[Ocfs2-devel] [PATCH] ocfs2/dlm: remove a potential deadlock

[Wengang Wang]

/*
 *
 * spinlock lock ordering: if multiple locks are needed, obey this ordering:
 *    dlm_domain_lock
 *    struct dlm_ctxt->spinlock
 *    struct dlm_lock_resource->spinlock
 *    struct dlm_ctxt->master_lock
 *    struct dlm_ctxt->ast_lock
 *    dlm_master_list_entry->spinlock
 *    dlm_lock->spinlock
 *
 */

There is a violation that locks on dlm_lock_resource->spinlock after locking
on dlm_ctxt->ast_lock. The violation is not detected because so far there is
only one place where both the two spinlock are taken at the same time. But it
is a potential deadlock in future(when the two lock are taken all together in
correct order).

This patch fixes the problem by changing the calling order.

Signed-off-by: Wengang Wang <wen.gang.wang at oracle.com>
---
 fs/ocfs2/dlm/dlmthread.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/ocfs2/dlm/dlmthread.c b/fs/ocfs2/dlm/dlmthread.c
index 2211acf..676ff3e 100644
--- a/fs/ocfs2/dlm/dlmthread.c
+++ b/fs/ocfs2/dlm/dlmthread.c
@@ -671,8 +671,8 @@ static int dlm_thread(void *data)
 		 	/* lockres can be re-dirtied/re-added to the
 			 * dirty_list in this gap, but that is ok */
 
-			spin_lock(&dlm->ast_lock);
 			spin_lock(&res->spinlock);
+			spin_lock(&dlm->ast_lock);
 			if (res->owner != dlm->node_num) {
 				__dlm_print_one_lock_resource(res);
 				mlog(ML_ERROR, "inprog:%s, mig:%s, reco:%s, dirty:%s\n",
@@ -691,8 +691,8 @@ static int dlm_thread(void *data)
 					  DLM_LOCK_RES_RECOVERING)) {
 				/* move it to the tail and keep going */
 				res->state &= ~DLM_LOCK_RES_DIRTY;
-				spin_unlock(&res->spinlock);
 				spin_unlock(&dlm->ast_lock);
+				spin_unlock(&res->spinlock);
 				mlog(0, "delaying list shuffling for in-"
 				     "progress lockres %.*s, state=%d\n",
 				     res->lockname.len, res->lockname.name,
@@ -713,8 +713,8 @@ static int dlm_thread(void *data)
 			/* called while holding lockres lock */
 			dlm_shuffle_lists(dlm, res);
 			res->state &= ~DLM_LOCK_RES_DIRTY;
-			spin_unlock(&res->spinlock);
 			spin_unlock(&dlm->ast_lock);
+			spin_unlock(&res->spinlock);
 
 			dlm_lockres_calc_usage(dlm, res);
 
-- 
1.7.2.2