init: Pin init task to the boot CPU, initially

Some of the boot code in init_kernel_freeable() which runs before SMP
bringup assumes (rightfully) that it runs on the boot CPU and therefore can
use smp_processor_id() in preemptible context.

That works so far because the smp_processor_id() check starts to be
effective after smp bringup. That's just wrong. Starting with SMP bringup
and the ability to move threads around, smp_processor_id() in preemptible
context is broken.

Aside of that it does not make sense to allow init to run on all CPUs
before sched_smp_init() has been run.

Pin the init to the boot CPU so the existing code can continue to use
smp_processor_id() without triggering the checks when the enabling of those
checks starts earlier.

Tested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20170516184734.943149935@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

init/main.c

@@ -389,6 +389,7 @@ static __initdata DECLARE_COMPLETION(kthreadd_done);
 
 static noinline void __ref rest_init(void)
 {
+	struct task_struct *tsk;
 	int pid;
 
 	rcu_scheduler_starting();
@@ -397,7 +398,17 @@ static noinline void __ref rest_init(void)
 	 * the init task will end up wanting to create kthreads, which, if
 	 * we schedule it before we create kthreadd, will OOPS.
 	 */
-	kernel_thread(kernel_init, NULL, CLONE_FS);
+	pid = kernel_thread(kernel_init, NULL, CLONE_FS);
+	/*
+	 * Pin init on the boot CPU. Task migration is not properly working
+	 * until sched_init_smp() has been run. It will set the allowed
+	 * CPUs for init to the non isolated CPUs.
+	 */
+	rcu_read_lock();
+	tsk = find_task_by_pid_ns(pid, &init_pid_ns);
+	set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id()));
+	rcu_read_unlock();
+
 	numa_default_policy();
 	pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
 	rcu_read_lock();
@@ -1015,10 +1026,6 @@ static noinline void __init kernel_init_freeable(void)
 	 * init can allocate pages on any node
 	 */
 	set_mems_allowed(node_states[N_MEMORY]);
-	/*
-	 * init can run on any cpu.
-	 */
-	set_cpus_allowed_ptr(current, cpu_all_mask);
 
 	cad_pid = task_pid(current);