nfsd: replace rp_mutex to avoid deadlock in move_to_close_lru()

move_to_close_lru() waits for sc_count to become zero while holding
rp_mutex.  This can deadlock if another thread holds a reference and is
waiting for rp_mutex.

By the time we get to move_to_close_lru() the openowner is unhashed and
cannot be found any more.  So code waiting for the mutex can safely
retry the lookup if move_to_close_lru() has started.

So change rp_mutex to an atomic_t with three states:

 RP_UNLOCK   - state is still hashed, not locked for reply
 RP_LOCKED   - state is still hashed, is locked for reply
 RP_UNHASHED - state is not hashed, no code can get a lock.

Use wait_var_event() to wait for either a lock, or for the owner to be
unhashed.  In the latter case, retry the lookup.

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

fs/nfsd/nfs4state.c

@@ -4650,21 +4650,32 @@ nfsd4_init_leases_net(struct nfsd_net *nn)
  atomic_set(&nn->nfsd_courtesy_clients, 0);
 }
 
+enum rp_lock {
+ RP_UNLOCKED,
+ RP_LOCKED,
+ RP_UNHASHED,
+};
+
 static void init_nfs4_replay(struct nfs4_replay *rp)
 {
  rp->rp_status = nfserr_serverfault;
  rp->rp_buflen = 0;
  rp->rp_buf = rp->rp_ibuf;
- mutex_init(&rp->rp_mutex);
+ atomic_set(&rp->rp_locked, RP_UNLOCKED);
 }
 
-static void nfsd4_cstate_assign_replay(struct nfsd4_compound_state *cstate,
- struct nfs4_stateowner *so)
+static int nfsd4_cstate_assign_replay(struct nfsd4_compound_state *cstate,
+  struct nfs4_stateowner *so)
 {
  if (!nfsd4_has_session(cstate)) {
- mutex_lock(&so->so_replay.rp_mutex);
+ wait_var_event(&so->so_replay.rp_locked,
+ atomic_cmpxchg(&so->so_replay.rp_locked,
+ RP_UNLOCKED, RP_LOCKED) != RP_LOCKED);
+ if (atomic_read(&so->so_replay.rp_locked) == RP_UNHASHED)
+ return -EAGAIN;
  cstate->replay_owner = nfs4_get_stateowner(so);
  }
+ return 0;
 }
 
 void nfsd4_cstate_clear_replay(struct nfsd4_compound_state *cstate)
@@ -4673,7 +4684,8 @@ void nfsd4_cstate_clear_replay(struct nfsd4_compound_state *cstate)
 
  if (so != NULL) {
  cstate->replay_owner = NULL;
- mutex_unlock(&so->so_replay.rp_mutex);
+ atomic_set(&so->so_replay.rp_locked, RP_UNLOCKED);
+ wake_up_var(&so->so_replay.rp_locked);
  nfs4_put_stateowner(so);
  }
 }
@@ -4969,7 +4981,11 @@ move_to_close_lru(struct nfs4_ol_stateid *s, struct net *net)
  * Wait for the refcount to drop to 2. Since it has been unhashed,
  * there should be no danger of the refcount going back up again at
  * this point.
+ * Some threads with a reference might be waiting for rp_locked,
+ * so tell them to stop waiting.
  */
+ atomic_set(&oo->oo_owner.so_replay.rp_locked, RP_UNHASHED);
+ wake_up_var(&oo->oo_owner.so_replay.rp_locked);
  wait_event(close_wq, refcount_read(&s->st_stid.sc_count) == 2);
 
  release_all_access(s);
@@ -5342,11 +5358,15 @@ nfsd4_process_open1(struct nfsd4_compound_state *cstate,
  clp = cstate->clp;
 
  strhashval = ownerstr_hashval(&open->op_owner);
+retry:
  oo = find_or_alloc_open_stateowner(strhashval, open, cstate);
  open->op_openowner = oo;
  if (!oo)
  return nfserr_jukebox;
- nfsd4_cstate_assign_replay(cstate, &oo->oo_owner);
+ if (nfsd4_cstate_assign_replay(cstate, &oo->oo_owner) == -EAGAIN) {
+ nfs4_put_stateowner(&oo->oo_owner);
+ goto retry;
+ }
  status = nfsd4_check_seqid(cstate, &oo->oo_owner, open->op_seqid);
  if (status)
  return status;
@@ -7186,12 +7206,16 @@ nfs4_preprocess_seqid_op(struct nfsd4_compound_state *cstate, u32 seqid,
  trace_nfsd_preprocess(seqid, stateid);
 
  *stpp = NULL;
+retry:
  status = nfsd4_lookup_stateid(cstate, stateid,
  typemask, statusmask, &s, nn);
  if (status)
  return status;
  stp = openlockstateid(s);
- nfsd4_cstate_assign_replay(cstate, stp->st_stateowner);
+ if (nfsd4_cstate_assign_replay(cstate, stp->st_stateowner) == -EAGAIN) {
+ nfs4_put_stateowner(stp->st_stateowner);
+ goto retry;
+ }
 
  status = nfs4_seqid_op_checks(cstate, stateid, seqid, stp);
  if (!status)

fs/nfsd/state.h

@@ -486,7 +486,7 @@ struct nfs4_replay {
  unsigned int rp_buflen;
  char *rp_buf;
  struct knfsd_fh rp_openfh;
- struct mutex rp_mutex;
+ atomic_t rp_locked;
  char rp_ibuf[NFSD4_REPLAY_ISIZE];
 };