[DLM] keep dlm from panicing when traversing rsb list in debugfs

This problem was originally reported against GFS6.1, but the same issue exists
in upstream DLM.  This patch keeps the rsb iterator assigning under the rsbtbl
list lock.  Each time we process an rsb we grab a reference to it to make sure
it is not freed out from underneath us, and then put it when we get the next rsb
in the list or move onto another list.
Signed-off-by: NJosef Bacik <jwhiter@redhat.com>
Signed-off-by: NSteven Whitehouse <swhiteho@redhat.com>

fs/dlm/debug_fs.c

@@ -17,6 +17,7 @@
 #include <linux/debugfs.h>
 
 #include "dlm_internal.h"
+#include "lock.h"
 
 #define DLM_DEBUG_BUF_LEN 4096
 static char debug_buf[DLM_DEBUG_BUF_LEN];
@@ -166,6 +167,9 @@ static int rsb_iter_next(struct rsb_iter *ri)
 			read_lock(&ls->ls_rsbtbl[i].lock);
 			if (!list_empty(&ls->ls_rsbtbl[i].list)) {
 				ri->next = ls->ls_rsbtbl[i].list.next;
+				ri->rsb = list_entry(ri->next, struct dlm_rsb,
+							res_hashchain);
+				dlm_hold_rsb(ri->rsb);
 				read_unlock(&ls->ls_rsbtbl[i].lock);
 				break;
 			}
@@ -176,6 +180,7 @@ static int rsb_iter_next(struct rsb_iter *ri)
 		if (ri->entry >= ls->ls_rsbtbl_size)
 			return 1;
 	} else {
+		struct dlm_rsb *old = ri->rsb;
 		i = ri->entry;
 		read_lock(&ls->ls_rsbtbl[i].lock);
 		ri->next = ri->next->next;
@@ -184,11 +189,13 @@ static int rsb_iter_next(struct rsb_iter *ri)
 			ri->next = NULL;
 			ri->entry++;
 			read_unlock(&ls->ls_rsbtbl[i].lock);
+			dlm_put_rsb(old);
 			goto top;
                 }
+		ri->rsb = list_entry(ri->next, struct dlm_rsb, res_hashchain);
 		read_unlock(&ls->ls_rsbtbl[i].lock);
+		dlm_put_rsb(old);
 	}
-	ri->rsb = list_entry(ri->next, struct dlm_rsb, res_hashchain);
 
 	return 0;
 }