/*
 *  Copyright (C) 2008 Red Hat, Inc., Eric Paris <eparis@redhat.com>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2, or (at your option)
 *  any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; see the file COPYING.  If not, write to
 *  the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
 */

#include <linux/list.h>
#include <linux/mutex.h>
#include <linux/slab.h>
#include <linux/srcu.h>
#include <linux/rculist.h>
#include <linux/wait.h>

#include <linux/fsnotify_backend.h>
#include "fsnotify.h"

#include <asm/atomic.h>

/* protects writes to fsnotify_groups and fsnotify_mask */
static DEFINE_MUTEX(fsnotify_grp_mutex);
/* protects reads while running the fsnotify_groups list */
struct srcu_struct fsnotify_grp_srcu;
/* all groups registered to receive filesystem notifications */
LIST_HEAD(fsnotify_groups);
/* bitwise OR of all events (FS_*) interesting to some group on this system */
__u32 fsnotify_mask;

/*
 * When a new group registers or changes it's set of interesting events
 * this function updates the fsnotify_mask to contain all interesting events
 */
void fsnotify_recalc_global_mask(void)
{
        struct fsnotify_group *group;
        __u32 mask = 0;
        int idx;

        idx = srcu_read_lock(&fsnotify_grp_srcu);
        list_for_each_entry_rcu(group, &fsnotify_groups, group_list)
                mask |= group->mask;
        srcu_read_unlock(&fsnotify_grp_srcu, idx);
        fsnotify_mask = mask;
}

/*
 * Update the group->mask by running all of the marks associated with this
 * group and finding the bitwise | of all of the mark->mask.  If we change
 * the group->mask we need to update the global mask of events interesting
 * to the system.
 */
void fsnotify_recalc_group_mask(struct fsnotify_group *group)
{
        __u32 mask = 0;
        __u32 old_mask = group->mask;
        struct fsnotify_mark_entry *entry;

        spin_lock(&group->mark_lock);
        list_for_each_entry(entry, &group->mark_entries, g_list)
                mask |= entry->mask;
        spin_unlock(&group->mark_lock);

        group->mask = mask;

        if (old_mask != mask)
                fsnotify_recalc_global_mask();
}

/*
 * Take a reference to a group so things found under the fsnotify_grp_mutex
 * can't get freed under us
 */
static void fsnotify_get_group(struct fsnotify_group *group)
{
        atomic_inc(&group->refcnt);
}

/*
 * Final freeing of a group
 */
void fsnotify_final_destroy_group(struct fsnotify_group *group)
{
        /* clear the notification queue of all events */
        fsnotify_flush_notify(group);

        if (group->ops->free_group_priv)
                group->ops->free_group_priv(group);

        kfree(group);
}

/*
 * Trying to get rid of a group.  We need to first get rid of any outstanding
 * allocations and then free the group.  Remember that fsnotify_clear_marks_by_group
 * could miss marks that are being freed by inode and those marks could still
 * hold a reference to this group (via group->num_marks)  If we get into that
 * situtation, the fsnotify_final_destroy_group will get called when that final
 * mark is freed.
 */
static void fsnotify_destroy_group(struct fsnotify_group *group)
{
        /* clear all inode mark entries for this group */
        fsnotify_clear_marks_by_group(group);

        /* past the point of no return, matches the initial value of 1 */
        if (atomic_dec_and_test(&group->num_marks))
                fsnotify_final_destroy_group(group);
}

/*
 * Remove this group from the global list of groups that will get events
 * this can be done even if there are still references and things still using
 * this group.  This just stops the group from getting new events.
 */
static void __fsnotify_evict_group(struct fsnotify_group *group)
{
        BUG_ON(!mutex_is_locked(&fsnotify_grp_mutex));

        if (group->on_group_list)
                list_del_rcu(&group->group_list);
        group->on_group_list = 0;
}

/*
 * Called when a group is no longer interested in getting events.  This can be
 * used if a group is misbehaving or if for some reason a group should no longer
 * get any filesystem events.
 */
void fsnotify_evict_group(struct fsnotify_group *group)
{
        mutex_lock(&fsnotify_grp_mutex);
        __fsnotify_evict_group(group);
        mutex_unlock(&fsnotify_grp_mutex);
}

/*
 * Drop a reference to a group.  Free it if it's through.
 */
void fsnotify_put_group(struct fsnotify_group *group)
{
        if (!atomic_dec_and_mutex_lock(&group->refcnt, &fsnotify_grp_mutex))
                return;

        /*
         * OK, now we know that there's no other users *and* we hold mutex,
         * so no new references will appear
         */
        __fsnotify_evict_group(group);

        /*
         * now it's off the list, so the only thing we might care about is
         * srcu access....
         */
        mutex_unlock(&fsnotify_grp_mutex);
        synchronize_srcu(&fsnotify_grp_srcu);

        /* and now it is really dead. _Nothing_ could be seeing it */
        fsnotify_recalc_global_mask();
        fsnotify_destroy_group(group);
}

/*
 * Simply run the fsnotify_groups list and find a group which matches
 * the given parameters.  If a group is found we take a reference to that
 * group.
 */
static struct fsnotify_group *fsnotify_find_group(unsigned int group_num, __u32 mask,
                                                  const struct fsnotify_ops *ops)
{
        struct fsnotify_group *group_iter;
        struct fsnotify_group *group = NULL;

        BUG_ON(!mutex_is_locked(&fsnotify_grp_mutex));

        list_for_each_entry_rcu(group_iter, &fsnotify_groups, group_list) {
                if (group_iter->group_num == group_num) {
                        if ((group_iter->mask == mask) &&
                            (group_iter->ops == ops)) {
                                fsnotify_get_group(group_iter);
                                group = group_iter;
                        } else
                                group = ERR_PTR(-EEXIST);
                }
        }
        return group;
}

/*
 * Either finds an existing group which matches the group_num, mask, and ops or
 * creates a new group and adds it to the global group list.  In either case we
 * take a reference for the group returned.
 */
struct fsnotify_group *fsnotify_obtain_group(unsigned int group_num, __u32 mask,
                                             const struct fsnotify_ops *ops)
{
        struct fsnotify_group *group, *tgroup;

        /* very low use, simpler locking if we just always alloc */
        group = kmalloc(sizeof(struct fsnotify_group), GFP_KERNEL);
        if (!group)
                return ERR_PTR(-ENOMEM);

        atomic_set(&group->refcnt, 1);

        group->on_group_list = 0;
        group->group_num = group_num;
        group->mask = mask;

        mutex_init(&group->notification_mutex);
        INIT_LIST_HEAD(&group->notification_list);
        init_waitqueue_head(&group->notification_waitq);
        group->q_len = 0;
        group->max_events = UINT_MAX;

        spin_lock_init(&group->mark_lock);
        atomic_set(&group->num_marks, 0);
        INIT_LIST_HEAD(&group->mark_entries);

        group->ops = ops;

        mutex_lock(&fsnotify_grp_mutex);
        tgroup = fsnotify_find_group(group_num, mask, ops);
        if (tgroup) {
                /* group already exists */
                mutex_unlock(&fsnotify_grp_mutex);
                /* destroy the new one we made */
                fsnotify_put_group(group);
                return tgroup;
        }

        /* group not found, add a new one */
        list_add_rcu(&group->group_list, &fsnotify_groups);
        group->on_group_list = 1;
        /* being on the fsnotify_groups list holds one num_marks */
        atomic_inc(&group->num_marks);

        mutex_unlock(&fsnotify_grp_mutex);

        if (mask)
                fsnotify_recalc_global_mask();

        return group;
}