linux-kernel - Re: [BUG?] [Ext4] INFO: suspicious rcu_dereference

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Date:	Tue, 23 Nov 2010 16:16:56 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	"Ted Ts'o" <tytso@....edu>, Minchan Kim <minchan.kim@...il.com>,
	linux-ext4@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Eric Sandeen <sandeen@...hat.com>
Cc:	kosaki.motohiro@...fujitsu.com
Subject: Re: [BUG?] [Ext4] INFO: suspicious rcu_dereference_check() usage

> 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@...il.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@...il.com>
Cc: "Paul E. McKenney" <paulmck@...ibm.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc: Christoph Lameter <cl@...ux-foundation.org>
Signed-off-by: Andrew Morton <akpm@...ux-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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