RE: [Xen-devel] xs transaction

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Keir Fraser [mailto:Keir.Fraser@xxxxxxxxxxxx]
>Sent: 2007年9月11日 15:32
>
>We do support concurrent transactions at the API level, but the
>implementation will currently allow only one concurrent transaction that
>modifies state to succeed. This is a detail of the implementation however
>(from a semantic point of view -- it may impact performance of course!).
>
>Nested transactions (transactions within transactions) are not supported,
>and attempts to create a nested transaction will be explicitly failed.
>
> -- Keir
>

Sorry that I may miss some important logic here. However from what I 
read, do_transaction_start doesn't prevent nest case:

void do_transaction_start(struct connection *conn, struct buffered_data
*in)
{
        ...
        /* We don't support nested transactions. */
        if (conn->transaction) {
                send_error(conn, EBUSY);
                return;
        }

Conn->transaction is only set in life cycle of each message process:
static void process_message(struct connection *conn, struct buffered_data *in)
{
        trans = transaction_lookup(conn, in->hdr.msg.tx_id);
        ...
        conn->transaction = trans;
        switch (in->hdr.msg.type) {
        ...
        conn->transaction = NULL;
}

So given following nest case, I'm not sure how it's prevented:
        Xs_transaction_start(xs_handle);
        <- at this point, conn->transaction is NULL
        Xs_transaction_start(xs_handle); 

Thanks,
Kevin

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