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

[David Gibson]

On Thu, Oct 24, 2019 at 08:28:54AM +0200, Greg Kurz wrote:
> On Thu, 24 Oct 2019 09:38:17 +1100
> David Gibson <address@hidden> 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 <address@hidden>
> > 
> > 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.

I suspect no particularly good reason, just history.  Everything's a
bit  different with LMBs because a single device_del will usually
remove a whole bunch of LMB DRCs.  In general the interfacing between
the qemu user side DIMM handling and the PAPR side LMB/DRC handling
is... pretty clunky.

> 
> > > ---
> > >  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,
> > > 
> > 
> 



-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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