[3.5.y.z extended stable] Patch "quota: Fix race between dqput() and dquot_scan_active()" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    quota: Fix race between dqput() and dquot_scan_active()

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From b3b3dd11544e58466557b383eb33df8b8687a7d1 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack at suse.cz>
Date: Thu, 20 Feb 2014 17:02:27 +0100
Subject: quota: Fix race between dqput() and dquot_scan_active()

commit 1362f4ea20fa63688ba6026e586d9746ff13a846 upstream.

Currently last dqput() can race with dquot_scan_active() causing it to
call callback for an already deactivated dquot. The race is as follows:

CPU1					CPU2
  dqput()
    spin_lock(&dq_list_lock);
    if (atomic_read(&dquot->dq_count) > 1) {
     - not taken
    if (test_bit(DQ_ACTIVE_B, &dquot->dq_flags)) {
      spin_unlock(&dq_list_lock);
      ->release_dquot(dquot);
        if (atomic_read(&dquot->dq_count) > 1)
         - not taken
					  dquot_scan_active()
					    spin_lock(&dq_list_lock);
					    if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags))
					     - not taken
					    atomic_inc(&dquot->dq_count);
					    spin_unlock(&dq_list_lock);
        - proceeds to release dquot
					    ret = fn(dquot, priv);
					     - called for inactive dquot

Fix the problem by making sure possible ->release_dquot() is finished by
the time we call the callback and new calls to it will notice reference
dquot_scan_active() has taken and bail out.

Signed-off-by: Jan Kara <jack at suse.cz>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/quota/dquot.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
index 10cbe84..000831b 100644
--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -580,9 +580,17 @@ int dquot_scan_active(struct super_block *sb,
 		dqstats_inc(DQST_LOOKUPS);
 		dqput(old_dquot);
 		old_dquot = dquot;
-		ret = fn(dquot, priv);
-		if (ret < 0)
-			goto out;
+		/*
+		 * ->release_dquot() can be racing with us. Our reference
+		 * protects us from new calls to it so just wait for any
+		 * outstanding call and recheck the DQ_ACTIVE_B after that.
+		 */
+		wait_on_dquot(dquot);
+		if (test_bit(DQ_ACTIVE_B, &dquot->dq_flags)) {
+			ret = fn(dquot, priv);
+			if (ret < 0)
+				goto out;
+		}
 		spin_lock(&dq_list_lock);
 		/* We are safe to continue now because our dquot could not
 		 * be moved out of the inuse list while we hold the reference */
--
1.9.0