nfsd: drop st_mutex before calling move_to_close_lru()

move_to_close_lru() is currently called with ->st_mutex held.
This can lead to a deadlock as move_to_close_lru() waits for sc_count to
drop to 2, and some threads holding a reference might be waiting for the
mutex.  These references will never be dropped so sc_count will never
reach 2.

There can be no harm in dropping ->st_mutex before
move_to_close_lru() because the only place that takes the mutex is
nfsd4_lock_ol_stateid(), and it quickly aborts if sc_type is
NFS4_CLOSED_STID, which it will be before move_to_close_lru() is called.

See also
 https://lore.kernel.org/lkml/[email protected]/T/
where this problem was raised but not successfully resolved.

Reviewed-by: Jeff Layton <[email protected]>
Signed-off-by: NeilBrown <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>

fs/nfsd/nfs4state.c

@@ -7357,7 +7357,7 @@ nfsd4_open_downgrade(struct svc_rqst *rqstp,
  return status;
 }
 
-static void nfsd4_close_open_stateid(struct nfs4_ol_stateid *s)
+static bool nfsd4_close_open_stateid(struct nfs4_ol_stateid *s)
 {
  struct nfs4_client *clp = s->st_stid.sc_client;
  bool unhashed;
@@ -7374,11 +7374,11 @@ static void nfsd4_close_open_stateid(struct nfs4_ol_stateid *s)
  list_for_each_entry(stp, &reaplist, st_locks)
  nfs4_free_cpntf_statelist(clp->net, &stp->st_stid);
  free_ol_stateid_reaplist(&reaplist);
+ return false;
  } else {
  spin_unlock(&clp->cl_lock);
  free_ol_stateid_reaplist(&reaplist);
- if (unhashed)
- move_to_close_lru(s, clp->net);
+ return unhashed;
  }
 }
 
@@ -7394,6 +7394,7 @@ nfsd4_close(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
  struct nfs4_ol_stateid *stp;
  struct net *net = SVC_NET(rqstp);
  struct nfsd_net *nn = net_generic(net, nfsd_net_id);
+ bool need_move_to_close_list;
 
  dprintk("NFSD: nfsd4_close on file %pd\n",
  cstate->current_fh.fh_dentry);
@@ -7418,8 +7419,10 @@ nfsd4_close(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
  */
  nfs4_inc_and_copy_stateid(&close->cl_stateid, &stp->st_stid);
 
- nfsd4_close_open_stateid(stp);
+ need_move_to_close_list = nfsd4_close_open_stateid(stp);
  mutex_unlock(&stp->st_mutex);
+ if (need_move_to_close_list)
+ move_to_close_lru(stp, net);
 
  /* v4.1+ suggests that we send a special stateid in here, since the
  * clients should just ignore this anyway. Since this is not useful