Re: [PATCH 2/5] spapr.c: check unplug_request flag in spapr_memory_unplug_request()

[Daniel Henrique Barboza]

On 3/1/21 11:03 PM, David Gibson wrote:
> On Fri, Feb 26, 2021 at 01:32:58PM -0300, Daniel Henrique Barboza wrote:
> > Now that we're asserting the first DRC LMB earlier, use it to query if
> > the DRC is already pending unplug and, in this case, issue the same
> > error we already do.
> >
> > The previous check was introduced in commit 2a129767ebb1 and it works,
> > but it's easier to check the unplug_requested  flag instead of looking
> > for the existence of the sPAPRDIMMState. It's also compliant with what
> > is already done in other unplug_request functions for other devices.
> >
> > Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
>
> I'm having some trouble completely convincing myself this is right.
>
> What about this situation:
>       1. We initiate a DIMM unplug
>         - unplug_request is set on all the LMBs
>         - all the LMBs go on the pending_unplug list
>       2. The guest encounters no problems, and starts issuing set
>          indicator calls to mark the LMBs unusable, starting from the
>         lowest address
>       3. On drc_set_unusable() for the first LMB, we see that unplug is
>          requested and call spapr_drc_release()
>       4. spapr_drc_release() on the first LMB clears unplug_requested
>       5. At this point, but before this is done on *all* of the DIMM's
>          LMBs, the user attempts another unplug triggering the code
>         below
>
> AFAICT this will now skip the error, since the first LMB is no longer
> in unplug_requested state, but there *are* still pending unplugs for
> some of the remaining LMBs, so the old code would have tripped the
> error.

Good point. Checking the existence of the sPAPRDIMMState struct at this
point is the same as checking for drc->unplug_requested for all the DRCs
of the DIMM.

I could check for drc->unplug_requested inside the loop where we instantiate
each DRC, but there is no gain in doing that instead of what we already have.

I'll drop this patch and change patch 1 to just remove the duplicated assert.


Thanks,


DHB

> > ---
> >   hw/ppc/spapr.c | 8 +-------
> >   1 file changed, 1 insertion(+), 7 deletions(-)
> >
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index 74e046b522..149dc2113f 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -3681,13 +3681,7 @@ static void spapr_memory_unplug_request(HotplugHandler 
> > *hotplug_dev,
> >                                   addr_start / SPAPR_MEMORY_BLOCK_SIZE);
> >       g_assert(drc_start);
> >   
> > -    /*
> > -     * 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)) {
> > +    if (spapr_drc_unplug_requested(drc_start)) {
> >           error_setg(errp, "Memory unplug already in progress for device %s",
> >                      dev->id);
> >           return;