Re: [PATCH] spapr: Don't request to unplug the same core twice

[Greg Kurz <groug@kaod.org>]

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On Thu, 24 Oct 2019 09:38:17 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Wed, Oct 23, 2019 at 09:17:40PM +0200, Greg Kurz wrote:
> > We must not call spapr_drc_detach() on a detached DRC otherwise bad things
> > can happen, ie. QEMU hangs or crashes. This is easily demonstrated with
> > a CPU hotplug/unplug loop using QMP.
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> 
> Ouch, good catch.  Applied.
> 
> I wonder if we have the same problem with other DRC types.
> 

We don't have it with PHB and PCI types, through the same use of
spapr_drc_unplug_requested().

LMBs see to avoid it by failing device_del early if an unplug
request is already in progress:

    /*
     * An existing pending dimm state for this DIMM means that there is an
     * unplug operation in progress, waiting for the spapr_lmb_release
     * callback to complete the job (BQL can't cover that far). In this case,
     * bail out to avoid detaching DRCs that were already released.
     */
    if (spapr_pending_dimm_unplugs_find(spapr, dimm)) {
        error_setg(&local_err,
                   "Memory unplug already in progress for device %s",
                   dev->id);
        goto out;
    }

Not sure why we error out in this case instead of ignoring the unplug 
request.

> > ---
> >  hw/ppc/spapr.c |    7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index f9410d390a07..94f9d27096af 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -3741,9 +3741,10 @@ void spapr_core_unplug_request(HotplugHandler *hotplug_dev, DeviceState *dev,
> >                            spapr_vcpu_id(spapr, cc->core_id));
> >      g_assert(drc);
> >  
> > -    spapr_drc_detach(drc);
> > -
> > -    spapr_hotplug_req_remove_by_index(drc);
> > +    if (!spapr_drc_unplug_requested(drc)) {
> > +        spapr_drc_detach(drc);
> > +        spapr_hotplug_req_remove_by_index(drc);
> > +    }
> >  }
> >  
> >  int spapr_core_dt_populate(SpaprDrc *drc, SpaprMachineState *spapr,
> > 
> 


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