[PATCH v3 5/5] powerpc/smp: add NULL guard for cause_ipi in smp_muxed_ipi_message_pass

smp_muxed_ipi_message_pass() calls smp_ops->cause_ipi() without
checking whether it has been set.

On platforms using muxed IPI (e.g. powernv/pseries), smp_ops->cause_ipi
is initialized to NULL in the static smp_ops and only assigned during
the platform smp_probe() handler.  If the IPI subsystem fails to
initialize -- for example when xive_init_ipis() fails and
xive_smp_probe() returns an error -- the probe handler returns early
and cause_ipi is never set.  Any subsequent IPI send (e.g.
arch_smp_send_reschedule()) would dereference the NULL pointer.

Add a NULL check to avoid the crash in that situation.

Fixes: 23d72bfd8f9f ("powerpc: Consolidate ipi message mux and demux")
Signed-off-by: Gou Hao <redacted>
Reviewed-by: jiazhenyuan <redacted>
---
 arch/powerpc/kernel/smp.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 3467f86fd78f..6a5a5469aaae 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c

@@ -289,6 +289,9 @@ void smp_muxed_ipi_set_message(int cpu, int msg)
 
 void smp_muxed_ipi_message_pass(int cpu, int msg)
 {
+	if (!smp_ops->cause_ipi)
+		return;
+
 	smp_muxed_ipi_set_message(cpu, msg);
 
 	/*

-- 
2.20.1