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

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH v7 14/24] nbd: Switch from close to eject notifier
Date: Mon, 30 Nov 2015 18:22:51 +0100
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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.

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).


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