acpi-cpufreq: fix 'smp_call_function_many()' confusion

It turns out that 'smp_call_function_many()' doesn't work at all like
'smp_call_function_single()', and my change to Andrew's patch to use it
rather than a loop over all CPU's acpi-cpufreq doesn't work.

My bad.

'smp_call_function_many()' has two "features" (aka "documented bugs"):

(a) it needs to be called with preemption disabled, because it uses
smp_processor_id() without guarding the CPU lookup with 'get_cpu()'
and 'put_cpu()' like the 'single' variant does.

(b) even if the current CPU is part of the CPU mask, it won't do the
call on that CPU.

Still, we're better off trying to use 'smp_call_function_many()' than
looping over CPU's, since it at least in theory allows us to use a
broadcast IPI and do it all in parallel. So let's just work around the
silly semantic bugs in that function.

Reported-and-tested-by: Ali Gholami Rudi <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Andrew Morton <[e-mail]>,
Cc: Rusty Russell <[e-mail]>
Cc: Dave Jones <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c

···
 
 static void drv_write(struct drv_cmd *cmd)
 {
+	int this_cpu;
+
+	this_cpu = get_cpu();
+	if (cpumask_test_cpu(this_cpu, cmd->mask))
+		do_drv_write(cmd);
 	smp_call_function_many(cmd->mask, do_drv_write, cmd, 1);
+	put_cpu();
 }
 
 static u32 get_cur_val(const struct cpumask *mask)