Merge tag 'kvm-4.15-1' of git://git.kernel.org/pub/scm/virt/kvm/kvm

[mirror_ubuntu-bionic-kernel.git] / fs / lockd / svclock.c

// SPDX-License-Identifier: GPL-2.0
/*
 * 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_xprt.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);

#if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
static const char *nlmdbg_cookie2a(const struct nlm_cookie *cookie)
{
        /*
         * We can get away with a static buffer because this is only called
         * from lockd, which is single-threaded.
         */
        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;

        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;
        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_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 */
        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);
}

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

/*
 * 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_inode(file->f_file)->i_sb->s_id,
                                file_inode(file->f_file)->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(SVC_NET(rqstp)) && !reclaim) {
                ret = nlm_lck_denied_grace_period;
                goto out;
        }
        if (reclaim && !locks_in_grace(SVC_NET(rqstp))) {
                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)
{
        int                     error;
        __be32                  ret;

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

        if (locks_in_grace(SVC_NET(rqstp))) {
                ret = nlm_lck_denied_grace_period;
                goto out;
        }

        error = vfs_test_lock(file->f_file, &lock->fl);
        if (error) {
                /* We can't currently deal with deferred test requests */
                if (error == FILE_LOCK_DEFERRED)
                        WARN_ON_ONCE(1);

                ret = nlm_lck_denied_nolocks;
                goto out;
        }

        if (lock->fl.fl_type == F_UNLCK) {
                ret = nlm_granted;
                goto out;
        }

        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;
        locks_release_private(&lock->fl);
        ret = nlm_lck_denied;
out:
        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 net *net, struct nlm_file *file, struct nlm_lock *lock)
{
        int     error;

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

        if (locks_in_grace(net))
                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 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, int result)
{
        block->b_flags |= B_GOT_CALLBACK;
        if (result == 0)
                block->b_granted = 1;
        else
                block->b_flags |= B_TIMED_OUT;
}

static int nlmsvc_grant_deferred(struct file_lock *fl, 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, 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;
}

/*
 * Since NLM uses two "keys" for tracking locks, we need to hash them down
 * to one for the blocked_hash. Here, we're just xor'ing the host address
 * with the pid in order to create a key value for picking a hash bucket.
 */
static unsigned long
nlmsvc_owner_key(struct file_lock *fl)
{
        return (unsigned long)fl->fl_owner ^ (unsigned long)fl->fl_pid;
}

const struct lock_manager_operations nlmsvc_lock_operations = {
        .lm_compare_owner = nlmsvc_same_owner,
        .lm_owner_key = nlmsvc_owner_key,
        .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;
        loff_t                  fl_start, fl_end;

        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 */
        /* vfs_lock_file() can mangle fl_start and fl_end, but we need
         * them unchanged for the GRANT_MSG
         */
        lock->fl.fl_flags |= FL_SLEEP;
        fl_start = lock->fl.fl_start;
        fl_end = lock->fl.fl_end;
        error = vfs_lock_file(file->f_file, F_SETLK, &lock->fl, NULL);
        lock->fl.fl_flags &= ~FL_SLEEP;
        lock->fl.fl_start = fl_start;
        lock->fl.fl_end = fl_end;

        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 (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;

        spin_lock(&nlm_blocked_lock);
        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;
                }
                spin_unlock(&nlm_blocked_lock);

                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);
                spin_lock(&nlm_blocked_lock);
        }
        spin_unlock(&nlm_blocked_lock);

        return timeout;
}