[PATCH 3/4] nfsd: replace rp_mutex to avoid deadlock in move_to_close_lru()

[NeilBrown <neilb@suse.de>]

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.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/nfsd/nfs4state.c | 38 +++++++++++++++++++++++++++++++-------
 fs/nfsd/state.h     |  2 +-
 2 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 690d0e697320..47e879d5d68a 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -4430,21 +4430,32 @@ 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)
@@ -4453,7 +4464,8 @@ 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);
 	}
 }
@@ -4691,7 +4703,11 @@ 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);
@@ -5064,11 +5080,15 @@ 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;
@@ -6828,11 +6848,15 @@ 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, &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)
diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
index 41bdc913fa71..6a3becd86112 100644
--- a/fs/nfsd/state.h
+++ b/fs/nfsd/state.h
@@ -446,7 +446,7 @@ 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];
 };
 
-- 
2.43.0