[Ocfs2-devel] May be deadlock for wrong locking order, patch request reviewed, thanks

Guozhonghua guozhonghua at h3c.com
Thu Sep 11 04:28:29 PDT 2014

As we test the ocfs2 cluster, the cluster is sometime hangs up.

I got some information about the dead lock, which cause the cluster hangs up, the sys dir / lock is held and the node did not release it which cause the cluster hangs up.
    root at cvknode-21:~# ps -e -o pid,stat,comm,wchan=WIDE-WCHAN-COLUMN | grep D
     7489 D jbd2/sdh-621 jbd2_journal_commit_transaction
    16218 D ls iterate_dir
    16533 D mkdir dlm_wait_for_lock_mastery
    31195 D+ ls iterate_dir

So the code reviewed, and I found the order of the lock may wrong.
In the function dlm_master_request_handler, the resource lock is held and so after the lock of &dlm->master_lock is locked.
But in the function dlm_get_lock_resource, the &dlm->master_lock is locked first and so resource lock.
They are different order in different function.
If there are two task, one holds the res->lock waiting for the dlm->master_lock, with the function dlm_master_request_handler.
Another task holds the &dlm->master_lock waiting for the res->lock with dlm_get_lock_resource.
So the deadlock may be up.

I changed some code, and the patch request reviews.

*** ocfs2-ko-3.16/dlm/dlmmaster.c      2014-09-11 12:45:45.821657634 +0800
--- ocfs2-ko-3.16_compared/dlm/dlmmaster.c      2014-09-11 18:54:34.970243238 +0800
*************** way_up_top:
*** 1506,1512 ****
--- 1506,1515 ----

              // mlog(0, "lockres is in progress...\n");
+             spin_unlock(&res->spinlock);
+             spin_lock(&res->spinlock);
              found = dlm_find_mle(dlm, &tmpmle, name, namelen);
              if (!found) {
                      mlog(ML_ERROR, "no mle found for this lock!\n");
*************** way_up_top:
*** 1551,1558 ****
                      set_bit(request->node_idx, tmpmle->maybe_map);

-              spin_unlock(&dlm->master_lock);
+             spin_unlock(&dlm->master_lock);

              /* keep the mle attached to heartbeat events */
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