workqueue: fix race condition in schedule_on_each_cpu()

Commit 65a64464 ("HWPOISON: Allow
schedule_on_each_cpu() from keventd") which allows schedule_on_each_cpu()
to be called from keventd added a race condition.  schedule_on_each_cpu()
may race with cpu hotplug and end up executing the function twice on a
cpu.

Fix it by moving direct execution into the section protected with
get/put_online_cpus().  While at it, update code such that direct
execution is done after works have been scheduled for all other cpus and
drop unnecessary cpu != orig test from flush loop.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/workqueue.c

@@ -692,31 +692,29 @@ int schedule_on_each_cpu(work_func_t func)
 	if (!works)
 		return -ENOMEM;
 
+	get_online_cpus();
+
 	/*
-	 * when running in keventd don't schedule a work item on itself.
-	 * Can just call directly because the work queue is already bound.
-	 * This also is faster.
-	 * Make this a generic parameter for other workqueues?
+	 * When running in keventd don't schedule a work item on
+	 * itself.  Can just call directly because the work queue is
+	 * already bound.  This also is faster.
 	 */
-	if (current_is_keventd()) {
+	if (current_is_keventd())
 		orig = raw_smp_processor_id();
-		INIT_WORK(per_cpu_ptr(works, orig), func);
-		func(per_cpu_ptr(works, orig));
-	}
 
-	get_online_cpus();
 	for_each_online_cpu(cpu) {
 		struct work_struct *work = per_cpu_ptr(works, cpu);
 
-		if (cpu == orig)
-			continue;
 		INIT_WORK(work, func);
+		if (cpu != orig)
 			schedule_work_on(cpu, work);
 	}
-	for_each_online_cpu(cpu) {
-		if (cpu != orig)
+	if (orig >= 0)
+		func(per_cpu_ptr(works, orig));
+
+	for_each_online_cpu(cpu)
 		flush_work(per_cpu_ptr(works, cpu));
-	}
+
 	put_online_cpus();
 	free_percpu(works);
 	return 0;