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 55cfaa3
  Author: Zeng Zhaoming <[email protected]>
  Date:   Thu Dec 2 14:31:13 2010 -0800

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

  commit 1e50df3
  Author: KOSAKI Motohiro <[email protected]>
  Date:   Thu Jan 13 15:46:14 2011 -0800

      mempolicy: remove tasklist_lock from migrate_pages

Signed-off-by: Greg Thelen <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Zeng Zhaoming <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: gthelen

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;