Re: [Xen-devel] [PATCH] libxl: trigger attach events for devices attached before xl devd startup

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jul 11, 2016 at 10:56:04AM +0200, Marek Marczykowski-Górecki wrote:
> On Mon, Jul 11, 2016 at 10:31:17AM +0200, Roger Pau Monné wrote:
> > On Sun, Jul 10, 2016 at 07:35:47PM +0200, Marek Marczykowski-Górecki wrote:
> > > When this daemon is started after creating backend device, that device
> > > will not be configured.
> > > 
> > > Racy situation:
> > > 1. driver domain is started
> > > 2. frontend domain is started (just after kicking driver domain off)
> > > 3. device in frontend domain is connected to the backend (as specified
> > >    in frontend domain configuration)
> > > 4. xl devd is started in driver domain
> > > 
> > > End result is that backend device in driver domain is not configured
> > > (like network interface is not enabled), so the device doesn't work.
> > > 
> > > Fix this by artifically triggering events for devices already present in
> > > xenstore before xl devd is started. Do this only after xenstore watch is
> > > already registered, and only for devices not already initialized (in
> > > XenbusStateInitWait state).
> > 
> > Thanks!
> > 
> > > Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> > > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > > Signed-off-by: Marek Marczykowski-Górecki 
> > > <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > > ---
> > >  tools/libxl/libxl.c | 40 ++++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 40 insertions(+)
> > > 
> > > diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
> > > index 1c81239..99815a7 100644
> > > --- a/tools/libxl/libxl.c
> > > +++ b/tools/libxl/libxl.c
> > > @@ -4743,8 +4743,16 @@ int libxl_device_events_handler(libxl_ctx *ctx,
> > >      uint32_t domid;
> > >      libxl__ddomain ddomain;
> > >      char *be_path;
> > > +    char **kinds = NULL, **domains = NULL, **devs = NULL;
> > > +    const char *sstate;
> > > +    char *state_path;
> > > +    int state;
> > > +    unsigned int nkinds, ndomains, ndevs;
> > > +    int i, j, k;
> > > +    xs_transaction_t t;
> > >  
> > >      ddomain.ao = ao;
> > > +    FILLZERO(ddomain.watch);
> > 
> > Is this a different bugfix or stray change?
> 
> To cleanly unregister watch and not do nothing if wasn't registered at
> all. If it isn't initialized, libxl__ev_xswatch_deregister call on
> not registered watch isn't harmless.

Right, I've realized that before your changes the function only registered 
the watch and exited, this is needed now.
 
> > >      LIBXL_SLIST_INIT(&ddomain.guests);
> > >  
> > >      rc = libxl__get_domid(gc, &domid);
> > > @@ -4762,9 +4770,41 @@ int libxl_device_events_handler(libxl_ctx *ctx,
> > >                                      be_path);
> > >      if (rc) goto out;
> > >  
> > > +    rc = libxl__xs_transaction_start(gc, &t);
> > > +    if (rc) goto out;
> > 
> > Why do you need to start a transaction here if you end up aborting it when 
> > finished?
> 
> Mostly to ease error checking. Because below code does three level
> listing, I don't want to deal with races where some entry was removed
> between those calls, at least not here. Like this:
> 
> xs_directory('backend/vif') -> 3, 4, 5
> xs_directory('backend/vif/3') -> 0, 1
> xs_read('backend/vif/3/0/state') -> ...
> xs_read('backend/vif/3/1/state') -> ...
> toolstack removes backend/vif/4 here
> xs_directory('backend/vif/4') ->  fail
> 
> Of course backend_watch_callback would fail anyway in such a case, which
> is ok. But having snapshot of xenstore during this multi-level listing
> looks like avoiding some corner cases during listing itself.

AFAICT your code seems to be prepared to deal with entries disappearing in 
the middle of the tree walk, so I would just remove the transaction.

> > > +    kinds = libxl__xs_directory(gc, t, be_path, &nkinds);
> > > +    if (kinds) {
> > > +        for (i = 0; i < nkinds; i++) {
> > > +            domains = libxl__xs_directory(gc, t,
> > > +                    GCSPRINTF("%s/%s", be_path, kinds[i]), &ndomains);
> > > +            if (!domains)
> > > +                continue;
> > > +            for (j = 0; j < ndomains; j++) {
> > > +                devs = libxl__xs_directory(gc, t,
> > > +                        GCSPRINTF("%s/%s/%s", be_path, kinds[i], 
> > > domains[j]), &ndevs);
> > > +                if (!devs)
> > > +                    continue;
> > > +                for (k = 0; k < ndevs; k++) {
> > > +                    state_path = GCSPRINTF("%s/%s/%s/%s/state",
> > > +                            be_path, kinds[i], domains[j], devs[k]);
> > > +                    rc = libxl__xs_read_checked(gc, t, state_path, 
> > > &sstate);
> > > +                    if (rc)
> > > +                        continue;
> > > +                    state = atoi(sstate);
> > > +                    if (state == XenbusStateInitWait)
> > > +                        backend_watch_callback(egc, &ddomain.watch,
> > > +                                be_path, state_path);
> > > +                }
> > > +            }
> > > +        }
> > > +    }
> > > +
> > > +    libxl__xs_transaction_abort(gc, &t);
> > > +
> > >      return AO_INPROGRESS;
> > >  
> > >  out:
> > > +    libxl__ev_xswatch_deregister(gc, &ddomain.watch);
> > 
> > This seems to be part of a different bugfix also.
> 
> No, this code previously wasn't reachable if xswatch was correctly
> registered.

Right, thanks.

Roger.

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