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[Xen-tools] Re: [PATCH 1/3] Recover transaction on restart, give transac

To: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Subject: [Xen-tools] Re: [PATCH 1/3] Recover transaction on restart, give transactions IDs
From: Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>
Date: Tue, 27 Sep 2005 05:58:15 +0100
Cc: Xen Tools <xen-tools@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
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On Tue, Sep 27, 2005 at 02:37:10PM +1000, Rusty Russell wrote:
> > I don't agree with this approach since it relies on the fact that
> > we won't suspend/resume within a transaction.
> If we suspend/resume over transactions, this mechanism will not work by
> itself, it will still work for local tools over xenstored restart.


> But transactions are a privileged operations (either in-kernel
> or /proc/xen/xenbus), and are short, so I don't see it as being an
> issue.  If it becomes an issue, I'd much rather do transaction migration
> than present a horrible API to xenstore users for such a corner case.

No, please don't go there, transaction migration is a horrible idea!

> As predicted, the change to have xs_transaction_end return EAGAIN made
> the kernel code uglier and caused bugs (thanks Keir for the fixes so
> far).  Returning EAGAIN from arbitrary operations is going to make it
> worse.  I'm convinced we don't want to go there.

That's because you can write all this code much simpler if you
do something like:
  err = xs_xxx(yyy);
  if (XS_CHECK_ERR(err, again))
    real error path

#define XS_CHECK_ERR(err, label) ({     \
        if ((err) == -EAGAIN)           \
                goto label;             \
        (err);                          \

That keeps all the code just as simple as it was before we had
to handle EAGAIN everywhere.


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