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 55cfaa3cbdd29c4919ecb5fb8965c310f357e48c
  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 1e50df39f6e2c3a4a3394df62baa8a213df16c54
  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: Greg 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 766115253807..352de555626c 100644
--- a/mm/migrate.c
+++ b/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;