mm: grab rcu read lock in move_pages()

The move_pages() usage of find_task_by_vpid() requires rcu_read_lock() to
prevent free_pid() from reclaiming the pid.

Without this patch, RCU warnings are printed in v2.6.38-rc4 move_pages()
with:

  CONFIG_LOCKUP_DETECTOR=y
  CONFIG_PREEMPT=y
  CONFIG_LOCKDEP=y
  CONFIG_PROVE_LOCKING=y
  CONFIG_PROVE_RCU=y

Previously, migrate_pages() went through a similar transformation
replacing usage of tasklist_lock with rcu read lock:

  commit 55cfaa3c
  Author: Zeng Zhaoming <zengzm.kernel@gmail.com>
  Date:   Thu Dec 2 14:31:13 2010 -0800

      mm/mempolicy.c: add rcu read lock to protect pid structure

  commit 1e50df39
  Author: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
  Date:   Thu Jan 13 15:46:14 2011 -0800

      mempolicy: remove tasklist_lock from migrate_pages
Signed-off-by: NGreg Thelen <gthelen@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Zeng Zhaoming <zengzm.kernel@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -1287,14 +1287,14 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_pages,
 		return -EPERM;
 
 	/* Find the mm_struct */
-	read_lock(&tasklist_lock);
+	rcu_read_lock();
 	task = pid ? find_task_by_vpid(pid) : current;
 	if (!task) {
-		read_unlock(&tasklist_lock);
+		rcu_read_unlock();
 		return -ESRCH;
 	}
 	mm = get_task_mm(task);
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 
 	if (!mm)
 		return -EINVAL;