[PATCH v2] powerpc: Fix smp_send_stop NMI IPI handling

[Michael Ellerman]

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.

cheers