Nicholas Piggin [off-list ref] 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 <redacted>
Signed-off-by: Nicholas Piggin <npiggin@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