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SubjectRe: [BUG?] [Ext4] INFO: suspicious rcu_dereference_check() usage
Date
> On Mon, Nov 22, 2010 at 12:39:49AM +0900, Minchan Kim wrote:
> >
> > I think it's no problem.
> >
> > That's because migration always holds lock_page on the file page.
> > So the page couldn't remove from radix.
>
> It may be "ok" in that it won't cause a race, but it still leaves an
> unsightly warning if LOCKDEP is enabled, and LOCKDEP warnings will
> cause /proc_lock_stat to be disabled. So I think it still needs to be
> fixed by adding rcu_read_lock()/rcu_read_unlock() to
> migrate_page_move_mapping().

Hi Ted,

Current mmotm has following patch and I think it should be fixed your
issue.

Thanks.




From: Zeng Zhaoming <zengzm.kernel@gmail.com>

find_task_by_vpid() should be protected by rcu_read_lock(), to prevent
free_pid() reclaiming pid.

Signed-off-by: Zeng Zhaoming <zengzm.kernel@gmail.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/mempolicy.c | 3 +++
1 file changed, 3 insertions(+)

diff -puN mm/mempolicy.c~mm-mempolicyc-add-rcu-read-lock-to-protect-pid-structure mm/mempolicy.c
--- a/mm/mempolicy.c~mm-mempolicyc-add-rcu-read-lock-to-protect-pid-structure
+++ a/mm/mempolicy.c
@@ -1307,15 +1307,18 @@ SYSCALL_DEFINE4(migrate_pages, pid_t, pi
goto out;

/* Find the mm_struct */
+ rcu_read_lock();
read_lock(&tasklist_lock);
task = pid ? find_task_by_vpid(pid) : current;
if (!task) {
read_unlock(&tasklist_lock);
+ rcu_read_unlock();
err = -ESRCH;
goto out;
}
mm = get_task_mm(task);
read_unlock(&tasklist_lock);
+ rcu_read_unlock();



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