[PATCH v2] powerpc: Fix smp_send_stop NMI IPI handling

[Nicholas Piggin]

On Thu, 26 Apr 2018 20:30:37 +1000
Michael Ellerman <mpe at ellerman.id.au> wrote:

> Nicholas Piggin <npiggin at gmail.com> writes:
> > The NMI IPI handler for a receiving CPU increments nmi_ipi_busy_count
> > over the handler function call, which causes later smp_send_nmi_ipi()
> > callers to spin until the call is finished.
> >
> > The smp_send_stop function never returns, so the busy count is never
> > decremeted, which can cause the system to hang in some cases. For
> > example panic() will call smp_send_stop early on, then later in the
> > reboot path, pnv_restart will call smp_send_stop again, which hangs.
> >
> > Fix this by adding a special case to the smp_send_stop handler to
> > decrement the busy count, because it will never return.
> >
> > Fixes: 6bed3237624e3 ("powerpc: use NMI IPI for smp_send_stop")
> > Reported-by: Abdul Haleem <abdhalee at linux.vnet.ibm.com>
> > Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> > ---
> > Changes since v1:
> > - Reduce #ifdef spaghetti suggested by mpe  
> 
> Sorry I missed this.
> 
> But then saw it in the patchwork listing, which is why I sent my version
> as v3. Which I also merged. Sorry for the confusion.
> 
> I'm pretty sure my version is equivalent to this, except for the way the
> #ifdefs are arranged.
> 
> The end result of this is nicer than my version, so I'd take a cleanup
> patch to get us to this final state.

That's okay. We end up needing to fix the !NMI case as well, with
"powerpc: Fix deadlock with multiple calls to smp_send_stop", so I
can rediff that and send it up.

Thanks,
Nick