Re: [BUG?] [Ext4] INFO: suspicious rcu_dereference_check() usage

[KOSAKI Motohiro]

> 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@xxxxxxxxx>

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

Signed-off-by: Zeng Zhaoming <zengzm.kernel@xxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 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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