MIPS: SMP: Drop stop_this_cpu() cpu_foreign_map hack

Commit cccf34e9 ("MIPS: c-r4k: Fix cache flushing for MT cores")
added the cpu_foreign_map cpumask containing a single VPE from each
online core, and recalculated it when secondary CPUs are brought up.

stop_this_cpu() was also updated to recalculate cpu_foreign_map, but
with an additional hack before marking the CPU as offline to copy
cpu_online_mask into cpu_foreign_map and perform an SMP memory barrier.

This appears to have been intended to prevent cache management IPIs
being missed when the VPE representing the core in cpu_foreign_map is
taken offline while other VPEs remain online. Unfortunately there is
nothing in this hack to prevent r4k_on_each_cpu() from reading the old
cpu_foreign_map, and smp_call_function_many() from reading that new
cpu_online_mask with the core's representative VPE marked offline. It
then wouldn't send an IPI to any online VPEs of that core.

stop_this_cpu() is only actually called in panic and system shutdown /
halt / reboot situations, in which case all CPUs are going down and we
don't really need to care about cache management, so drop this hack.

Note that the __cpu_disable() case for CPU hotplug is handled in the
previous commit, and no synchronisation is needed there due to the use
of stop_machine() which prevents hotplug from taking place while any CPU
has disabled preemption (as r4k_on_each_cpu() does).
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Leonid Yegoshin <leonid.yegoshin@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13796/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/smp.c

@@ -344,16 +344,9 @@ asmlinkage void start_secondary(void)
 static void stop_this_cpu(void *dummy)
 {
 	/*
-	 * Remove this CPU. Be a bit slow here and
-	 * set the bits for every online CPU so we don't miss
-	 * any IPI whilst taking this VPE down.
+	 * Remove this CPU:
 	 */
 
-	cpumask_copy(&cpu_foreign_map, cpu_online_mask);
-
-	/* Make it visible to every other CPU */
-	smp_mb();
-
 	set_cpu_online(smp_processor_id(), false);
 	calculate_cpu_foreign_map();
 	local_irq_disable();