dlm: move dlm_search_rsb_tree() out of lock

The rhashtable structure is lockless for readers such as
rhashtable_lookup_fast(). It should be save to call this lookup
functionality out of holding ls_rsbtbl_lock to get the rsb pointer out
of the hash. This reduce the contention time of ls_rsbtbl_lock in some
cases. We still need to check if the rsb is part of the check as this
state can be changed while ls_rsbtbl_lock is not held. If its part of
the rhashtable data structure we take a reference to be sure it will not
be freed after we drop the ls_rsbtbl_lock read lock.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>

Author: Alexander Aring

fs/dlm/lock.c

@@ -733,11 +733,13 @@ static int find_rsb_dir(struct dlm_ls *ls, const void *name, int len,
 	}
 
  retry:
+	error = dlm_search_rsb_tree(&ls->ls_rsbtbl, name, len, &r);
+	if (error)
+		goto do_new;
 
 	/* check if the rsb is active under read lock - likely path */
 	read_lock_bh(&ls->ls_rsbtbl_lock);
-	error = dlm_search_rsb_tree(&ls->ls_rsbtbl, name, len, &r);
-	if (error) {
+	if (!rsb_flag(r, RSB_HASHED)) {
 		read_unlock_bh(&ls->ls_rsbtbl_lock);
 		goto do_new;
 	}
@@ -918,11 +920,13 @@ static int find_rsb_nodir(struct dlm_ls *ls, const void *name, int len,
 	int error;
 
  retry:
+	error = dlm_search_rsb_tree(&ls->ls_rsbtbl, name, len, &r);
+	if (error)
+		goto do_new;
 
 	/* check if the rsb is in active state under read lock - likely path */
 	read_lock_bh(&ls->ls_rsbtbl_lock);
-	error = dlm_search_rsb_tree(&ls->ls_rsbtbl, name, len, &r);
-	if (error) {
+	if (!rsb_flag(r, RSB_HASHED)) {
 		read_unlock_bh(&ls->ls_rsbtbl_lock);
 		goto do_new;
 	}
@@ -1276,36 +1280,38 @@ static int _dlm_master_lookup(struct dlm_ls *ls, int from_nodeid, const char *na
 	}
 
  retry:
+	error = dlm_search_rsb_tree(&ls->ls_rsbtbl, name, len, &r);
+	if (error)
+		goto not_found;
 
 	/* check if the rsb is active under read lock - likely path */
 	read_lock_bh(&ls->ls_rsbtbl_lock);
-	error = dlm_search_rsb_tree(&ls->ls_rsbtbl, name, len, &r);
-	if (!error) {
-		if (rsb_flag(r, RSB_INACTIVE)) {
-			read_unlock_bh(&ls->ls_rsbtbl_lock);
-			goto do_inactive;
-		}
-
-		/* because the rsb is active, we need to lock_rsb before
-		 * checking/changing re_master_nodeid
-		 */
+	if (!rsb_flag(r, RSB_HASHED)) {
+		read_unlock_bh(&ls->ls_rsbtbl_lock);
+		goto not_found;
+	}
 
-		hold_rsb(r);
+	if (rsb_flag(r, RSB_INACTIVE)) {
 		read_unlock_bh(&ls->ls_rsbtbl_lock);
-		lock_rsb(r);
+		goto do_inactive;
+	}
 
-		__dlm_master_lookup(ls, r, our_nodeid, from_nodeid, false,
-				    flags, r_nodeid, result);
+	/* because the rsb is active, we need to lock_rsb before
+	 * checking/changing re_master_nodeid
+	 */
 
-		/* the rsb was active */
-		unlock_rsb(r);
-		put_rsb(r);
+	hold_rsb(r);
+	read_unlock_bh(&ls->ls_rsbtbl_lock);
+	lock_rsb(r);
 
-		return 0;
-	} else {
-		read_unlock_bh(&ls->ls_rsbtbl_lock);
-		goto not_found;
-	}
+	__dlm_master_lookup(ls, r, our_nodeid, from_nodeid, false,
+			    flags, r_nodeid, result);
+
+	/* the rsb was active */
+	unlock_rsb(r);
+	put_rsb(r);
+
+	return 0;
 
  do_inactive:
 	/* unlikely path - check if still part of ls_rsbtbl */
@@ -1403,14 +1409,14 @@ void dlm_dump_rsb_name(struct dlm_ls *ls, const char *name, int len)
 	struct dlm_rsb *r = NULL;
 	int error;
 
-	read_lock_bh(&ls->ls_rsbtbl_lock);
+	rcu_read_lock();
 	error = dlm_search_rsb_tree(&ls->ls_rsbtbl, name, len, &r);
 	if (!error)
 		goto out;
 
 	dlm_dump_rsb(r);
  out:
-	read_unlock_bh(&ls->ls_rsbtbl_lock);
+	rcu_read_unlock();
 }
 
 static void deactivate_rsb(struct kref *kref)
@@ -4309,16 +4315,27 @@ static void receive_remove(struct dlm_ls *ls, const struct dlm_message *ms)
 	memset(name, 0, sizeof(name));
 	memcpy(name, ms->m_extra, len);
 
-	write_lock_bh(&ls->ls_rsbtbl_lock);
-
+	rcu_read_lock();
 	rv = dlm_search_rsb_tree(&ls->ls_rsbtbl, name, len, &r);
 	if (rv) {
+		rcu_read_unlock();
 		/* should not happen */
 		log_error(ls, "%s from %d not found %s", __func__,
 			  from_nodeid, name);
+		return;
+	}
+
+	write_lock_bh(&ls->ls_rsbtbl_lock);
+	if (!rsb_flag(r, RSB_HASHED)) {
+		rcu_read_unlock();
 		write_unlock_bh(&ls->ls_rsbtbl_lock);
+		/* should not happen */
+		log_error(ls, "%s from %d got removed during removal %s",
+			  __func__, from_nodeid, name);
 		return;
 	}
+	/* at this stage the rsb can only being freed here */
+	rcu_read_unlock();
 
 	if (!rsb_flag(r, RSB_INACTIVE)) {
 		if (r->res_master_nodeid != from_nodeid) {