ocfs2/dlm: Migrate lockres with no locks if it has a reference

o2dlm was not migrating resources with zero locks because it assumed that that
resource would get purged by dlm_thread. However, some usage patterns involve
creating and dropping locks at a high rate leading to the migrate thread seeing
zero locks but the purge thread seeing an active reference. When this happens,
the dlm_thread cannot purge the resource and the migrate thread sees no reason
to migrate that resource. The spell is broken when the migrate thread catches
the resource with a lock.

The fix is to make the migrate thread also consider the reference map.

This usage pattern can be triggered by userspace on userdlm locks and flocks.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

1 files changed, 27 insertions, 13 deletions

diff --git a/fs/ocfs2/dlm/dlmmaster.c b/fs/ocfs2/dlm/dlmmaster.c
index f564b0e5f80d..59f0f6bdfc62 100644
--- a/fs/ocfs2/dlm/dlmmaster.c
+++ b/fs/ocfs2/dlm/dlmmaster.c
@@ -2346,7 +2346,8 @@ static void dlm_deref_lockres_worker(struct dlm_work_item *item, void *data)
  */
 static int dlm_is_lockres_migrateable(struct dlm_ctxt *dlm,
                                       struct dlm_lock_resource *res,
-                                      int *numlocks)
+                                      int *numlocks,
+                                      int *hasrefs)
 {
         int ret;
         int i;
@@ -2356,6 +2357,9 @@ static int dlm_is_lockres_migrateable(struct dlm_ctxt *dlm,
 
         assert_spin_locked(&res->spinlock);
 
+        *numlocks = 0;
+        *hasrefs = 0;
+
         ret = -EINVAL;
         if (res->owner == DLM_LOCK_RES_OWNER_UNKNOWN) {
                 mlog(0, "cannot migrate lockres with unknown owner!\n");
@@ -2386,7 +2390,13 @@ static int dlm_is_lockres_migrateable(struct dlm_ctxt *dlm,
         }
 
         *numlocks = count;
-        mlog(0, "migrateable lockres having %d locks\n", *numlocks);
+
+        count = find_next_bit(res->refmap, O2NM_MAX_NODES, 0);
+        if (count < O2NM_MAX_NODES)
+                *hasrefs = 1;
+
+        mlog(0, "%s: res %.*s, Migrateable, locks %d, refs %d\n", dlm->name,
+             res->lockname.len, res->lockname.name, *numlocks, *hasrefs);
 
 leave:
         return ret;
@@ -2408,7 +2418,7 @@ static int dlm_migrate_lockres(struct dlm_ctxt *dlm,
         const char *name;
         unsigned int namelen;
         int mle_added = 0;
-        int numlocks;
+        int numlocks, hasrefs;
         int wake = 0;
 
         if (!dlm_grab(dlm))
@@ -2417,13 +2427,13 @@ static int dlm_migrate_lockres(struct dlm_ctxt *dlm,
         name = res->lockname.name;
         namelen = res->lockname.len;
 
-        mlog(0, "migrating %.*s to %u\n", namelen, name, target);
+        mlog(0, "%s: Migrating %.*s to %u\n", dlm->name, namelen, name, target);
 
         /*
          * ensure this lockres is a proper candidate for migration
          */
         spin_lock(&res->spinlock);
-        ret = dlm_is_lockres_migrateable(dlm, res, &numlocks);
+        ret = dlm_is_lockres_migrateable(dlm, res, &numlocks, &hasrefs);
         if (ret < 0) {
                 spin_unlock(&res->spinlock);
                 goto leave;
@@ -2431,10 +2441,8 @@ static int dlm_migrate_lockres(struct dlm_ctxt *dlm,
         spin_unlock(&res->spinlock);
 
         /* no work to do */
-        if (numlocks == 0) {
-                mlog(0, "no locks were found on this lockres! done!\n");
+        if (numlocks == 0 && !hasrefs)
                 goto leave;
-        }
 
         /*
          * preallocate up front
@@ -2459,14 +2467,14 @@ static int dlm_migrate_lockres(struct dlm_ctxt *dlm,
          * find a node to migrate the lockres to
          */
 
-        mlog(0, "picking a migration node\n");
         spin_lock(&dlm->spinlock);
         /* pick a new node */
         if (!test_bit(target, dlm->domain_map) ||
             target >= O2NM_MAX_NODES) {
                 target = dlm_pick_migration_target(dlm, res);
         }
-        mlog(0, "node %u chosen for migration\n", target);
+        mlog(0, "%s: res %.*s, Node %u chosen for migration\n", dlm->name,
+             namelen, name, target);
 
         if (target >= O2NM_MAX_NODES ||
             !test_bit(target, dlm->domain_map)) {
@@ -2667,7 +2675,7 @@ int dlm_empty_lockres(struct dlm_ctxt *dlm, struct dlm_lock_resource *res)
 {
         int ret;
         int lock_dropped = 0;
-        int numlocks;
+        int numlocks, hasrefs;
 
         spin_lock(&res->spinlock);
         if (res->owner != dlm->node_num) {
@@ -2681,8 +2689,8 @@ int dlm_empty_lockres(struct dlm_ctxt *dlm, struct dlm_lock_resource *res)
         }
 
         /* No need to migrate a lockres having no locks */
-        ret = dlm_is_lockres_migrateable(dlm, res, &numlocks);
-        if (ret >= 0 && numlocks == 0) {
+        ret = dlm_is_lockres_migrateable(dlm, res, &numlocks, &hasrefs);
+        if (ret >= 0 && numlocks == 0 && !hasrefs) {
                 spin_unlock(&res->spinlock);
                 goto leave;
         }
@@ -2915,6 +2923,12 @@ static u8 dlm_pick_migration_target(struct dlm_ctxt *dlm,
                 }
                 queue++;
         }
+
+        nodenum = find_next_bit(res->refmap, O2NM_MAX_NODES, 0);
+        if (nodenum < O2NM_MAX_NODES) {
+                spin_unlock(&res->spinlock);
+                return nodenum;
+        }
         spin_unlock(&res->spinlock);
         mlog(0, "have not found a suitable target yet! checking domain map\n");