xref: /openbmc/linux/fs/lockd/svclock.c (revision 1eb4c977)

/*
 * linux/fs/lockd/svclock.c
 *
 * Handling of server-side locks, mostly of the blocked variety.
 * This is the ugliest part of lockd because we tread on very thin ice.
 * GRANT and CANCEL calls may get stuck, meet in mid-flight, etc.
 * IMNSHO introducing the grant callback into the NLM protocol was one
 * of the worst ideas Sun ever had. Except maybe for the idea of doing
 * NFS file locking at all.
 *
 * I'm trying hard to avoid race conditions by protecting most accesses
 * to a file's list of blocked locks through a semaphore. The global
 * list of blocked locks is not protected in this fashion however.
 * Therefore, some functions (such as the RPC callback for the async grant
 * call) move blocked locks towards the head of the list *while some other
 * process might be traversing it*. This should not be a problem in
 * practice, because this will only cause functions traversing the list
 * to visit some blocks twice.
 *
 * Copyright (C) 1996, Olaf Kirch <okir@monad.swb.de>
 */

#include <linux/types.h>
#include <linux/slab.h>
#include <linux/errno.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/sunrpc/clnt.h>
#include <linux/sunrpc/svc.h>
#include <linux/lockd/nlm.h>
#include <linux/lockd/lockd.h>
#include <linux/kthread.h>

#define NLMDBG_FACILITY		NLMDBG_SVCLOCK

#ifdef CONFIG_LOCKD_V4
#define nlm_deadlock	nlm4_deadlock
#else
#define nlm_deadlock	nlm_lck_denied
#endif

static void nlmsvc_release_block(struct nlm_block *block);
static void	nlmsvc_insert_block(struct nlm_block *block, unsigned long);
static void	nlmsvc_remove_block(struct nlm_block *block);

static int nlmsvc_setgrantargs(struct nlm_rqst *call, struct nlm_lock *lock);
static void nlmsvc_freegrantargs(struct nlm_rqst *call);
static const struct rpc_call_ops nlmsvc_grant_ops;

/*
 * The list of blocked locks to retry
 */
static LIST_HEAD(nlm_blocked);
static DEFINE_SPINLOCK(nlm_blocked_lock);

#ifdef LOCKD_DEBUG
static const char *nlmdbg_cookie2a(const struct nlm_cookie *cookie)
{
	/*
	 * We can get away with a static buffer because we're only
	 * called with BKL held.
	 */
	static char buf[2*NLM_MAXCOOKIELEN+1];
	unsigned int i, len = sizeof(buf);
	char *p = buf;

	len--;	/* allow for trailing \0 */
	if (len < 3)
		return "???";
	for (i = 0 ; i < cookie->len ; i++) {
		if (len < 2) {
			strcpy(p-3, "...");
			break;
		}
		sprintf(p, "%02x", cookie->data[i]);
		p += 2;
		len -= 2;
	}
	*p = '\0';

	return buf;
}
#endif

/*
 * Insert a blocked lock into the global list
 */
static void
nlmsvc_insert_block_locked(struct nlm_block *block, unsigned long when)
{
	struct nlm_block *b;
	struct list_head *pos;

	dprintk("lockd: nlmsvc_insert_block(%p, %ld)\n", block, when);
	if (list_empty(&block->b_list)) {
		kref_get(&block->b_count);
	} else {
		list_del_init(&block->b_list);
	}

	pos = &nlm_blocked;
	if (when != NLM_NEVER) {
		if ((when += jiffies) == NLM_NEVER)
			when ++;
		list_for_each(pos, &nlm_blocked) {
			b = list_entry(pos, struct nlm_block, b_list);
			if (time_after(b->b_when,when) || b->b_when == NLM_NEVER)
				break;
		}
		/* On normal exit from the loop, pos == &nlm_blocked,
		 * so we will be adding to the end of the list - good
		 */
	}

	list_add_tail(&block->b_list, pos);
	block->b_when = when;
}

static void nlmsvc_insert_block(struct nlm_block *block, unsigned long when)
{
	spin_lock(&nlm_blocked_lock);
	nlmsvc_insert_block_locked(block, when);
	spin_unlock(&nlm_blocked_lock);
}

/*
 * Remove a block from the global list
 */
static inline void
nlmsvc_remove_block(struct nlm_block *block)
{
	if (!list_empty(&block->b_list)) {
		spin_lock(&nlm_blocked_lock);
		list_del_init(&block->b_list);
		spin_unlock(&nlm_blocked_lock);
		nlmsvc_release_block(block);
	}
}

/*
 * Find a block for a given lock
 */
static struct nlm_block *
nlmsvc_lookup_block(struct nlm_file *file, struct nlm_lock *lock)
{
	struct nlm_block	*block;
	struct file_lock	*fl;

	dprintk("lockd: nlmsvc_lookup_block f=%p pd=%d %Ld-%Ld ty=%d\n",
				file, lock->fl.fl_pid,
				(long long)lock->fl.fl_start,
				(long long)lock->fl.fl_end, lock->fl.fl_type);
	list_for_each_entry(block, &nlm_blocked, b_list) {
		fl = &block->b_call->a_args.lock.fl;
		dprintk("lockd: check f=%p pd=%d %Ld-%Ld ty=%d cookie=%s\n",
				block->b_file, fl->fl_pid,
				(long long)fl->fl_start,
				(long long)fl->fl_end, fl->fl_type,
				nlmdbg_cookie2a(&block->b_call->a_args.cookie));
		if (block->b_file == file && nlm_compare_locks(fl, &lock->fl)) {
			kref_get(&block->b_count);
			return block;
		}
	}

	return NULL;
}

static inline int nlm_cookie_match(struct nlm_cookie *a, struct nlm_cookie *b)
{
	if (a->len != b->len)
		return 0;
	if (memcmp(a->data, b->data, a->len))
		return 0;
	return 1;
}

/*
 * Find a block with a given NLM cookie.
 */
static inline struct nlm_block *
nlmsvc_find_block(struct nlm_cookie *cookie)
{
	struct nlm_block *block;

	list_for_each_entry(block, &nlm_blocked, b_list) {
		if (nlm_cookie_match(&block->b_call->a_args.cookie,cookie))
			goto found;
	}

	return NULL;

found:
	dprintk("nlmsvc_find_block(%s): block=%p\n", nlmdbg_cookie2a(cookie), block);
	kref_get(&block->b_count);
	return block;
}

/*
 * Create a block and initialize it.
 *
 * Note: we explicitly set the cookie of the grant reply to that of
 * the blocked lock request. The spec explicitly mentions that the client
 * should _not_ rely on the callback containing the same cookie as the
 * request, but (as I found out later) that's because some implementations
 * do just this. Never mind the standards comittees, they support our
 * logging industries.
 *
 * 10 years later: I hope we can safely ignore these old and broken
 * clients by now. Let's fix this so we can uniquely identify an incoming
 * GRANTED_RES message by cookie, without having to rely on the client's IP
 * address. --okir
 */
static struct nlm_block *
nlmsvc_create_block(struct svc_rqst *rqstp, struct nlm_host *host,
		    struct nlm_file *file, struct nlm_lock *lock,
		    struct nlm_cookie *cookie)
{
	struct nlm_block	*block;
	struct nlm_rqst		*call = NULL;

	nlm_get_host(host);
	call = nlm_alloc_call(host);
	if (call == NULL)
		return NULL;

	/* Allocate memory for block, and initialize arguments */
	block = kzalloc(sizeof(*block), GFP_KERNEL);
	if (block == NULL)
		goto failed;
	kref_init(&block->b_count);
	INIT_LIST_HEAD(&block->b_list);
	INIT_LIST_HEAD(&block->b_flist);

	if (!nlmsvc_setgrantargs(call, lock))
		goto failed_free;

	/* Set notifier function for VFS, and init args */
	call->a_args.lock.fl.fl_flags |= FL_SLEEP;
	call->a_args.lock.fl.fl_lmops = &nlmsvc_lock_operations;
	nlmclnt_next_cookie(&call->a_args.cookie);

	dprintk("lockd: created block %p...\n", block);

	/* Create and initialize the block */
	block->b_daemon = rqstp->rq_server;
	block->b_host   = host;
	block->b_file   = file;
	block->b_fl = NULL;
	file->f_count++;

	/* Add to file's list of blocks */
	list_add(&block->b_flist, &file->f_blocks);

	/* Set up RPC arguments for callback */
	block->b_call = call;
	call->a_flags   = RPC_TASK_ASYNC;
	call->a_block = block;

	return block;

failed_free:
	kfree(block);
failed:
	nlmsvc_release_call(call);
	return NULL;
}

/*
 * Delete a block.
 * It is the caller's responsibility to check whether the file
 * can be closed hereafter.
 */
static int nlmsvc_unlink_block(struct nlm_block *block)
{
	int status;
	dprintk("lockd: unlinking block %p...\n", block);

	/* Remove block from list */
	status = posix_unblock_lock(block->b_file->f_file, &block->b_call->a_args.lock.fl);
	nlmsvc_remove_block(block);
	return status;
}

static void nlmsvc_free_block(struct kref *kref)
{
	struct nlm_block *block = container_of(kref, struct nlm_block, b_count);
	struct nlm_file		*file = block->b_file;

	dprintk("lockd: freeing block %p...\n", block);

	/* Remove block from file's list of blocks */
	mutex_lock(&file->f_mutex);
	list_del_init(&block->b_flist);
	mutex_unlock(&file->f_mutex);

	nlmsvc_freegrantargs(block->b_call);
	nlmsvc_release_call(block->b_call);
	nlm_release_file(block->b_file);
	kfree(block->b_fl);
	kfree(block);
}

static void nlmsvc_release_block(struct nlm_block *block)
{
	if (block != NULL)
		kref_put(&block->b_count, nlmsvc_free_block);
}

/*
 * Loop over all blocks and delete blocks held by
 * a matching host.
 */
void nlmsvc_traverse_blocks(struct nlm_host *host,
			struct nlm_file *file,
			nlm_host_match_fn_t match)
{
	struct nlm_block *block, *next;

restart:
	mutex_lock(&file->f_mutex);
	list_for_each_entry_safe(block, next, &file->f_blocks, b_flist) {
		if (!match(block->b_host, host))
			continue;
		/* Do not destroy blocks that are not on
		 * the global retry list - why? */
		if (list_empty(&block->b_list))
			continue;
		kref_get(&block->b_count);
		mutex_unlock(&file->f_mutex);
		nlmsvc_unlink_block(block);
		nlmsvc_release_block(block);
		goto restart;
	}
	mutex_unlock(&file->f_mutex);
}

/*
 * Initialize arguments for GRANTED call. The nlm_rqst structure
 * has been cleared already.
 */
static int nlmsvc_setgrantargs(struct nlm_rqst *call, struct nlm_lock *lock)
{
	locks_copy_lock(&call->a_args.lock.fl, &lock->fl);
	memcpy(&call->a_args.lock.fh, &lock->fh, sizeof(call->a_args.lock.fh));
	call->a_args.lock.caller = utsname()->nodename;
	call->a_args.lock.oh.len = lock->oh.len;

	/* set default data area */
	call->a_args.lock.oh.data = call->a_owner;
	call->a_args.lock.svid = lock->fl.fl_pid;

	if (lock->oh.len > NLMCLNT_OHSIZE) {
		void *data = kmalloc(lock->oh.len, GFP_KERNEL);
		if (!data)
			return 0;
		call->a_args.lock.oh.data = (u8 *) data;
	}

	memcpy(call->a_args.lock.oh.data, lock->oh.data, lock->oh.len);
	return 1;
}

static void nlmsvc_freegrantargs(struct nlm_rqst *call)
{
	if (call->a_args.lock.oh.data != call->a_owner)
		kfree(call->a_args.lock.oh.data);

	locks_release_private(&call->a_args.lock.fl);
}

/*
 * Deferred lock request handling for non-blocking lock
 */
static __be32
nlmsvc_defer_lock_rqst(struct svc_rqst *rqstp, struct nlm_block *block)
{
	__be32 status = nlm_lck_denied_nolocks;

	block->b_flags |= B_QUEUED;

	nlmsvc_insert_block(block, NLM_TIMEOUT);

	block->b_cache_req = &rqstp->rq_chandle;
	if (rqstp->rq_chandle.defer) {
		block->b_deferred_req =
			rqstp->rq_chandle.defer(block->b_cache_req);
		if (block->b_deferred_req != NULL)
			status = nlm_drop_reply;
	}
	dprintk("lockd: nlmsvc_defer_lock_rqst block %p flags %d status %d\n",
		block, block->b_flags, ntohl(status));

	return status;
}

/*
 * Attempt to establish a lock, and if it can't be granted, block it
 * if required.
 */
__be32
nlmsvc_lock(struct svc_rqst *rqstp, struct nlm_file *file,
	    struct nlm_host *host, struct nlm_lock *lock, int wait,
	    struct nlm_cookie *cookie, int reclaim)
{
	struct nlm_block	*block = NULL;
	int			error;
	__be32			ret;

	dprintk("lockd: nlmsvc_lock(%s/%ld, ty=%d, pi=%d, %Ld-%Ld, bl=%d)\n",
				file->f_file->f_path.dentry->d_inode->i_sb->s_id,
				file->f_file->f_path.dentry->d_inode->i_ino,
				lock->fl.fl_type, lock->fl.fl_pid,
				(long long)lock->fl.fl_start,
				(long long)lock->fl.fl_end,
				wait);

	/* Lock file against concurrent access */
	mutex_lock(&file->f_mutex);
	/* Get existing block (in case client is busy-waiting)
	 * or create new block
	 */
	block = nlmsvc_lookup_block(file, lock);
	if (block == NULL) {
		block = nlmsvc_create_block(rqstp, host, file, lock, cookie);
		ret = nlm_lck_denied_nolocks;
		if (block == NULL)
			goto out;
		lock = &block->b_call->a_args.lock;
	} else
		lock->fl.fl_flags &= ~FL_SLEEP;

	if (block->b_flags & B_QUEUED) {
		dprintk("lockd: nlmsvc_lock deferred block %p flags %d\n",
							block, block->b_flags);
		if (block->b_granted) {
			nlmsvc_unlink_block(block);
			ret = nlm_granted;
			goto out;
		}
		if (block->b_flags & B_TIMED_OUT) {
			nlmsvc_unlink_block(block);
			ret = nlm_lck_denied;
			goto out;
		}
		ret = nlm_drop_reply;
		goto out;
	}

	if (locks_in_grace() && !reclaim) {
		ret = nlm_lck_denied_grace_period;
		goto out;
	}
	if (reclaim && !locks_in_grace()) {
		ret = nlm_lck_denied_grace_period;
		goto out;
	}

	if (!wait)
		lock->fl.fl_flags &= ~FL_SLEEP;
	error = vfs_lock_file(file->f_file, F_SETLK, &lock->fl, NULL);
	lock->fl.fl_flags &= ~FL_SLEEP;

	dprintk("lockd: vfs_lock_file returned %d\n", error);
	switch (error) {
		case 0:
			ret = nlm_granted;
			goto out;
		case -EAGAIN:
			/*
			 * If this is a blocking request for an
			 * already pending lock request then we need
			 * to put it back on lockd's block list
			 */
			if (wait)
				break;
			ret = nlm_lck_denied;
			goto out;
		case FILE_LOCK_DEFERRED:
			if (wait)
				break;
			/* Filesystem lock operation is in progress
			   Add it to the queue waiting for callback */
			ret = nlmsvc_defer_lock_rqst(rqstp, block);
			goto out;
		case -EDEADLK:
			ret = nlm_deadlock;
			goto out;
		default:			/* includes ENOLCK */
			ret = nlm_lck_denied_nolocks;
			goto out;
	}

	ret = nlm_lck_blocked;

	/* Append to list of blocked */
	nlmsvc_insert_block(block, NLM_NEVER);
out:
	mutex_unlock(&file->f_mutex);
	nlmsvc_release_block(block);
	dprintk("lockd: nlmsvc_lock returned %u\n", ret);
	return ret;
}

/*
 * Test for presence of a conflicting lock.
 */
__be32
nlmsvc_testlock(struct svc_rqst *rqstp, struct nlm_file *file,
		struct nlm_host *host, struct nlm_lock *lock,
		struct nlm_lock *conflock, struct nlm_cookie *cookie)
{
	struct nlm_block 	*block = NULL;
	int			error;
	__be32			ret;

	dprintk("lockd: nlmsvc_testlock(%s/%ld, ty=%d, %Ld-%Ld)\n",
				file->f_file->f_path.dentry->d_inode->i_sb->s_id,
				file->f_file->f_path.dentry->d_inode->i_ino,
				lock->fl.fl_type,
				(long long)lock->fl.fl_start,
				(long long)lock->fl.fl_end);

	/* Get existing block (in case client is busy-waiting) */
	block = nlmsvc_lookup_block(file, lock);

	if (block == NULL) {
		struct file_lock *conf = kzalloc(sizeof(*conf), GFP_KERNEL);

		if (conf == NULL)
			return nlm_granted;
		block = nlmsvc_create_block(rqstp, host, file, lock, cookie);
		if (block == NULL) {
			kfree(conf);
			return nlm_granted;
		}
		block->b_fl = conf;
	}
	if (block->b_flags & B_QUEUED) {
		dprintk("lockd: nlmsvc_testlock deferred block %p flags %d fl %p\n",
			block, block->b_flags, block->b_fl);
		if (block->b_flags & B_TIMED_OUT) {
			nlmsvc_unlink_block(block);
			ret = nlm_lck_denied;
			goto out;
		}
		if (block->b_flags & B_GOT_CALLBACK) {
			nlmsvc_unlink_block(block);
			if (block->b_fl != NULL
					&& block->b_fl->fl_type != F_UNLCK) {
				lock->fl = *block->b_fl;
				goto conf_lock;
			} else {
				ret = nlm_granted;
				goto out;
			}
		}
		ret = nlm_drop_reply;
		goto out;
	}

	if (locks_in_grace()) {
		ret = nlm_lck_denied_grace_period;
		goto out;
	}
	error = vfs_test_lock(file->f_file, &lock->fl);
	if (error == FILE_LOCK_DEFERRED) {
		ret = nlmsvc_defer_lock_rqst(rqstp, block);
		goto out;
	}
	if (error) {
		ret = nlm_lck_denied_nolocks;
		goto out;
	}
	if (lock->fl.fl_type == F_UNLCK) {
		ret = nlm_granted;
		goto out;
	}

conf_lock:
	dprintk("lockd: conflicting lock(ty=%d, %Ld-%Ld)\n",
		lock->fl.fl_type, (long long)lock->fl.fl_start,
		(long long)lock->fl.fl_end);
	conflock->caller = "somehost";	/* FIXME */
	conflock->len = strlen(conflock->caller);
	conflock->oh.len = 0;		/* don't return OH info */
	conflock->svid = lock->fl.fl_pid;
	conflock->fl.fl_type = lock->fl.fl_type;
	conflock->fl.fl_start = lock->fl.fl_start;
	conflock->fl.fl_end = lock->fl.fl_end;
	ret = nlm_lck_denied;
out:
	if (block)
		nlmsvc_release_block(block);
	return ret;
}

/*
 * Remove a lock.
 * This implies a CANCEL call: We send a GRANT_MSG, the client replies
 * with a GRANT_RES call which gets lost, and calls UNLOCK immediately
 * afterwards. In this case the block will still be there, and hence
 * must be removed.
 */
__be32
nlmsvc_unlock(struct nlm_file *file, struct nlm_lock *lock)
{
	int	error;

	dprintk("lockd: nlmsvc_unlock(%s/%ld, pi=%d, %Ld-%Ld)\n",
				file->f_file->f_path.dentry->d_inode->i_sb->s_id,
				file->f_file->f_path.dentry->d_inode->i_ino,
				lock->fl.fl_pid,
				(long long)lock->fl.fl_start,
				(long long)lock->fl.fl_end);

	/* First, cancel any lock that might be there */
	nlmsvc_cancel_blocked(file, lock);

	lock->fl.fl_type = F_UNLCK;
	error = vfs_lock_file(file->f_file, F_SETLK, &lock->fl, NULL);

	return (error < 0)? nlm_lck_denied_nolocks : nlm_granted;
}

/*
 * Cancel a previously blocked request.
 *
 * A cancel request always overrides any grant that may currently
 * be in progress.
 * The calling procedure must check whether the file can be closed.
 */
__be32
nlmsvc_cancel_blocked(struct nlm_file *file, struct nlm_lock *lock)
{
	struct nlm_block	*block;
	int status = 0;

	dprintk("lockd: nlmsvc_cancel(%s/%ld, pi=%d, %Ld-%Ld)\n",
				file->f_file->f_path.dentry->d_inode->i_sb->s_id,
				file->f_file->f_path.dentry->d_inode->i_ino,
				lock->fl.fl_pid,
				(long long)lock->fl.fl_start,
				(long long)lock->fl.fl_end);

	if (locks_in_grace())
		return nlm_lck_denied_grace_period;

	mutex_lock(&file->f_mutex);
	block = nlmsvc_lookup_block(file, lock);
	mutex_unlock(&file->f_mutex);
	if (block != NULL) {
		vfs_cancel_lock(block->b_file->f_file,
				&block->b_call->a_args.lock.fl);
		status = nlmsvc_unlink_block(block);
		nlmsvc_release_block(block);
	}
	return status ? nlm_lck_denied : nlm_granted;
}

/*
 * This is a callback from the filesystem for VFS file lock requests.
 * It will be used if lm_grant is defined and the filesystem can not
 * respond to the request immediately.
 * For GETLK request it will copy the reply to the nlm_block.
 * For SETLK or SETLKW request it will get the local posix lock.
 * In all cases it will move the block to the head of nlm_blocked q where
 * nlmsvc_retry_blocked() can send back a reply for SETLKW or revisit the
 * deferred rpc for GETLK and SETLK.
 */
static void
nlmsvc_update_deferred_block(struct nlm_block *block, struct file_lock *conf,
			     int result)
{
	block->b_flags |= B_GOT_CALLBACK;
	if (result == 0)
		block->b_granted = 1;
	else
		block->b_flags |= B_TIMED_OUT;
	if (conf) {
		if (block->b_fl)
			__locks_copy_lock(block->b_fl, conf);
	}
}

static int nlmsvc_grant_deferred(struct file_lock *fl, struct file_lock *conf,
					int result)
{
	struct nlm_block *block;
	int rc = -ENOENT;

	spin_lock(&nlm_blocked_lock);
	list_for_each_entry(block, &nlm_blocked, b_list) {
		if (nlm_compare_locks(&block->b_call->a_args.lock.fl, fl)) {
			dprintk("lockd: nlmsvc_notify_blocked block %p flags %d\n",
							block, block->b_flags);
			if (block->b_flags & B_QUEUED) {
				if (block->b_flags & B_TIMED_OUT) {
					rc = -ENOLCK;
					break;
				}
				nlmsvc_update_deferred_block(block, conf, result);
			} else if (result == 0)
				block->b_granted = 1;

			nlmsvc_insert_block_locked(block, 0);
			svc_wake_up(block->b_daemon);
			rc = 0;
			break;
		}
	}
	spin_unlock(&nlm_blocked_lock);
	if (rc == -ENOENT)
		printk(KERN_WARNING "lockd: grant for unknown block\n");
	return rc;
}

/*
 * Unblock a blocked lock request. This is a callback invoked from the
 * VFS layer when a lock on which we blocked is removed.
 *
 * This function doesn't grant the blocked lock instantly, but rather moves
 * the block to the head of nlm_blocked where it can be picked up by lockd.
 */
static void
nlmsvc_notify_blocked(struct file_lock *fl)
{
	struct nlm_block	*block;

	dprintk("lockd: VFS unblock notification for block %p\n", fl);
	spin_lock(&nlm_blocked_lock);
	list_for_each_entry(block, &nlm_blocked, b_list) {
		if (nlm_compare_locks(&block->b_call->a_args.lock.fl, fl)) {
			nlmsvc_insert_block_locked(block, 0);
			spin_unlock(&nlm_blocked_lock);
			svc_wake_up(block->b_daemon);
			return;
		}
	}
	spin_unlock(&nlm_blocked_lock);
	printk(KERN_WARNING "lockd: notification for unknown block!\n");
}

static int nlmsvc_same_owner(struct file_lock *fl1, struct file_lock *fl2)
{
	return fl1->fl_owner == fl2->fl_owner && fl1->fl_pid == fl2->fl_pid;
}

const struct lock_manager_operations nlmsvc_lock_operations = {
	.lm_compare_owner = nlmsvc_same_owner,
	.lm_notify = nlmsvc_notify_blocked,
	.lm_grant = nlmsvc_grant_deferred,
};

/*
 * Try to claim a lock that was previously blocked.
 *
 * Note that we use both the RPC_GRANTED_MSG call _and_ an async
 * RPC thread when notifying the client. This seems like overkill...
 * Here's why:
 *  -	we don't want to use a synchronous RPC thread, otherwise
 *	we might find ourselves hanging on a dead portmapper.
 *  -	Some lockd implementations (e.g. HP) don't react to
 *	RPC_GRANTED calls; they seem to insist on RPC_GRANTED_MSG calls.
 */
static void
nlmsvc_grant_blocked(struct nlm_block *block)
{
	struct nlm_file		*file = block->b_file;
	struct nlm_lock		*lock = &block->b_call->a_args.lock;
	int			error;

	dprintk("lockd: grant blocked lock %p\n", block);

	kref_get(&block->b_count);

	/* Unlink block request from list */
	nlmsvc_unlink_block(block);

	/* If b_granted is true this means we've been here before.
	 * Just retry the grant callback, possibly refreshing the RPC
	 * binding */
	if (block->b_granted) {
		nlm_rebind_host(block->b_host);
		goto callback;
	}

	/* Try the lock operation again */
	lock->fl.fl_flags |= FL_SLEEP;
	error = vfs_lock_file(file->f_file, F_SETLK, &lock->fl, NULL);
	lock->fl.fl_flags &= ~FL_SLEEP;

	switch (error) {
	case 0:
		break;
	case FILE_LOCK_DEFERRED:
		dprintk("lockd: lock still blocked error %d\n", error);
		nlmsvc_insert_block(block, NLM_NEVER);
		nlmsvc_release_block(block);
		return;
	default:
		printk(KERN_WARNING "lockd: unexpected error %d in %s!\n",
				-error, __func__);
		nlmsvc_insert_block(block, 10 * HZ);
		nlmsvc_release_block(block);
		return;
	}

callback:
	/* Lock was granted by VFS. */
	dprintk("lockd: GRANTing blocked lock.\n");
	block->b_granted = 1;

	/* keep block on the list, but don't reattempt until the RPC
	 * completes or the submission fails
	 */
	nlmsvc_insert_block(block, NLM_NEVER);

	/* Call the client -- use a soft RPC task since nlmsvc_retry_blocked
	 * will queue up a new one if this one times out
	 */
	error = nlm_async_call(block->b_call, NLMPROC_GRANTED_MSG,
				&nlmsvc_grant_ops);

	/* RPC submission failed, wait a bit and retry */
	if (error < 0)
		nlmsvc_insert_block(block, 10 * HZ);
}

/*
 * This is the callback from the RPC layer when the NLM_GRANTED_MSG
 * RPC call has succeeded or timed out.
 * Like all RPC callbacks, it is invoked by the rpciod process, so it
 * better not sleep. Therefore, we put the blocked lock on the nlm_blocked
 * chain once more in order to have it removed by lockd itself (which can
 * then sleep on the file semaphore without disrupting e.g. the nfs client).
 */
static void nlmsvc_grant_callback(struct rpc_task *task, void *data)
{
	struct nlm_rqst		*call = data;
	struct nlm_block	*block = call->a_block;
	unsigned long		timeout;

	dprintk("lockd: GRANT_MSG RPC callback\n");

	spin_lock(&nlm_blocked_lock);
	/* if the block is not on a list at this point then it has
	 * been invalidated. Don't try to requeue it.
	 *
	 * FIXME: it's possible that the block is removed from the list
	 * after this check but before the nlmsvc_insert_block. In that
	 * case it will be added back. Perhaps we need better locking
	 * for nlm_blocked?
	 */
	if (list_empty(&block->b_list))
		goto out;

	/* Technically, we should down the file semaphore here. Since we
	 * move the block towards the head of the queue only, no harm
	 * can be done, though. */
	if (task->tk_status < 0) {
		/* RPC error: Re-insert for retransmission */
		timeout = 10 * HZ;
	} else {
		/* Call was successful, now wait for client callback */
		timeout = 60 * HZ;
	}
	nlmsvc_insert_block_locked(block, timeout);
	svc_wake_up(block->b_daemon);
out:
	spin_unlock(&nlm_blocked_lock);
}

/*
 * FIXME: nlmsvc_release_block() grabs a mutex.  This is not allowed for an
 * .rpc_release rpc_call_op
 */
static void nlmsvc_grant_release(void *data)
{
	struct nlm_rqst		*call = data;
	nlmsvc_release_block(call->a_block);
}

static const struct rpc_call_ops nlmsvc_grant_ops = {
	.rpc_call_done = nlmsvc_grant_callback,
	.rpc_release = nlmsvc_grant_release,
};

/*
 * We received a GRANT_RES callback. Try to find the corresponding
 * block.
 */
void
nlmsvc_grant_reply(struct nlm_cookie *cookie, __be32 status)
{
	struct nlm_block	*block;

	dprintk("grant_reply: looking for cookie %x, s=%d \n",
		*(unsigned int *)(cookie->data), status);
	if (!(block = nlmsvc_find_block(cookie)))
		return;

	if (block) {
		if (status == nlm_lck_denied_grace_period) {
			/* Try again in a couple of seconds */
			nlmsvc_insert_block(block, 10 * HZ);
		} else {
			/* Lock is now held by client, or has been rejected.
			 * In both cases, the block should be removed. */
			nlmsvc_unlink_block(block);
		}
	}
	nlmsvc_release_block(block);
}

/* Helper function to handle retry of a deferred block.
 * If it is a blocking lock, call grant_blocked.
 * For a non-blocking lock or test lock, revisit the request.
 */
static void
retry_deferred_block(struct nlm_block *block)
{
	if (!(block->b_flags & B_GOT_CALLBACK))
		block->b_flags |= B_TIMED_OUT;
	nlmsvc_insert_block(block, NLM_TIMEOUT);
	dprintk("revisit block %p flags %d\n",	block, block->b_flags);
	if (block->b_deferred_req) {
		block->b_deferred_req->revisit(block->b_deferred_req, 0);
		block->b_deferred_req = NULL;
	}
}

/*
 * Retry all blocked locks that have been notified. This is where lockd
 * picks up locks that can be granted, or grant notifications that must
 * be retransmitted.
 */
unsigned long
nlmsvc_retry_blocked(void)
{
	unsigned long	timeout = MAX_SCHEDULE_TIMEOUT;
	struct nlm_block *block;

	while (!list_empty(&nlm_blocked) && !kthread_should_stop()) {
		block = list_entry(nlm_blocked.next, struct nlm_block, b_list);

		if (block->b_when == NLM_NEVER)
			break;
		if (time_after(block->b_when, jiffies)) {
			timeout = block->b_when - jiffies;
			break;
		}

		dprintk("nlmsvc_retry_blocked(%p, when=%ld)\n",
			block, block->b_when);
		if (block->b_flags & B_QUEUED) {
			dprintk("nlmsvc_retry_blocked delete block (%p, granted=%d, flags=%d)\n",
				block, block->b_granted, block->b_flags);
			retry_deferred_block(block);
		} else
			nlmsvc_grant_blocked(block);
	}

	return timeout;
}