[3.13.y-ckt stable] Patch "ocfs2: dlm: fix race between purge and get lock resource" has been added to staging queue

[Kamal Mostafa]

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

    ocfs2: dlm: fix race between purge and get lock resource

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt22.

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

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 1d718ecf377e3f9e1dcd0f4f90f3dc0f231fe091 Mon Sep 17 00:00:00 2001
From: Junxiao Bi <junxiao.bi at oracle.com>
Date: Tue, 5 May 2015 16:24:02 -0700
Subject: ocfs2: dlm: fix race between purge and get lock resource

commit b1432a2a35565f538586774a03bf277c27fc267d upstream.

There is a race window in dlm_get_lock_resource(), which may return a
lock resource which has been purged.  This will cause the process to
hang forever in dlmlock() as the ast msg can't be handled due to its
lock resource not existing.

    dlm_get_lock_resource {
        ...
        spin_lock(&dlm->spinlock);
        tmpres = __dlm_lookup_lockres_full(dlm, lockid, namelen, hash);
        if (tmpres) {
             spin_unlock(&dlm->spinlock);
             >>>>>>>> race window, dlm_run_purge_list() may run and purge
                              the lock resource
             spin_lock(&tmpres->spinlock);
             ...
             spin_unlock(&tmpres->spinlock);
        }
    }

Signed-off-by: Junxiao Bi <junxiao.bi at oracle.com>
Cc: Joseph Qi <joseph.qi at huawei.com>
Cc: Mark Fasheh <mfasheh at suse.com>
Cc: Joel Becker <jlbec at evilplan.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/ocfs2/dlm/dlmmaster.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/fs/ocfs2/dlm/dlmmaster.c b/fs/ocfs2/dlm/dlmmaster.c
index 1be3398..1dd0bcc 100644
--- a/fs/ocfs2/dlm/dlmmaster.c
+++ b/fs/ocfs2/dlm/dlmmaster.c
@@ -726,6 +726,19 @@ lookup:
 	if (tmpres) {
 		spin_unlock(&dlm->spinlock);
 		spin_lock(&tmpres->spinlock);
+
+		/*
+		 * Right after dlm spinlock was released, dlm_thread could have
+		 * purged the lockres. Check if lockres got unhashed. If so
+		 * start over.
+		 */
+		if (hlist_unhashed(&tmpres->hash_node)) {
+			spin_unlock(&tmpres->spinlock);
+			dlm_lockres_put(tmpres);
+			tmpres = NULL;
+			goto lookup;
+		}
+
 		/* Wait on the thread that is mastering the resource */
 		if (tmpres->owner == DLM_LOCK_RES_OWNER_UNKNOWN) {
 			__dlm_wait_on_lockres(tmpres);
--
1.9.1