Re: [PATCH] cxenstored: wait until after reset to notify dom0less domains

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 19 Oct 2023, Jan Beulich wrote:
> On 18.10.2023 23:31, Stefano Stabellini wrote:
> > On Wed, 17 Oct 2023, Jan Beulich wrote:
> >>
> >> On 14.10.2023 01:06, Stefano Stabellini wrote:
> >>> From: George Dunlap <george.dunlap@xxxxxxxxx>
> >>>
> >>> Commit fc2b57c9a ("xenstored: send an evtchn notification on
> >>> introduce_domain") introduced the sending of an event channel to the
> >>> guest when first introduced, so that dom0less domains waiting for the
> >>> connection would know that xenstore was ready to use.
> >>>
> >>> Unfortunately, it was introduced in introduce_domain(), which 1) is
> >>> called by other functions, where such functionality is unneeded, and
> >>> 2) after the main XS_INTRODUCE call, calls domain_conn_reset().  This
> >>> introduces a race condition, whereby if xenstored is delayed, a domain
> >>> can wake up, send messages to the buffer, only to have them deleted by
> >>> xenstore before finishing its processing of the XS_INTRODUCE message.
> >>>
> >>> Move the connect-and-notfy call into do_introduce() instead, after the
> >>> domain_conn_rest(); predicated on the state being in the
> >>> XENSTORE_RECONNECT state.
> >>>
> >>> (We don't need to check for "restoring", since that value is always
> >>> passed as "false" from do_domain_introduce()).
> >>>
> >>> Also take the opportunity to add a missing wmb barrier after resetting
> >>> the indexes of the ring in domain_conn_reset.
> >>>
> >>> This change will also remove an extra event channel notification for
> >>> dom0 (because the notification is now done by do_introduce which is not
> >>> called for dom0.) The extra dom0 event channel notification was only
> >>> introduced by fc2b57c9a and was never present before. It is not needed
> >>> because dom0 is the one to tell xenstored the connection parameters, so
> >>> dom0 has to know that the ring page is setup correctly by the time
> >>> xenstored starts looking at it. It is dom0 that performs the ring page
> >>> init.
> >>>
> >>> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxx>
> >>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
> >>
> >> Should this have had a Fixes: tag thus marking it to pick up for
> >> backport?
> > 
> > Sorry this was committed already
> 
> That's why I used "have had". I still need an answer to the question
> though; your reply only hints towards "yes".

Yes, I would add the Fixes tag