On Thu, 2018-04-26 at 10:19:02 UTC, Michael Ellerman wrote:

From: Nicholas Piggin <npiggin@gmail.com>

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 stop_this_cpu() 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 which calls
stop_this_cpu() on other CPUs, 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 stop_this_cpu() handler to
decrement the busy count, because it will never return.

Now that the NMI/non-NMI versions of stop_this_cpu() are different,
split them out into separate functions rather than doing #ifdef tricks
to share the body between the two functions.

Fixes: 6bed3237624e3 ("powerpc: use NMI IPI for smp_send_stop")
Reported-by: Abdul Haleem <redacted>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Split out the functions, tweak change log a bit]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Applied to powerpc fixes.

https://git.kernel.org/powerpc/c/ac61c1156623455c46701654abd8c9

cheers