[PATCH] dlm: don't set RSB_MASTER_UNCERTAIN on rsb with no master

[Ameer Hamza]

When a local request reuses an inactive rsb, find_rsb_dir() sets
RSB_MASTER_UNCERTAIN if res_master_nodeid is not our nodeid. This also
matches an rsb that never got a master (res_master_nodeid 0, res_nodeid
-1). set_master() then trusts the "cached" master, copies res_nodeid -1
into lkb_nodeid and skips the lookup, and _request_lock() hits the
DLM_ASSERT(r->res_nodeid >= 0) in is_remote():

  rsb: nodeid -1 master 0 dir 1 flags 200 first 1 rlc 0 name pr
  Kernel panic - not syncing: DLM:  Record message above and reboot.
  Call Trace:
   _request_lock.cold+0x3e/0x3e [dlm]
   request_lock+0xcc/0x110 [dlm]
   dlm_user_request+0x110/0x340 [dlm]
   device_write+0x605/0x6d0 [dlm]

Such an rsb is left behind when a request is cancelled while its lookup
is still outstanding and the lockspace then recovers, e.g. because the
dir node died: dlm_recover_waiters_post() completes the cancelled lkb
instead of resending it, so no master is ever set. Once the rsb is
inactive, the next request for the name revives it and crashes as
above. If we became the dir node in that recovery, deactivate_rsb()
even keeps the rsb as a dir record, so it does not age out.

Before commit c04fecb4d9f7 ("dlm: use rsbtbl as resource directory")
the flag was only set when res_nodeid was > 0. Restore that and set the
flag only when a master is actually cached, so that set_master() does a
normal lookup (or becomes the master itself as dir node) for an rsb
that has none. res_first_lkid is still cleared, as the cancelled lkb
leaves a stale id behind.

Fixes: c04fecb4d9f7 ("dlm: use rsbtbl as resource directory")
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Ameer Hamza <ameer.hamza@xxxxxxxxxxx>
---
Reproduced with two nodes: pause the dir node for a name, request the
name, cancel the request once the lookup is out, remove the paused node,
request the name again. The second request asserts without this patch
and is granted with it, also when the dir node moves to a third live
node.

 fs/dlm/lock.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/dlm/lock.c b/fs/dlm/lock.c
index c715e171bab7..2323c7709aa1 100644
--- a/fs/dlm/lock.c
+++ b/fs/dlm/lock.c
@@ -826,8 +826,12 @@ static int find_rsb_dir(struct dlm_ls *ls, const void *name, int len,
 
 	if (from_local && (r->res_master_nodeid != our_nodeid)) {
 		/* Because we have held no locks on this rsb,
-		   res_master_nodeid could have become stale. */
-		rsb_set_flag(r, RSB_MASTER_UNCERTAIN);
+		 * res_master_nodeid could have become stale.  If no master
+		 * was ever set, there is nothing to confirm and set_master()
+		 * must resolve it as for a new rsb.
+		 */
+		if (r->res_master_nodeid)
+			rsb_set_flag(r, RSB_MASTER_UNCERTAIN);
 		r->res_first_lkid = 0;
 	}
 
-- 
2.53.0