >> Signed-off-by: Max Reitz <address@hidden>
> >> ---
> >>  blockdev-nbd.c | 37 +------------------------------------
> >>  nbd.c          | 13 +++++++++++++
> >>  2 files changed, 14 insertions(+), 36 deletions(-)
> >>
> >> diff --git a/blockdev-nbd.c b/blockdev-nbd.c
> >> index bcdd18b..b28a55b 100644
> >> --- a/blockdev-nbd.c
> >> +++ b/blockdev-nbd.c
> >> @@ -46,37 +46,11 @@ void qmp_nbd_server_start(SocketAddress *addr, Error 
> >> **errp)
> >>      }
> >>  }
> >>  
> >> -/*
> >> - * Hook into the BlockBackend notifiers to close the export when the
> >> - * backend is closed.
> >> - */
> >> -typedef struct NBDCloseNotifier {
> >> -    Notifier n;
> >> -    NBDExport *exp;
> >> -    QTAILQ_ENTRY(NBDCloseNotifier) next;
> >> -} NBDCloseNotifier;
> >> -
> >> -static QTAILQ_HEAD(, NBDCloseNotifier) close_notifiers =
> >> -    QTAILQ_HEAD_INITIALIZER(close_notifiers);
> >> -
> >> -static void nbd_close_notifier(Notifier *n, void *data)
> >> -{
> >> -    NBDCloseNotifier *cn = DO_UPCAST(NBDCloseNotifier, n, n);
> >> -
> >> -    notifier_remove(&cn->n);
> >> -    QTAILQ_REMOVE(&close_notifiers, cn, next);
> >> -
> >> -    nbd_export_close(cn->exp);
> >> -    nbd_export_put(cn->exp);
> > 
> > Here you remove a close/put pair, but in the new code you only add the
> > close call. Why is this correct?
> 
> Thanks for putting so much unfounded faith in me. :-)
> 
> After having put quite some time into looking into it, first I have to
> say that it's a very good question.
> 
> First off, it's wrong. This is because I thought every export would have
> a refcount of one. Turns out this is wrong, they have a refcount of two:
> It is created with a refcount of one, and then it gains another by
> giving it a name and entering it into the export list.
> 
> I did know about the second but I completely forgot about the former.
> And indeed I do think it is wrong. qmp_nbd_server_add() does not keep
> the reference to the export, once the function returns, it is gone.
> Therefore, it should call nbd_export_put() before returning.
> 
> So in my opinion the current code is wrong and I didn't fix it enough.
> *cough*
> 
> So, with the nbd_export_put() added to qmp_nbd_server_add(), every
> export will have a refcount of 1 + |clients|. The eject notifier will
> call nbd_close_notifier(), which first disconnects the clients, thus
> reducing the refcount by |clients|, and then sets the name to NULL, thus
> dropping the final refcount and destroying the export.
> 
> In the old code, we needed another nbd_export_put() because of the
> superfluous reference from nbd_export_new(), which doesn't actually
> exist for blockdev-nbd (it does for qemu-nbd, though).

Okay, that makes sense to me. So first a patch that moves the existing
nbd_export_put() call from nbd_close_notifier() to qmp_nbd_server_add()
and then this one on top?

Kevin

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