cpu/hotplug: Read_lock(tasklist_lock) doesn't need to disable irqs

check_for_tasks() doesn't need to disable irqs, recursive
read_lock() from interrupt is fine.

While at it, s/do_each_thread/for_each_process_thread/.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Kirill Tkhai <ktkhai@odin.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Kirill Tkhai <ktkhai@parallels.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150910130750.GA20055@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/cpu.c

@@ -304,8 +304,8 @@ static inline void check_for_tasks(int dead_cpu)
 {
 	struct task_struct *g, *p;
 
-	read_lock_irq(&tasklist_lock);
-	do_each_thread(g, p) {
+	read_lock(&tasklist_lock);
+	for_each_process_thread(g, p) {
 		if (!p->on_rq)
 			continue;
 		/*
@@ -320,8 +320,8 @@ static inline void check_for_tasks(int dead_cpu)
 
 		pr_warn("Task %s (pid=%d) is on cpu %d (state=%ld, flags=%x)\n",
 			p->comm, task_pid_nr(p), dead_cpu, p->state, p->flags);
-	} while_each_thread(g, p);
-	read_unlock_irq(&tasklist_lock);
+	}
+	read_unlock(&tasklist_lock);
 }
 
 struct take_cpu_down_param {