linux/fs/afs/cell.c

// SPDX-License-Identifier: GPL-2.0-or-later
/* AFS cell and server record management
 *
 * Copyright (C) 2002, 2017 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 */

#include <linux/slab.h>
#include <linux/key.h>
#include <linux/ctype.h>
#include <linux/dns_resolver.h>
#include <linux/sched.h>
#include <linux/inet.h>
#include <linux/namei.h>
#include <keys/rxrpc-type.h>
#include "internal.h"

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 void afs_manage_cell(struct work_struct *);

static void afs_dec_cells_outstanding(struct afs_net *net)
{
	if (atomic_dec_and_test(&net->cells_outstanding))
		wake_up_var(&net->cells_outstanding);
}

/*
 * Set the cell timer to fire after a given delay, assuming it's not already
 * set for an earlier time.
 */
static void afs_set_cell_timer(struct afs_net *net, time64_t delay)
{
	if (net->live) {
		atomic_inc(&net->cells_outstanding);
		if (timer_reduce(&net->cells_timer, jiffies + delay * HZ))
			afs_dec_cells_outstanding(net);
	}
}

/*
 * Look up and get an activation reference on a cell record under RCU
 * conditions.  The caller must hold the RCU read lock.
 */
struct afs_cell *afs_lookup_cell_rcu(struct afs_net *net,
				     const char *name, unsigned int namesz)
{
	struct afs_cell *cell = NULL;
	struct rb_node *p;
	int n, seq = 0, ret = 0;

	_enter("%*.*s", namesz, namesz, name);

	if (name && namesz == 0)
		return ERR_PTR(-EINVAL);
	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;
		}

		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;
		}

	} while (need_seqretry(&net->cells_lock, seq));

	done_seqretry(&net->cells_lock, seq);

	if (ret != 0 && cell)
		afs_put_cell(net, cell);

	return ret == 0 ? cell : ERR_PTR(ret);
}

/*
 * Set up a cell record and fill in its name, VL server address list and
 * allocate an anonymous key
 */
static struct afs_cell *afs_alloc_cell(struct afs_net *net,
				       const char *name, unsigned int namelen,
				       const char *addresses)
{
	struct afs_vlserver_list *vllist;
	struct afs_cell *cell;
	int i, ret;

	ASSERT(name);
	if (namelen == 0)
		return ERR_PTR(-EINVAL);
	if (namelen > AFS_MAXCELLNAME) {
		_leave(" = -ENAMETOOLONG");
		return ERR_PTR(-ENAMETOOLONG);
	}
	if (namelen == 5 && memcmp(name, "@cell", 5) == 0)
		return ERR_PTR(-EINVAL);

	_enter("%*.*s,%s", namelen, namelen, name, addresses);

	cell = kzalloc(sizeof(struct afs_cell), GFP_KERNEL);
	if (!cell) {
		_leave(" = -ENOMEM");
		return ERR_PTR(-ENOMEM);
	}

	cell->net = net;
	cell->name_len = namelen;
	for (i = 0; i < namelen; i++)
		cell->name[i] = tolower(name[i]);

	atomic_set(&cell->usage, 2);
	INIT_WORK(&cell->manager, afs_manage_cell);
	INIT_LIST_HEAD(&cell->proc_volumes);
	rwlock_init(&cell->proc_lock);
	rwlock_init(&cell->vl_servers_lock);

	/* Provide a VL server list, filling it in if we were given a list of
	 * addresses to use.
	 */
	if (addresses) {
		vllist = afs_parse_text_addrs(net,
					      addresses, strlen(addresses), ':',
					      VL_SERVICE, AFS_VL_PORT);
		if (IS_ERR(vllist)) {
			ret = PTR_ERR(vllist);
			goto parse_failed;
		}

		vllist->source = DNS_RECORD_FROM_CONFIG;
		vllist->status = DNS_LOOKUP_NOT_DONE;
		cell->dns_expiry = TIME64_MAX;
	} else {
		ret = -ENOMEM;
		vllist = afs_alloc_vlserver_list(0);
		if (!vllist)
			goto error;
		vllist->source = DNS_RECORD_UNAVAILABLE;
		vllist->status = DNS_LOOKUP_NOT_DONE;
		cell->dns_expiry = ktime_get_real_seconds();
	}

	rcu_assign_pointer(cell->vl_servers, vllist);

	cell->dns_source = vllist->source;
	cell->dns_status = vllist->status;
	smp_store_release(&cell->dns_lookup_count, 1); /* vs source/status */

	_leave(" = %p", cell);
	return cell;

parse_failed:
	if (ret == -EINVAL)
		printk(KERN_ERR "kAFS: bad VL server IP address\n");
error:
	kfree(cell);
	_leave(" = %d", ret);
	return ERR_PTR(ret);
}

/*
 * afs_lookup_cell - Look up or create a cell record.
 * @net:	The network namespace
 * @name:	The name of the cell.
 * @namesz:	The strlen of the cell name.
 * @vllist:	A colon/comma separated list of numeric IP addresses or NULL.
 * @excl:	T if an error should be given if the cell name already exists.
 *
 * Look up a cell record by name and query the DNS for VL server addresses if
 * needed.  Note that that actual DNS query is punted off to the manager thread
 * so that this function can return immediately if interrupted whilst allowing
 * cell records to be shared even if not yet fully constructed.
 */
struct afs_cell *afs_lookup_cell(struct afs_net *net,
				 const char *name, unsigned int namesz,
				 const char *vllist, bool excl)
{
	struct afs_cell *cell, *candidate, *cursor;
	struct rb_node *parent, **pp;
	enum afs_cell_state state;
	int ret, n;

	_enter("%s,%s", name, vllist);

	if (!excl) {
		rcu_read_lock();
		cell = afs_lookup_cell_rcu(net, name, namesz);
		rcu_read_unlock();
		if (!IS_ERR(cell))
			goto wait_for_cell;
	}

	/* Assume we're probably going to create a cell and preallocate and
	 * mostly set up a candidate record.  We can then use this to stash the
	 * name, the net namespace and VL server addresses.
	 *
	 * We also want to do this before we hold any locks as it may involve
	 * upcalling to userspace to make DNS queries.
	 */
	candidate = afs_alloc_cell(net, name, namesz, vllist);
	if (IS_ERR(candidate)) {
		_leave(" = %ld", PTR_ERR(candidate));
		return candidate;
	}

	/* Find the insertion point and check to see if someone else added a
	 * cell whilst we were allocating.
	 */
	write_seqlock(&net->cells_lock);

	pp = &net->cells.rb_node;
	parent = NULL;
	while (*pp) {
		parent = *pp;
		cursor = rb_entry(parent, struct afs_cell, net_node);

		n = strncasecmp(cursor->name, name,
				min_t(size_t, cursor->name_len, namesz));
		if (n == 0)
			n = cursor->name_len - namesz;
		if (n < 0)
			pp = &(*pp)->rb_left;
		else if (n > 0)
			pp = &(*pp)->rb_right;
		else
			goto cell_already_exists;
	}

	cell = candidate;
	candidate = NULL;
	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);

	queue_work(afs_wq, &cell->manager);

wait_for_cell:
	_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;
		       }));

	/* Check the state obtained from the wait check. */
	if (state == AFS_CELL_FAILED) {
		ret = cell->error;
		goto error;
	}

	_leave(" = %p [cell]", cell);
	return cell;

cell_already_exists:
	_debug("cell exists");
	cell = cursor;
	if (excl) {
		ret = -EEXIST;
	} else {
		afs_get_cell(cursor);
		ret = 0;
	}
	write_sequnlock(&net->cells_lock);
	kfree(candidate);
	if (ret == 0)
		goto wait_for_cell;
	goto error_noput;
error:
	afs_put_cell(net, cell);
error_noput:
	_leave(" = %d [error]", ret);
	return ERR_PTR(ret);
}

/*
 * set the root cell information
 * - can be called with a module parameter string
 * - can be called from a write to /proc/fs/afs/rootcell
 */
int afs_cell_init(struct afs_net *net, const char *rootcell)
{
	struct afs_cell *old_root, *new_root;
	const char *cp, *vllist;
	size_t len;

	_enter("");

	if (!rootcell) {
		/* module is loaded with no parameters, or built statically.
		 * - in the future we might initialize cell DB here.
		 */
		_leave(" = 0 [no root]");
		return 0;
	}

	cp = strchr(rootcell, ':');
	if (!cp) {
		_debug("kAFS: no VL server IP addresses specified");
		vllist = NULL;
		len = strlen(rootcell);
	} else {
		vllist = cp + 1;
		len = cp - rootcell;
	}

	/* allocate a cell record for the root cell */
	new_root = afs_lookup_cell(net, rootcell, len, vllist, false);
	if (IS_ERR(new_root)) {
		_leave(" = %ld", PTR_ERR(new_root));
		return PTR_ERR(new_root);
	}

	if (!test_and_set_bit(AFS_CELL_FL_NO_GC, &new_root->flags))
		afs_get_cell(new_root);

	/* 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);

	afs_put_cell(net, old_root);
	_leave(" = 0");
	return 0;
}

/*
 * Update a cell's VL server address list from the DNS.
 */
static int afs_update_cell(struct afs_cell *cell)
{
	struct afs_vlserver_list *vllist, *old = NULL, *p;
	unsigned int min_ttl = READ_ONCE(afs_cell_min_ttl);
	unsigned int max_ttl = READ_ONCE(afs_cell_max_ttl);
	time64_t now, expiry = 0;
	int ret = 0;

	_enter("%s", cell->name);

	vllist = afs_dns_query(cell, &expiry);
	if (IS_ERR(vllist)) {
		ret = PTR_ERR(vllist);

		_debug("%s: fail %d", cell->name, ret);
		if (ret == -ENOMEM)
			goto out_wake;

		ret = -ENOMEM;
		vllist = afs_alloc_vlserver_list(0);
		if (!vllist)
			goto out_wake;

		switch (ret) {
		case -ENODATA:
		case -EDESTADDRREQ:
			vllist->status = DNS_LOOKUP_GOT_NOT_FOUND;
			break;
		case -EAGAIN:
		case -ECONNREFUSED:
			vllist->status = DNS_LOOKUP_GOT_TEMP_FAILURE;
			break;
		default:
			vllist->status = DNS_LOOKUP_GOT_LOCAL_FAILURE;
			break;
		}
	}

	_debug("%s: got list %d %d", cell->name, vllist->source, vllist->status);
	cell->dns_status = vllist->status;

	now = ktime_get_real_seconds();
	if (min_ttl > max_ttl)
		max_ttl = min_ttl;
	if (expiry < now + min_ttl)
		expiry = now + min_ttl;
	else if (expiry > now + max_ttl)
		expiry = now + max_ttl;

	_debug("%s: status %d", cell->name, vllist->status);
	if (vllist->source == DNS_RECORD_UNAVAILABLE) {
		switch (vllist->status) {
		case DNS_LOOKUP_GOT_NOT_FOUND:
			/* The DNS said that the cell does not exist or there
			 * weren't any addresses to be had.
			 */
			cell->dns_expiry = expiry;
			break;

		case DNS_LOOKUP_BAD:
		case DNS_LOOKUP_GOT_LOCAL_FAILURE:
		case DNS_LOOKUP_GOT_TEMP_FAILURE:
		case DNS_LOOKUP_GOT_NS_FAILURE:
		default:
			cell->dns_expiry = now + 10;
			break;
		}
	} else {
		cell->dns_expiry = expiry;
	}

	/* Replace the VL server list if the new record has servers or the old
	 * record doesn't.
	 */
	write_lock(&cell->vl_servers_lock);
	p = rcu_dereference_protected(cell->vl_servers, true);
	if (vllist->nr_servers > 0 || p->nr_servers == 0) {
		rcu_assign_pointer(cell->vl_servers, vllist);
		cell->dns_source = vllist->source;
		old = p;
	}
	write_unlock(&cell->vl_servers_lock);
	afs_put_vlserverlist(cell->net, old);

out_wake:
	smp_store_release(&cell->dns_lookup_count,
			  cell->dns_lookup_count + 1); /* vs source/status */
	wake_up_var(&cell->dns_lookup_count);
	_leave(" = %d", ret);
	return ret;
}

/*
 * Destroy a cell record
 */
static void afs_cell_destroy(struct rcu_head *rcu)
{
	struct afs_cell *cell = container_of(rcu, struct afs_cell, rcu);

	_enter("%p{%s}", cell, cell->name);

	ASSERTCMP(atomic_read(&cell->usage), ==, 0);

	afs_put_vlserverlist(cell->net, rcu_access_pointer(cell->vl_servers));
	key_put(cell->anonymous_key);
	kfree(cell);

	_leave(" [destroyed]");
}

/*
 * Queue the cell manager.
 */
static void afs_queue_cell_manager(struct afs_net *net)
{
	int outstanding = atomic_inc_return(&net->cells_outstanding);

	_enter("%d", outstanding);

	if (!queue_work(afs_wq, &net->cells_manager))
		afs_dec_cells_outstanding(net);
}

/*
 * Cell management timer.  We have an increment on cells_outstanding that we
 * need to pass along to the work item.
 */
void afs_cells_timer(struct timer_list *timer)
{
	struct afs_net *net = container_of(timer, struct afs_net, cells_timer);

	_enter("");
	if (!queue_work(afs_wq, &net->cells_manager))
		afs_dec_cells_outstanding(net);
}

/*
 * Get a reference on a cell record.
 */
struct afs_cell *afs_get_cell(struct afs_cell *cell)
{
	atomic_inc(&cell->usage);
	return cell;
}

/*
 * Drop a reference on a cell record.
 */
void afs_put_cell(struct afs_net *net, struct afs_cell *cell)
{
	time64_t now, expire_delay;

	if (!cell)
		return;

	_enter("%s", cell->name);

	now = ktime_get_real_seconds();
	cell->last_inactive = now;
	expire_delay = 0;
	if (cell->vl_servers->nr_servers)
		expire_delay = afs_cell_gc_delay;

	if (atomic_dec_return(&cell->usage) > 1)
		return;

	/* 'cell' may now be garbage collected. */
	afs_set_cell_timer(net, expire_delay);
}

/*
 * Allocate a key to use as a placeholder for anonymous user security.
 */
static int afs_alloc_anon_key(struct afs_cell *cell)
{
	struct key *key;
	char keyname[4 + AFS_MAXCELLNAME + 1], *cp, *dp;

	/* Create a key to represent an anonymous user. */
	memcpy(keyname, "afs@", 4);
	dp = keyname + 4;
	cp = cell->name;
	do {
		*dp++ = tolower(*cp);
	} while (*cp++);

	key = rxrpc_get_null_key(keyname);
	if (IS_ERR(key))
		return PTR_ERR(key);

	cell->anonymous_key = key;

	_debug("anon key %p{%x}",
	       cell->anonymous_key, key_serial(cell->anonymous_key));
	return 0;
}

/*
 * Activate a cell.
 */
static int afs_activate_cell(struct afs_net *net, struct afs_cell *cell)
{
	struct hlist_node **p;
	struct afs_cell *pcell;
	int ret;

	if (!cell->anonymous_key) {
		ret = afs_alloc_anon_key(cell);
		if (ret < 0)
			return ret;
	}

#ifdef CONFIG_AFS_FSCACHE
	cell->cache = fscache_acquire_cookie(afs_cache_netfs.primary_index,
					     &afs_cell_cache_index_def,
					     cell->name, strlen(cell->name),
					     NULL, 0,
					     cell, 0, true);
#endif
	ret = afs_proc_cell_setup(cell);
	if (ret < 0)
		return ret;

	mutex_lock(&net->proc_cells_lock);
	for (p = &net->proc_cells.first; *p; p = &(*p)->next) {
		pcell = hlist_entry(*p, struct afs_cell, proc_link);
		if (strcmp(cell->name, pcell->name) < 0)
			break;
	}

	cell->proc_link.pprev = p;
	cell->proc_link.next = *p;
	rcu_assign_pointer(*p, &cell->proc_link.next);
	if (cell->proc_link.next)
		cell->proc_link.next->pprev = &cell->proc_link.next;

	afs_dynroot_mkdir(net, cell);
	mutex_unlock(&net->proc_cells_lock);
	return 0;
}

/*
 * Deactivate a cell.
 */
static void afs_deactivate_cell(struct afs_net *net, struct afs_cell *cell)
{
	_enter("%s", cell->name);

	afs_proc_cell_remove(cell);

	mutex_lock(&net->proc_cells_lock);
	hlist_del_rcu(&cell->proc_link);
	afs_dynroot_rmdir(net, cell);
	mutex_unlock(&net->proc_cells_lock);

#ifdef CONFIG_AFS_FSCACHE
	fscache_relinquish_cookie(cell->cache, NULL, false);
	cell->cache = NULL;
#endif

	_leave("");
}

/*
 * Manage a cell record, initialising and destroying it, maintaining its DNS
 * records.
 */
static void afs_manage_cell(struct work_struct *work)
{
	struct afs_cell *cell = container_of(work, struct afs_cell, manager);
	struct afs_net *net = cell->net;
	bool deleted;
	int ret, usage;

	_enter("%s", cell->name);

again:
	_debug("state %u", cell->state);
	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)
			rb_erase(&cell->net_node, &net->cells);
		write_sequnlock(&net->cells_lock);
		if (deleted)
			goto final_destruction;
		if (cell->state == AFS_CELL_FAILED)
			goto done;
		smp_store_release(&cell->state, AFS_CELL_UNSET);
		wake_up_var(&cell->state);
		goto again;

	case AFS_CELL_UNSET:
		smp_store_release(&cell->state, AFS_CELL_ACTIVATING);
		wake_up_var(&cell->state);
		goto again;

	case AFS_CELL_ACTIVATING:
		ret = afs_activate_cell(net, cell);
		if (ret < 0)
			goto activation_failed;

		smp_store_release(&cell->state, AFS_CELL_ACTIVE);
		wake_up_var(&cell->state);
		goto again;

	case AFS_CELL_ACTIVE:
		if (atomic_read(&cell->usage) > 1) {
			if (test_and_clear_bit(AFS_CELL_FL_DO_LOOKUP, &cell->flags)) {
				ret = afs_update_cell(cell);
				if (ret < 0)
					cell->error = ret;
			}
			goto done;
		}
		smp_store_release(&cell->state, AFS_CELL_DEACTIVATING);
		wake_up_var(&cell->state);
		goto again;

	case AFS_CELL_DEACTIVATING:
		if (atomic_read(&cell->usage) > 1)
			goto reverse_deactivation;
		afs_deactivate_cell(net, cell);
		smp_store_release(&cell->state, AFS_CELL_INACTIVE);
		wake_up_var(&cell->state);
		goto again;

	default:
		break;
	}
	_debug("bad state %u", cell->state);
	BUG(); /* Unhandled state */

activation_failed:
	cell->error = ret;
	afs_deactivate_cell(net, cell);

	smp_store_release(&cell->state, AFS_CELL_FAILED); /* vs error */
	wake_up_var(&cell->state);
	goto again;

reverse_deactivation:
	smp_store_release(&cell->state, AFS_CELL_ACTIVE);
	wake_up_var(&cell->state);
	_leave(" [deact->act]");
	return;

done:
	_leave(" [done %u]", cell->state);
	return;

final_destruction:
	call_rcu(&cell->rcu, afs_cell_destroy);
	afs_dec_cells_outstanding(net);
	_leave(" [destruct %d]", atomic_read(&net->cells_outstanding));
}

/*
 * Manage the records of cells known to a network namespace.  This includes
 * updating the DNS records and garbage collecting unused cells that were
 * automatically added.
 *
 * Note that constructed cell records may only be removed from net->cells by
 * this work item, so it is safe for this work item to stash a cursor pointing
 * into the tree and then return to caller (provided it skips cells that are
 * still under construction).
 *
 * Note also that we were given an increment on net->cells_outstanding by
 * whoever queued us that we need to deal with before returning.
 */
void afs_manage_cells(struct work_struct *work)
{
	struct afs_net *net = container_of(work, struct afs_net, cells_manager);
	struct rb_node *cursor;
	time64_t now = ktime_get_real_seconds(), next_manage = TIME64_MAX;
	bool purging = !net->live;

	_enter("");

	/* Trawl the cell database looking for cells that have expired from
	 * lack of use and cells whose DNS results have expired and dispatch
	 * their managers.
	 */
	read_seqlock_excl(&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;
		bool sched_cell = false;

		usage = atomic_read(&cell->usage);
		_debug("manage %s %u", cell->name, usage);

		ASSERTCMP(usage, >=, 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 (usage == 1) {
			struct afs_vlserver_list *vllist;
			time64_t expire_at = cell->last_inactive;

			read_lock(&cell->vl_servers_lock);
			vllist = rcu_dereference_protected(
				cell->vl_servers,
				lockdep_is_held(&cell->vl_servers_lock));
			if (vllist->nr_servers > 0)
				expire_at += afs_cell_gc_delay;
			read_unlock(&cell->vl_servers_lock);
			if (purging || expire_at <= now)
				sched_cell = true;
			else if (expire_at < next_manage)
				next_manage = expire_at;
		}

		if (!purging) {
			if (test_bit(AFS_CELL_FL_DO_LOOKUP, &cell->flags))
				sched_cell = true;
		}

		if (sched_cell)
			queue_work(afs_wq, &cell->manager);
	}

	read_sequnlock_excl(&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
	 * the work scheduler.
	 */
	if (!purging && next_manage < TIME64_MAX) {
		now = ktime_get_real_seconds();

		if (next_manage - now <= 0) {
			if (queue_work(afs_wq, &net->cells_manager))
				atomic_inc(&net->cells_outstanding);
		} else {
			afs_set_cell_timer(net, next_manage - now);
		}
	}

	afs_dec_cells_outstanding(net);
	_leave(" [%d]", atomic_read(&net->cells_outstanding));
}

/*
 * Purge in-memory cell database.
 */
void afs_cell_purge(struct afs_net *net)
{
	struct afs_cell *ws;

	_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);

	_debug("del timer");
	if (del_timer_sync(&net->cells_timer))
		atomic_dec(&net->cells_outstanding);

	_debug("kick mgr");
	afs_queue_cell_manager(net);

	_debug("wait");
	wait_var_event(&net->cells_outstanding,
		       !atomic_read(&net->cells_outstanding));
	_leave("");
}