Merge tag 'afs-fixes-20201016' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs

Pull afs updates from David Howells:
 "A collection of fixes to fix afs_cell struct refcounting, thereby
  fixing a slew of related syzbot bugs:

   - Fix the cell tree in the netns to use an rwsem rather than RCU.

     There seem to be some problems deriving from the use of RCU and a
     seqlock to walk the rbtree, but it's not entirely clear what since
     there are several different failures being seen.

     Changing things to use an rwsem instead makes it more robust. The
     extra performance derived from using RCU isn't necessary in this
     case since the only time we're looking up a cell is during mount or
     when cells are being manually added.

   - Fix the refcounting by splitting the usage counter into a memory
     refcount and an active users counter. The usage counter was doing
     double duty, keeping track of whether a cell is still in use and
     keeping track of when it needs to be destroyed - but this makes the
     clean up tricky. Separating these out simplifies the logic.

   - Fix purging a cell that has an alias. A cell alias pins the cell
     it's an alias of, but the alias is always later in the list. Trying
     to purge in a single pass causes rmmod to hang in such a case.

   - Fix cell removal. If a cell's manager is requeued whilst it's
     removing itself, the manager will run again and re-remove itself,
     causing problems in various places. Follow Hillf Danton's
     suggestion to insert a more terminal state that causes the manager
     to do nothing post-removal.

  In additional to the above, two other changes:

   - Add a tracepoint for the cell refcount and active users count. This
     helped with debugging the above and may be useful again in future.

   - Downgrade an assertion to a print when a still-active server is
     seen during purging. This was happening as a consequence of
     incomplete cell removal before the servers were cleaned up"

* tag 'afs-fixes-20201016' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs:
  afs: Don't assert on unpurgeable server records
  afs: Add tracing for cell refcount and active user count
  afs: Fix cell removal
  afs: Fix cell purging with aliases
  afs: Fix cell refcounting by splitting the usage counter
  afs: Fix rapid cell addition/removal by not using RCU on cells tree

11 files changed, 269 insertions, 172 deletions

diff --git a/fs/afs/cell.c b/fs/afs/cell.c
index 5b79cdceefa0..52233fa6195f 100644
--- a/fs/afs/cell.c
+++ b/fs/afs/cell.c
@@ -18,8 +18,10 @@
 static unsigned __read_mostly afs_cell_gc_delay = 10;
 static unsigned __read_mostly afs_cell_min_ttl = 10 * 60;
 static unsigned __read_mostly afs_cell_max_ttl = 24 * 60 * 60;
+static atomic_t cell_debug_id;
 
-static void afs_manage_cell(struct work_struct *);
+static void afs_queue_cell_manager(struct afs_net *);
+static void afs_manage_cell_work(struct work_struct *);
 
 static void afs_dec_cells_outstanding(struct afs_net *net)
 {
@@ -37,19 +39,22 @@ static void afs_set_cell_timer(struct afs_net *net, time64_t delay)
 		atomic_inc(&net->cells_outstanding);
 		if (timer_reduce(&net->cells_timer, jiffies + delay * HZ))
 			afs_dec_cells_outstanding(net);
+	} else {
+		afs_queue_cell_manager(net);
 	}
 }
 
 /*
- * Look up and get an activation reference on a cell record under RCU
- * conditions.  The caller must hold the RCU read lock.
+ * Look up and get an activation reference on a cell record.  The caller must
+ * hold net->cells_lock at least read-locked.
  */
-struct afs_cell *afs_lookup_cell_rcu(struct afs_net *net,
-				     const char *name, unsigned int namesz)
+static struct afs_cell *afs_find_cell_locked(struct afs_net *net,
+					     const char *name, unsigned int namesz,
+					     enum afs_cell_trace reason)
 {
 	struct afs_cell *cell = NULL;
 	struct rb_node *p;
-	int n, seq = 0, ret = 0;
+	int n;
 
 	_enter("%*.*s", namesz, namesz, name);
 
@@ -58,61 +63,48 @@ struct afs_cell *afs_lookup_cell_rcu(struct afs_net *net,
 	if (namesz > AFS_MAXCELLNAME)
 		return ERR_PTR(-ENAMETOOLONG);
 
-	do {
-		/* Unfortunately, rbtree walking doesn't give reliable results
-		 * under just the RCU read lock, so we have to check for
-		 * changes.
-		 */
-		if (cell)
-			afs_put_cell(net, cell);
-		cell = NULL;
-		ret = -ENOENT;
-
-		read_seqbegin_or_lock(&net->cells_lock, &seq);
-
-		if (!name) {
-			cell = rcu_dereference_raw(net->ws_cell);
-			if (cell) {
-				afs_get_cell(cell);
-				ret = 0;
-				break;
-			}
-			ret = -EDESTADDRREQ;
-			continue;
-		}
+	if (!name) {
+		cell = net->ws_cell;
+		if (!cell)
+			return ERR_PTR(-EDESTADDRREQ);
+		goto found;
+	}
 
-		p = rcu_dereference_raw(net->cells.rb_node);
-		while (p) {
-			cell = rb_entry(p, struct afs_cell, net_node);
-
-			n = strncasecmp(cell->name, name,
-					min_t(size_t, cell->name_len, namesz));
-			if (n == 0)
-				n = cell->name_len - namesz;
-			if (n < 0) {
-				p = rcu_dereference_raw(p->rb_left);
-			} else if (n > 0) {
-				p = rcu_dereference_raw(p->rb_right);
-			} else {
-				if (atomic_inc_not_zero(&cell->usage)) {
-					ret = 0;
-					break;
-				}
-				/* We want to repeat the search, this time with
-				 * the lock properly locked.
-				 */
-			}
-			cell = NULL;
-		}
+	p = net->cells.rb_node;
+	while (p) {
+		cell = rb_entry(p, struct afs_cell, net_node);
 
-	} while (need_seqretry(&net->cells_lock, seq));
+		n = strncasecmp(cell->name, name,
+				min_t(size_t, cell->name_len, namesz));
+		if (n == 0)
+			n = cell->name_len - namesz;
+		if (n < 0)
+			p = p->rb_left;
+		else if (n > 0)
+			p = p->rb_right;
+		else
+			goto found;
+	}
 
-	done_seqretry(&net->cells_lock, seq);
+	return ERR_PTR(-ENOENT);
 
-	if (ret != 0 && cell)
-		afs_put_cell(net, cell);
+found:
+	return afs_use_cell(cell, reason);
+}
 
-	return ret == 0 ? cell : ERR_PTR(ret);
+/*
+ * Look up and get an activation reference on a cell record.
+ */
+struct afs_cell *afs_find_cell(struct afs_net *net,
+			       const char *name, unsigned int namesz,
+			       enum afs_cell_trace reason)
+{
+	struct afs_cell *cell;
+
+	down_read(&net->cells_lock);
+	cell = afs_find_cell_locked(net, name, namesz, reason);
+	up_read(&net->cells_lock);
+	return cell;
 }
 
 /*
@@ -166,8 +158,9 @@ static struct afs_cell *afs_alloc_cell(struct afs_net *net,
 		cell->name[i] = tolower(name[i]);
 	cell->name[i] = 0;
 
-	atomic_set(&cell->usage, 2);
-	INIT_WORK(&cell->manager, afs_manage_cell);
+	atomic_set(&cell->ref, 1);
+	atomic_set(&cell->active, 0);
+	INIT_WORK(&cell->manager, afs_manage_cell_work);
 	cell->volumes = RB_ROOT;
 	INIT_HLIST_HEAD(&cell->proc_volumes);
 	seqlock_init(&cell->volume_lock);
@@ -206,6 +199,9 @@ static struct afs_cell *afs_alloc_cell(struct afs_net *net,
 	cell->dns_source = vllist->source;
 	cell->dns_status = vllist->status;
 	smp_store_release(&cell->dns_lookup_count, 1); /* vs source/status */
+	atomic_inc(&net->cells_outstanding);
+	cell->debug_id = atomic_inc_return(&cell_debug_id);
+	trace_afs_cell(cell->debug_id, 1, 0, afs_cell_trace_alloc);
 
 	_leave(" = %p", cell);
 	return cell;
@@ -245,9 +241,7 @@ struct afs_cell *afs_lookup_cell(struct afs_net *net,
 	_enter("%s,%s", name, vllist);
 
 	if (!excl) {
-		rcu_read_lock();
-		cell = afs_lookup_cell_rcu(net, name, namesz);
-		rcu_read_unlock();
+		cell = afs_find_cell(net, name, namesz, afs_cell_trace_use_lookup);
 		if (!IS_ERR(cell))
 			goto wait_for_cell;
 	}
@@ -268,7 +262,7 @@ struct afs_cell *afs_lookup_cell(struct afs_net *net,
 	/* Find the insertion point and check to see if someone else added a
 	 * cell whilst we were allocating.
 	 */
-	write_seqlock(&net->cells_lock);
+	down_write(&net->cells_lock);
 
 	pp = &net->cells.rb_node;
 	parent = NULL;
@@ -290,23 +284,26 @@ struct afs_cell *afs_lookup_cell(struct afs_net *net,
 
 	cell = candidate;
 	candidate = NULL;
+	atomic_set(&cell->active, 2);
+	trace_afs_cell(cell->debug_id, atomic_read(&cell->ref), 2, afs_cell_trace_insert);
 	rb_link_node_rcu(&cell->net_node, parent, pp);
 	rb_insert_color(&cell->net_node, &net->cells);
-	atomic_inc(&net->cells_outstanding);
-	write_sequnlock(&net->cells_lock);
+	up_write(&net->cells_lock);
 
-	queue_work(afs_wq, &cell->manager);
+	afs_queue_cell(cell, afs_cell_trace_get_queue_new);
 
 wait_for_cell:
+	trace_afs_cell(cell->debug_id, atomic_read(&cell->ref), atomic_read(&cell->active),
+		       afs_cell_trace_wait);
 	_debug("wait_for_cell");
 	wait_var_event(&cell->state,
 		       ({
 			       state = smp_load_acquire(&cell->state); /* vs error */
-			       state == AFS_CELL_ACTIVE || state == AFS_CELL_FAILED;
+			       state == AFS_CELL_ACTIVE || state == AFS_CELL_REMOVED;
 		       }));
 
 	/* Check the state obtained from the wait check. */
-	if (state == AFS_CELL_FAILED) {
+	if (state == AFS_CELL_REMOVED) {
 		ret = cell->error;
 		goto error;
 	}
@@ -320,16 +317,17 @@ cell_already_exists:
 	if (excl) {
 		ret = -EEXIST;
 	} else {
-		afs_get_cell(cursor);
+		afs_use_cell(cursor, afs_cell_trace_use_lookup);
 		ret = 0;
 	}
-	write_sequnlock(&net->cells_lock);
-	kfree(candidate);
+	up_write(&net->cells_lock);
+	if (candidate)
+		afs_put_cell(candidate, afs_cell_trace_put_candidate);
 	if (ret == 0)
 		goto wait_for_cell;
 	goto error_noput;
 error:
-	afs_put_cell(net, cell);
+	afs_unuse_cell(net, cell, afs_cell_trace_unuse_lookup);
 error_noput:
 	_leave(" = %d [error]", ret);
 	return ERR_PTR(ret);
@@ -374,15 +372,16 @@ int afs_cell_init(struct afs_net *net, const char *rootcell)
 	}
 
 	if (!test_and_set_bit(AFS_CELL_FL_NO_GC, &new_root->flags))
-		afs_get_cell(new_root);
+		afs_use_cell(new_root, afs_cell_trace_use_pin);
 
 	/* install the new cell */
-	write_seqlock(&net->cells_lock);
-	old_root = rcu_access_pointer(net->ws_cell);
-	rcu_assign_pointer(net->ws_cell, new_root);
-	write_sequnlock(&net->cells_lock);
+	down_write(&net->cells_lock);
+	afs_see_cell(new_root, afs_cell_trace_see_ws);
+	old_root = net->ws_cell;
+	net->ws_cell = new_root;
+	up_write(&net->cells_lock);
 
-	afs_put_cell(net, old_root);
+	afs_unuse_cell(net, old_root, afs_cell_trace_unuse_ws);
 	_leave(" = 0");
 	return 0;
 }
@@ -488,18 +487,22 @@ out_wake:
 static void afs_cell_destroy(struct rcu_head *rcu)
 {
 	struct afs_cell *cell = container_of(rcu, struct afs_cell, rcu);
+	struct afs_net *net = cell->net;
+	int u;
 
 	_enter("%p{%s}", cell, cell->name);
 
-	ASSERTCMP(atomic_read(&cell->usage), ==, 0);
+	u = atomic_read(&cell->ref);
+	ASSERTCMP(u, ==, 0);
+	trace_afs_cell(cell->debug_id, u, atomic_read(&cell->active), afs_cell_trace_free);
 
-	afs_put_volume(cell->net, cell->root_volume, afs_volume_trace_put_cell_root);
-	afs_put_vlserverlist(cell->net, rcu_access_pointer(cell->vl_servers));
-	afs_put_cell(cell->net, cell->alias_of);
+	afs_put_vlserverlist(net, rcu_access_pointer(cell->vl_servers));
+	afs_unuse_cell(net, cell->alias_of, afs_cell_trace_unuse_alias);
 	key_put(cell->anonymous_key);
 	kfree(cell->name);
 	kfree(cell);
 
+	afs_dec_cells_outstanding(net);
 	_leave(" [destroyed]");
 }
 
@@ -532,18 +535,63 @@ void afs_cells_timer(struct timer_list *timer)
 /*
  * Get a reference on a cell record.
  */
-struct afs_cell *afs_get_cell(struct afs_cell *cell)
+struct afs_cell *afs_get_cell(struct afs_cell *cell, enum afs_cell_trace reason)
 {
-	atomic_inc(&cell->usage);
+	int u;
+
+	if (atomic_read(&cell->ref) <= 0)
+		BUG();
+
+	u = atomic_inc_return(&cell->ref);
+	trace_afs_cell(cell->debug_id, u, atomic_read(&cell->active), reason);
 	return cell;
 }
 
 /*
  * Drop a reference on a cell record.
  */
-void afs_put_cell(struct afs_net *net, struct afs_cell *cell)
+void afs_put_cell(struct afs_cell *cell, enum afs_cell_trace reason)
 {
+	if (cell) {
+		unsigned int debug_id = cell->debug_id;
+		unsigned int u, a;
+
+		a = atomic_read(&cell->active);
+		u = atomic_dec_return(&cell->ref);
+		trace_afs_cell(debug_id, u, a, reason);
+		if (u == 0) {
+			a = atomic_read(&cell->active);
+			WARN(a != 0, "Cell active count %u > 0\n", a);
+			call_rcu(&cell->rcu, afs_cell_destroy);
+		}
+	}
+}
+
+/*
+ * Note a cell becoming more active.
+ */
+struct afs_cell *afs_use_cell(struct afs_cell *cell, enum afs_cell_trace reason)
+{
+	int u, a;
+
+	if (atomic_read(&cell->ref) <= 0)
+		BUG();
+
+	u = atomic_read(&cell->ref);
+	a = atomic_inc_return(&cell->active);
+	trace_afs_cell(cell->debug_id, u, a, reason);
+	return cell;
+}
+
+/*
+ * Record a cell becoming less active.  When the active counter reaches 1, it
+ * is scheduled for destruction, but may get reactivated.
+ */
+void afs_unuse_cell(struct afs_net *net, struct afs_cell *cell, enum afs_cell_trace reason)
+{
+	unsigned int debug_id = cell->debug_id;
 	time64_t now, expire_delay;
+	int u, a;
 
 	if (!cell)
 		return;
@@ -556,11 +604,35 @@ void afs_put_cell(struct afs_net *net, struct afs_cell *cell)
 	if (cell->vl_servers->nr_servers)
 		expire_delay = afs_cell_gc_delay;
 
-	if (atomic_dec_return(&cell->usage) > 1)
-		return;
+	u = atomic_read(&cell->ref);
+	a = atomic_dec_return(&cell->active);
+	trace_afs_cell(debug_id, u, a, reason);
+	WARN_ON(a == 0);
+	if (a == 1)
+		/* 'cell' may now be garbage collected. */
+		afs_set_cell_timer(net, expire_delay);
+}
+
+/*
+ * Note that a cell has been seen.
+ */
+void afs_see_cell(struct afs_cell *cell, enum afs_cell_trace reason)
+{
+	int u, a;
+
+	u = atomic_read(&cell->ref);
+	a = atomic_read(&cell->active);
+	trace_afs_cell(cell->debug_id, u, a, reason);
+}
 
-	/* 'cell' may now be garbage collected. */
-	afs_set_cell_timer(net, expire_delay);
+/*
+ * Queue a cell for management, giving the workqueue a ref to hold.
+ */
+void afs_queue_cell(struct afs_cell *cell, enum afs_cell_trace reason)
+{
+	afs_get_cell(cell, reason);
+	if (!queue_work(afs_wq, &cell->manager))
+		afs_put_cell(cell, afs_cell_trace_put_queue_fail);
 }
 
 /*
@@ -660,12 +732,10 @@ static void afs_deactivate_cell(struct afs_net *net, struct afs_cell *cell)
  * Manage a cell record, initialising and destroying it, maintaining its DNS
  * records.
  */
-static void afs_manage_cell(struct work_struct *work)
+static void afs_manage_cell(struct afs_cell *cell)
 {
-	struct afs_cell *cell = container_of(work, struct afs_cell, manager);
 	struct afs_net *net = cell->net;
-	bool deleted;
-	int ret, usage;
+	int ret, active;
 
 	_enter("%s", cell->name);
 
@@ -674,14 +744,19 @@ again:
 	switch (cell->state) {
 	case AFS_CELL_INACTIVE:
 	case AFS_CELL_FAILED:
-		write_seqlock(&net->cells_lock);
-		usage = 1;
-		deleted = atomic_try_cmpxchg_relaxed(&cell->usage, &usage, 0);
-		if (deleted)
+		down_write(&net->cells_lock);
+		active = 1;
+		if (atomic_try_cmpxchg_relaxed(&cell->active, &active, 0)) {
 			rb_erase(&cell->net_node, &net->cells);
-		write_sequnlock(&net->cells_lock);
-		if (deleted)
+			trace_afs_cell(cell->debug_id, atomic_read(&cell->ref), 0,
+				       afs_cell_trace_unuse_delete);
+			smp_store_release(&cell->state, AFS_CELL_REMOVED);
+		}
+		up_write(&net->cells_lock);
+		if (cell->state == AFS_CELL_REMOVED) {
+			wake_up_var(&cell->state);
 			goto final_destruction;
+		}
 		if (cell->state == AFS_CELL_FAILED)
 			goto done;
 		smp_store_release(&cell->state, AFS_CELL_UNSET);
@@ -703,7 +778,7 @@ again:
 		goto again;
 
 	case AFS_CELL_ACTIVE:
-		if (atomic_read(&cell->usage) > 1) {
+		if (atomic_read(&cell->active) > 1) {
 			if (test_and_clear_bit(AFS_CELL_FL_DO_LOOKUP, &cell->flags)) {
 				ret = afs_update_cell(cell);
 				if (ret < 0)
@@ -716,13 +791,16 @@ again:
 		goto again;
 
 	case AFS_CELL_DEACTIVATING:
-		if (atomic_read(&cell->usage) > 1)
+		if (atomic_read(&cell->active) > 1)
 			goto reverse_deactivation;
 		afs_deactivate_cell(net, cell);
 		smp_store_release(&cell->state, AFS_CELL_INACTIVE);
 		wake_up_var(&cell->state);
 		goto again;
 
+	case AFS_CELL_REMOVED:
+		goto done;
+
 	default:
 		break;
 	}
@@ -748,9 +826,18 @@ done:
 	return;
 
 final_destruction:
-	call_rcu(&cell->rcu, afs_cell_destroy);
-	afs_dec_cells_outstanding(net);
-	_leave(" [destruct %d]", atomic_read(&net->cells_outstanding));
+	/* The root volume is pinning the cell */
+	afs_put_volume(cell->net, cell->root_volume, afs_volume_trace_put_cell_root);
+	cell->root_volume = NULL;
+	afs_put_cell(cell, afs_cell_trace_put_destroy);
+}
+
+static void afs_manage_cell_work(struct work_struct *work)
+{
+	struct afs_cell *cell = container_of(work, struct afs_cell, manager);
+
+	afs_manage_cell(cell);
+	afs_put_cell(cell, afs_cell_trace_put_queue_work);
 }
 
 /*
@@ -779,26 +866,29 @@ void afs_manage_cells(struct work_struct *work)
 	 * lack of use and cells whose DNS results have expired and dispatch
 	 * their managers.
 	 */
-	read_seqlock_excl(&net->cells_lock);
+	down_read(&net->cells_lock);
 
 	for (cursor = rb_first(&net->cells); cursor; cursor = rb_next(cursor)) {
 		struct afs_cell *cell =
 			rb_entry(cursor, struct afs_cell, net_node);
-		unsigned usage;
+		unsigned active;
 		bool sched_cell = false;
 
-		usage = atomic_read(&cell->usage);
-		_debug("manage %s %u", cell->name, usage);
+		active = atomic_read(&cell->active);
+		trace_afs_cell(cell->debug_id, atomic_read(&cell->ref),
+			       active, afs_cell_trace_manage);
 
-		ASSERTCMP(usage, >=, 1);
+		ASSERTCMP(active, >=, 1);
 
 		if (purging) {
-			if (test_and_clear_bit(AFS_CELL_FL_NO_GC, &cell->flags))
-				usage = atomic_dec_return(&cell->usage);
-			ASSERTCMP(usage, ==, 1);
+			if (test_and_clear_bit(AFS_CELL_FL_NO_GC, &cell->flags)) {
+				active = atomic_dec_return(&cell->active);
+				trace_afs_cell(cell->debug_id, atomic_read(&cell->ref),
+					       active, afs_cell_trace_unuse_pin);
+			}
 		}
 
-		if (usage == 1) {
+		if (active == 1) {
 			struct afs_vlserver_list *vllist;
 			time64_t expire_at = cell->last_inactive;
 
@@ -821,10 +911,10 @@ void afs_manage_cells(struct work_struct *work)
 		}
 
 		if (sched_cell)
-			queue_work(afs_wq, &cell->manager);
+			afs_queue_cell(cell, afs_cell_trace_get_queue_manage);
 	}
 
-	read_sequnlock_excl(&net->cells_lock);
+	up_read(&net->cells_lock);
 
 	/* Update the timer on the way out.  We have to pass an increment on
 	 * cells_outstanding in the namespace that we are in to the timer or
@@ -854,11 +944,11 @@ void afs_cell_purge(struct afs_net *net)
 
 	_enter("");
 
-	write_seqlock(&net->cells_lock);
-	ws = rcu_access_pointer(net->ws_cell);
-	RCU_INIT_POINTER(net->ws_cell, NULL);
-	write_sequnlock(&net->cells_lock);
-	afs_put_cell(net, ws);
+	down_write(&net->cells_lock);
+	ws = net->ws_cell;
+	net->ws_cell = NULL;
+	up_write(&net->cells_lock);
+	afs_unuse_cell(net, ws, afs_cell_trace_unuse_ws);
 
 	_debug("del timer");
 	if (del_timer_sync(&net->cells_timer))
diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
index 7b784af604fd..db832cc931c8 100644
--- a/fs/afs/dynroot.c
+++ b/fs/afs/dynroot.c
@@ -123,9 +123,9 @@ static int afs_probe_cell_name(struct dentry *dentry)
 		len--;
 	}
 
-	cell = afs_lookup_cell_rcu(net, name, len);
+	cell = afs_find_cell(net, name, len, afs_cell_trace_use_probe);
 	if (!IS_ERR(cell)) {
-		afs_put_cell(net, cell);
+		afs_unuse_cell(net, cell, afs_cell_trace_unuse_probe);
 		return 0;
 	}
 
@@ -179,7 +179,6 @@ static struct dentry *afs_lookup_atcell(struct dentry *dentry)
 	struct afs_cell *cell;
 	struct afs_net *net = afs_d2net(dentry);
 	struct dentry *ret;
-	unsigned int seq = 0;
 	char *name;
 	int len;
 
@@ -191,17 +190,13 @@ static struct dentry *afs_lookup_atcell(struct dentry *dentry)
 	if (!name)
 		goto out_p;
 
-	rcu_read_lock();
-	do {
-		read_seqbegin_or_lock(&net->cells_lock, &seq);
-		cell = rcu_dereference_raw(net->ws_cell);
-		if (cell) {
-			len = cell->name_len;
-			memcpy(name, cell->name, len + 1);
-		}
-	} while (need_seqretry(&net->cells_lock, seq));
-	done_seqretry(&net->cells_lock, seq);
-	rcu_read_unlock();
+	down_read(&net->cells_lock);
+	cell = net->ws_cell;
+	if (cell) {
+		len = cell->name_len;
+		memcpy(name, cell->name, len + 1);
+	}
+	up_read(&net->cells_lock);
 
 	ret = ERR_PTR(-ENOENT);
 	if (!cell)
diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index e5f0446f27e5..81b0485fd22a 100644
--- a/fs/afs/internal.h
+++ b/fs/afs/internal.h
@@ -263,11 +263,11 @@ struct afs_net {
 
 	/* Cell database */
 	struct rb_root		cells;
-	struct afs_cell __rcu	*ws_cell;
+	struct afs_cell		*ws_cell;
 	struct work_struct	cells_manager;
 	struct timer_list	cells_timer;
 	atomic_t		cells_outstanding;
-	seqlock_t		cells_lock;
+	struct rw_semaphore	cells_lock;
 	struct mutex		cells_alias_lock;
 
 	struct mutex		proc_cells_lock;
@@ -326,6 +326,7 @@ enum afs_cell_state {
 	AFS_CELL_DEACTIVATING,
 	AFS_CELL_INACTIVE,
 	AFS_CELL_FAILED,
+	AFS_CELL_REMOVED,
 };
 
 /*
@@ -363,7 +364,8 @@ struct afs_cell {
 #endif
 	time64_t		dns_expiry;	/* Time AFSDB/SRV record expires */
 	time64_t		last_inactive;	/* Time of last drop of usage count */
-	atomic_t		usage;
+	atomic_t		ref;		/* Struct refcount */
+	atomic_t		active;		/* Active usage counter */
 	unsigned long		flags;
 #define AFS_CELL_FL_NO_GC	0		/* The cell was added manually, don't auto-gc */
 #define AFS_CELL_FL_DO_LOOKUP	1		/* DNS lookup requested */
@@ -373,6 +375,7 @@ struct afs_cell {
 	enum dns_record_source	dns_source:8;	/* Latest source of data from lookup */
 	enum dns_lookup_status	dns_status:8;	/* Latest status of data from lookup */
 	unsigned int		dns_lookup_count; /* Counter of DNS lookups */
+	unsigned int		debug_id;
 
 	/* The volumes belonging to this cell */
 	struct rb_root		volumes;	/* Tree of volumes on this server */
@@ -917,11 +920,16 @@ static inline bool afs_cb_is_broken(unsigned int cb_break,
  * cell.c
  */
 extern int afs_cell_init(struct afs_net *, const char *);
-extern struct afs_cell *afs_lookup_cell_rcu(struct afs_net *, const char *, unsigned);
+extern struct afs_cell *afs_find_cell(struct afs_net *, const char *, unsigned,
+				      enum afs_cell_trace);
 extern struct afs_cell *afs_lookup_cell(struct afs_net *, const char *, unsigned,
 					const char *, bool);
-extern struct afs_cell *afs_get_cell(struct afs_cell *);
-extern void afs_put_cell(struct afs_net *, struct afs_cell *);
+extern struct afs_cell *afs_use_cell(struct afs_cell *, enum afs_cell_trace);
+extern void afs_unuse_cell(struct afs_net *, struct afs_cell *, enum afs_cell_trace);
+extern struct afs_cell *afs_get_cell(struct afs_cell *, enum afs_cell_trace);
+extern void afs_see_cell(struct afs_cell *, enum afs_cell_trace);
+extern void afs_put_cell(struct afs_cell *, enum afs_cell_trace);
+extern void afs_queue_cell(struct afs_cell *, enum afs_cell_trace);
 extern void afs_manage_cells(struct work_struct *);
 extern void afs_cells_timer(struct timer_list *);
 extern void __net_exit afs_cell_purge(struct afs_net *);
diff --git a/fs/afs/main.c b/fs/afs/main.c
index 31b472f7c734..accdd8970e7c 100644
--- a/fs/afs/main.c
+++ b/fs/afs/main.c
@@ -78,7 +78,7 @@ static int __net_init afs_net_init(struct net *net_ns)
 	mutex_init(&net->socket_mutex);
 
 	net->cells = RB_ROOT;
-	seqlock_init(&net->cells_lock);
+	init_rwsem(&net->cells_lock);
 	INIT_WORK(&net->cells_manager, afs_manage_cells);
 	timer_setup(&net->cells_timer, afs_cells_timer, 0);
 
diff --git a/fs/afs/mntpt.c b/fs/afs/mntpt.c
index 79bc5f1338ed..052dab2f5c03 100644
--- a/fs/afs/mntpt.c
+++ b/fs/afs/mntpt.c
@@ -88,7 +88,7 @@ static int afs_mntpt_set_params(struct fs_context *fc, struct dentry *mntpt)
 		ctx->force = true;
 	}
 	if (ctx->cell) {
-		afs_put_cell(ctx->net, ctx->cell);
+		afs_unuse_cell(ctx->net, ctx->cell, afs_cell_trace_unuse_mntpt);
 		ctx->cell = NULL;
 	}
 	if (test_bit(AFS_VNODE_PSEUDODIR, &vnode->flags)) {
@@ -124,7 +124,7 @@ static int afs_mntpt_set_params(struct fs_context *fc, struct dentry *mntpt)
 		char *buf;
 
 		if (src_as->cell)
-			ctx->cell = afs_get_cell(src_as->cell);
+			ctx->cell = afs_use_cell(src_as->cell, afs_cell_trace_use_mntpt);
 
 		if (size < 2 || size > PAGE_SIZE - 1)
 			return -EINVAL;
diff --git a/fs/afs/proc.c b/fs/afs/proc.c
index e8babb62ed44..065a28bfa3f1 100644
--- a/fs/afs/proc.c
+++ b/fs/afs/proc.c
@@ -38,7 +38,7 @@ static int afs_proc_cells_show(struct seq_file *m, void *v)
 
 	if (v == SEQ_START_TOKEN) {
 		/* display header on line 1 */
-		seq_puts(m, "USE    TTL SV ST NAME\n");
+		seq_puts(m, "USE ACT    TTL SV ST NAME\n");
 		return 0;
 	}
 
@@ -46,10 +46,11 @@ static int afs_proc_cells_show(struct seq_file *m, void *v)
 	vllist = rcu_dereference(cell->vl_servers);
 
 	/* display one cell per line on subsequent lines */
-	seq_printf(m, "%3u %6lld %2u %2u %s\n",
-		   atomic_read(&cell->usage),
+	seq_printf(m, "%3u %3u %6lld %2u %2u %s\n",
+		   atomic_read(&cell->ref),
+		   atomic_read(&cell->active),
 		   cell->dns_expiry - ktime_get_real_seconds(),
-		   vllist->nr_servers,
+		   vllist ? vllist->nr_servers : 0,
 		   cell->state,
 		   cell->name);
 	return 0;
@@ -128,7 +129,7 @@ static int afs_proc_cells_write(struct file *file, char *buf, size_t size)
 		}
 
 		if (test_and_set_bit(AFS_CELL_FL_NO_GC, &cell->flags))
-			afs_put_cell(net, cell);
+			afs_unuse_cell(net, cell, afs_cell_trace_unuse_no_pin);
 	} else {
 		goto inval;
 	}
@@ -154,13 +155,11 @@ static int afs_proc_rootcell_show(struct seq_file *m, void *v)
 	struct afs_net *net;
 
 	net = afs_seq2net_single(m);
-	if (rcu_access_pointer(net->ws_cell)) {
-		rcu_read_lock();
-		cell = rcu_dereference(net->ws_cell);
-		if (cell)
-			seq_printf(m, "%s\n", cell->name);
-		rcu_read_unlock();
-	}
+	down_read(&net->cells_lock);
+	cell = net->ws_cell;
+	if (cell)
+		seq_printf(m, "%s\n", cell->name);
+	up_read(&net->cells_lock);
 	return 0;
 }
 
diff --git a/fs/afs/server.c b/fs/afs/server.c
index e82e452e2612..684a2b02b9ff 100644
--- a/fs/afs/server.c
+++ b/fs/afs/server.c
@@ -550,7 +550,12 @@ void afs_manage_servers(struct work_struct *work)
 
 		_debug("manage %pU %u", &server->uuid, active);
 
-		ASSERTIFCMP(purging, active, ==, 0);
+		if (purging) {
+			trace_afs_server(server, atomic_read(&server->ref),
+					 active, afs_server_trace_purging);
+			if (active != 0)
+				pr_notice("Can't purge s=%08x\n", server->debug_id);
+		}
 
 		if (active == 0) {
 			time64_t expire_at = server->unuse_time;
diff --git a/fs/afs/super.c b/fs/afs/super.c
index 3a40ee752c1e..6c5900df6aa5 100644
--- a/fs/afs/super.c
+++ b/fs/afs/super.c
@@ -294,7 +294,8 @@ static int afs_parse_source(struct fs_context *fc, struct fs_parameter *param)
 			       cellnamesz, cellnamesz, cellname ?: "");
 			return PTR_ERR(cell);
 		}
-		afs_put_cell(ctx->net, ctx->cell);
+		afs_unuse_cell(ctx->net, ctx->cell, afs_cell_trace_unuse_parse);
+		afs_see_cell(cell, afs_cell_trace_see_source);
 		ctx->cell = cell;
 	}
 
@@ -389,8 +390,9 @@ static int afs_validate_fc(struct fs_context *fc)
 				_debug("switch to alias");
 				key_put(ctx->key);
 				ctx->key = NULL;
-				cell = afs_get_cell(ctx->cell->alias_of);
-				afs_put_cell(ctx->net, ctx->cell);
+				cell = afs_use_cell(ctx->cell->alias_of,
+						    afs_cell_trace_use_fc_alias);
+				afs_unuse_cell(ctx->net, ctx->cell, afs_cell_trace_unuse_fc);
 				ctx->cell = cell;
 				goto reget_key;
 			}
@@ -507,7 +509,7 @@ static struct afs_super_info *afs_alloc_sbi(struct fs_context *fc)
 		if (ctx->dyn_root) {
 			as->dyn_root = true;
 		} else {
-			as->cell = afs_get_cell(ctx->cell);
+			as->cell = afs_use_cell(ctx->cell, afs_cell_trace_use_sbi);
 			as->volume = afs_get_volume(ctx->volume,
 						    afs_volume_trace_get_alloc_sbi);
 		}
@@ -520,7 +522,7 @@ static void afs_destroy_sbi(struct afs_super_info *as)
 	if (as) {
 		struct afs_net *net = afs_net(as->net_ns);
 		afs_put_volume(net, as->volume, afs_volume_trace_put_destroy_sbi);
-		afs_put_cell(net, as->cell);
+		afs_unuse_cell(net, as->cell, afs_cell_trace_unuse_sbi);
 		put_net(as->net_ns);
 		kfree(as);
 	}
@@ -606,7 +608,7 @@ static void afs_free_fc(struct fs_context *fc)
 
 	afs_destroy_sbi(fc->s_fs_info);
 	afs_put_volume(ctx->net, ctx->volume, afs_volume_trace_put_free_fc);
-	afs_put_cell(ctx->net, ctx->cell);
+	afs_unuse_cell(ctx->net, ctx->cell, afs_cell_trace_unuse_fc);
 	key_put(ctx->key);
 	kfree(ctx);
 }
@@ -633,9 +635,7 @@ static int afs_init_fs_context(struct fs_context *fc)
 	ctx->net = afs_net(fc->net_ns);
 
 	/* Default to the workstation cell. */
-	rcu_read_lock();
-	cell = afs_lookup_cell_rcu(ctx->net, NULL, 0);
-	rcu_read_unlock();
+	cell = afs_find_cell(ctx->net, NULL, 0, afs_cell_trace_use_fc);
 	if (IS_ERR(cell))
 		cell = NULL;
 	ctx->cell = cell;
diff --git a/fs/afs/vl_alias.c b/fs/afs/vl_alias.c
index 5082ef04e99c..f04a80e4f5c3 100644
--- a/fs/afs/vl_alias.c
+++ b/fs/afs/vl_alias.c
@@ -177,7 +177,7 @@ static int afs_compare_cell_roots(struct afs_cell *cell)
 
 is_alias:
 	rcu_read_unlock();
-	cell->alias_of = afs_get_cell(p);
+	cell->alias_of = afs_use_cell(p, afs_cell_trace_use_alias);
 	return 1;
 }
 
@@ -247,18 +247,18 @@ static int afs_query_for_alias(struct afs_cell *cell, struct key *key)
 			continue;
 		if (p->root_volume)
 			continue; /* Ignore cells that have a root.cell volume. */
-		afs_get_cell(p);
+		afs_use_cell(p, afs_cell_trace_use_check_alias);
 		mutex_unlock(&cell->net->proc_cells_lock);
 
 		if (afs_query_for_alias_one(cell, key, p) != 0)
 			goto is_alias;
 
 		if (mutex_lock_interruptible(&cell->net->proc_cells_lock) < 0) {
-			afs_put_cell(cell->net, p);
+			afs_unuse_cell(cell->net, p, afs_cell_trace_unuse_check_alias);
 			return -ERESTARTSYS;
 		}
 
-		afs_put_cell(cell->net, p);
+		afs_unuse_cell(cell->net, p, afs_cell_trace_unuse_check_alias);
 	}
 
 	mutex_unlock(&cell->net->proc_cells_lock);
diff --git a/fs/afs/vl_rotate.c b/fs/afs/vl_rotate.c
index c0458c903b31..488e58490b16 100644
--- a/fs/afs/vl_rotate.c
+++ b/fs/afs/vl_rotate.c
@@ -45,7 +45,7 @@ static bool afs_start_vl_iteration(struct afs_vl_cursor *vc)
 	    cell->dns_expiry <= ktime_get_real_seconds()) {
 		dns_lookup_count = smp_load_acquire(&cell->dns_lookup_count);
 		set_bit(AFS_CELL_FL_DO_LOOKUP, &cell->flags);
-		queue_work(afs_wq, &cell->manager);
+		afs_queue_cell(cell, afs_cell_trace_get_queue_dns);
 
 		if (cell->dns_source == DNS_RECORD_UNAVAILABLE) {
 			if (wait_var_event_interruptible(
diff --git a/fs/afs/volume.c b/fs/afs/volume.c
index 9bc0509e3634..f84194b791d3 100644
--- a/fs/afs/volume.c
+++ b/fs/afs/volume.c
@@ -83,7 +83,7 @@ static struct afs_volume *afs_alloc_volume(struct afs_fs_context *params,
 
 	volume->vid		= vldb->vid[params->type];
 	volume->update_at	= ktime_get_real_seconds() + afs_volume_record_life;
-	volume->cell		= afs_get_cell(params->cell);
+	volume->cell		= afs_get_cell(params->cell, afs_cell_trace_get_vol);
 	volume->type		= params->type;
 	volume->type_force	= params->force;
 	volume->name_len	= vldb->name_len;
@@ -106,7 +106,7 @@ static struct afs_volume *afs_alloc_volume(struct afs_fs_context *params,
 	return volume;
 
 error_1:
-	afs_put_cell(params->net, volume->cell);
+	afs_put_cell(volume->cell, afs_cell_trace_put_vol);
 	kfree(volume);
 error_0:
 	return ERR_PTR(ret);
@@ -228,7 +228,7 @@ static void afs_destroy_volume(struct afs_net *net, struct afs_volume *volume)
 
 	afs_remove_volume_from_cell(volume);
 	afs_put_serverlist(net, rcu_access_pointer(volume->servers));
-	afs_put_cell(net, volume->cell);
+	afs_put_cell(volume->cell, afs_cell_trace_put_vol);
 	trace_afs_volume(volume->vid, atomic_read(&volume->usage),
 			 afs_volume_trace_free);
 	kfree_rcu(volume, rcu);