Documentation: rw_lock lessons learned

[wandboard.git] / fs / notify / notification.c

/*
 *  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.
 */

/*
 * Basic idea behind the notification queue: An fsnotify group (like inotify)
 * sends the userspace notification about events asyncronously some time after
 * the event happened.  When inotify gets an event it will need to add that
 * event to the group notify queue.  Since a single event might need to be on
 * multiple group's notification queues we can't add the event directly to each
 * queue and instead add a small "event_holder" to each queue.  This event_holder
 * has a pointer back to the original event.  Since the majority of events are
 * going to end up on one, and only one, notification queue we embed one
 * event_holder into each event.  This means we have a single allocation instead
 * of always needing two.  If the embedded event_holder is already in use by
 * another group a new event_holder (from fsnotify_event_holder_cachep) will be
 * allocated and used.
 */

#include <linux/fs.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/list.h>
#include <linux/module.h>
#include <linux/mount.h>
#include <linux/mutex.h>
#include <linux/namei.h>
#include <linux/path.h>
#include <linux/slab.h>
#include <linux/spinlock.h>

#include <asm/atomic.h>

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

static struct kmem_cache *fsnotify_event_cachep;
static struct kmem_cache *fsnotify_event_holder_cachep;
/*
 * This is a magic event we send when the q is too full.  Since it doesn't
 * hold real event information we just keep one system wide and use it any time
 * it is needed.  It's refcnt is set 1 at kernel init time and will never
 * get set to 0 so it will never get 'freed'
 */
static struct fsnotify_event q_overflow_event;
static atomic_t fsnotify_sync_cookie = ATOMIC_INIT(0);

/**
 * fsnotify_get_cookie - return a unique cookie for use in synchronizing events.
 * Called from fsnotify_move, which is inlined into filesystem modules.
 */
u32 fsnotify_get_cookie(void)
{
        return atomic_inc_return(&fsnotify_sync_cookie);
}
EXPORT_SYMBOL_GPL(fsnotify_get_cookie);

/* return true if the notify queue is empty, false otherwise */
bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group)
{
        BUG_ON(!mutex_is_locked(&group->notification_mutex));
        return list_empty(&group->notification_list) ? true : false;
}

void fsnotify_get_event(struct fsnotify_event *event)
{
        atomic_inc(&event->refcnt);
}

void fsnotify_put_event(struct fsnotify_event *event)
{
        if (!event)
                return;

        if (atomic_dec_and_test(&event->refcnt)) {
                if (event->data_type == FSNOTIFY_EVENT_PATH)
                        path_put(&event->path);

                BUG_ON(!list_empty(&event->private_data_list));

                kfree(event->file_name);
                kmem_cache_free(fsnotify_event_cachep, event);
        }
}

struct fsnotify_event_holder *fsnotify_alloc_event_holder(void)
{
        return kmem_cache_alloc(fsnotify_event_holder_cachep, GFP_KERNEL);
}

void fsnotify_destroy_event_holder(struct fsnotify_event_holder *holder)
{
        kmem_cache_free(fsnotify_event_holder_cachep, holder);
}

/*
 * Find the private data that the group previously attached to this event when
 * the group added the event to the notification queue (fsnotify_add_notify_event)
 */
struct fsnotify_event_private_data *fsnotify_remove_priv_from_event(struct fsnotify_group *group, struct fsnotify_event *event)
{
        struct fsnotify_event_private_data *lpriv;
        struct fsnotify_event_private_data *priv = NULL;

        assert_spin_locked(&event->lock);

        list_for_each_entry(lpriv, &event->private_data_list, event_list) {
                if (lpriv->group == group) {
                        priv = lpriv;
                        list_del(&priv->event_list);
                        break;
                }
        }
        return priv;
}

/*
 * Check if 2 events contain the same information.  We do not compare private data
 * but at this moment that isn't a problem for any know fsnotify listeners.
 */
static bool event_compare(struct fsnotify_event *old, struct fsnotify_event *new)
{
        if ((old->mask == new->mask) &&
            (old->to_tell == new->to_tell) &&
            (old->data_type == new->data_type) &&
            (old->name_len == new->name_len)) {
                switch (old->data_type) {
                case (FSNOTIFY_EVENT_INODE):
                        /* remember, after old was put on the wait_q we aren't
                         * allowed to look at the inode any more, only thing
                         * left to check was if the file_name is the same */
                        if (!old->name_len ||
                            !strcmp(old->file_name, new->file_name))
                                return true;
                        break;
                case (FSNOTIFY_EVENT_PATH):
                        if ((old->path.mnt == new->path.mnt) &&
                            (old->path.dentry == new->path.dentry))
                                return true;
                        break;
                case (FSNOTIFY_EVENT_NONE):
                        if (old->mask & FS_Q_OVERFLOW)
                                return true;
                        else if (old->mask & FS_IN_IGNORED)
                                return false;
                        return false;
                };
        }
        return false;
}

/*
 * Add an event to the group notification queue.  The group can later pull this
 * event off the queue to deal with.  If the event is successfully added to the
 * group's notification queue, a reference is taken on event.
 */
int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_event *event,
                              struct fsnotify_event_private_data *priv)
{
        struct fsnotify_event_holder *holder = NULL;
        struct list_head *list = &group->notification_list;
        struct fsnotify_event_holder *last_holder;
        struct fsnotify_event *last_event;
        int ret = 0;

        /*
         * There is one fsnotify_event_holder embedded inside each fsnotify_event.
         * Check if we expect to be able to use that holder.  If not alloc a new
         * holder.
         * For the overflow event it's possible that something will use the in
         * event holder before we get the lock so we may need to jump back and
         * alloc a new holder, this can't happen for most events...
         */
        if (!list_empty(&event->holder.event_list)) {
alloc_holder:
                holder = fsnotify_alloc_event_holder();
                if (!holder)
                        return -ENOMEM;
        }

        mutex_lock(&group->notification_mutex);

        if (group->q_len >= group->max_events) {
                event = &q_overflow_event;
                ret = -EOVERFLOW;
                /* sorry, no private data on the overflow event */
                priv = NULL;
        }

        spin_lock(&event->lock);

        if (list_empty(&event->holder.event_list)) {
                if (unlikely(holder))
                        fsnotify_destroy_event_holder(holder);
                holder = &event->holder;
        } else if (unlikely(!holder)) {
                /* between the time we checked above and got the lock the in
                 * event holder was used, go back and get a new one */
                spin_unlock(&event->lock);
                mutex_unlock(&group->notification_mutex);
                goto alloc_holder;
        }

        if (!list_empty(list)) {
                last_holder = list_entry(list->prev, struct fsnotify_event_holder, event_list);
                last_event = last_holder->event;
                if (event_compare(last_event, event)) {
                        spin_unlock(&event->lock);
                        mutex_unlock(&group->notification_mutex);
                        if (holder != &event->holder)
                                fsnotify_destroy_event_holder(holder);
                        return -EEXIST;
                }
        }

        group->q_len++;
        holder->event = event;

        fsnotify_get_event(event);
        list_add_tail(&holder->event_list, list);
        if (priv)
                list_add_tail(&priv->event_list, &event->private_data_list);
        spin_unlock(&event->lock);
        mutex_unlock(&group->notification_mutex);

        wake_up(&group->notification_waitq);
        return ret;
}

/*
 * Remove and return the first event from the notification list.  There is a
 * reference held on this event since it was on the list.  It is the responsibility
 * of the caller to drop this reference.
 */
struct fsnotify_event *fsnotify_remove_notify_event(struct fsnotify_group *group)
{
        struct fsnotify_event *event;
        struct fsnotify_event_holder *holder;

        BUG_ON(!mutex_is_locked(&group->notification_mutex));

        holder = list_first_entry(&group->notification_list, struct fsnotify_event_holder, event_list);

        event = holder->event;

        spin_lock(&event->lock);
        holder->event = NULL;
        list_del_init(&holder->event_list);
        spin_unlock(&event->lock);

        /* event == holder means we are referenced through the in event holder */
        if (holder != &event->holder)
                fsnotify_destroy_event_holder(holder);

        group->q_len--;

        return event;
}

/*
 * This will not remove the event, that must be done with fsnotify_remove_notify_event()
 */
struct fsnotify_event *fsnotify_peek_notify_event(struct fsnotify_group *group)
{
        struct fsnotify_event *event;
        struct fsnotify_event_holder *holder;

        BUG_ON(!mutex_is_locked(&group->notification_mutex));

        holder = list_first_entry(&group->notification_list, struct fsnotify_event_holder, event_list);
        event = holder->event;

        return event;
}

/*
 * Called when a group is being torn down to clean up any outstanding
 * event notifications.
 */
void fsnotify_flush_notify(struct fsnotify_group *group)
{
        struct fsnotify_event *event;
        struct fsnotify_event_private_data *priv;

        mutex_lock(&group->notification_mutex);
        while (!fsnotify_notify_queue_is_empty(group)) {
                event = fsnotify_remove_notify_event(group);
                /* if they don't implement free_event_priv they better not have attached any */
                if (group->ops->free_event_priv) {
                        spin_lock(&event->lock);
                        priv = fsnotify_remove_priv_from_event(group, event);
                        spin_unlock(&event->lock);
                        if (priv)
                                group->ops->free_event_priv(priv);
                }
                fsnotify_put_event(event); /* matches fsnotify_add_notify_event */
        }
        mutex_unlock(&group->notification_mutex);
}

static void initialize_event(struct fsnotify_event *event)
{
        event->holder.event = NULL;
        INIT_LIST_HEAD(&event->holder.event_list);
        atomic_set(&event->refcnt, 1);

        spin_lock_init(&event->lock);

        event->path.dentry = NULL;
        event->path.mnt = NULL;
        event->inode = NULL;
        event->data_type = FSNOTIFY_EVENT_NONE;

        INIT_LIST_HEAD(&event->private_data_list);

        event->to_tell = NULL;

        event->file_name = NULL;
        event->name_len = 0;

        event->sync_cookie = 0;
}

/*
 * fsnotify_create_event - Allocate a new event which will be sent to each
 * group's handle_event function if the group was interested in this
 * particular event.
 *
 * @to_tell the inode which is supposed to receive the event (sometimes a
 *      parent of the inode to which the event happened.
 * @mask what actually happened.
 * @data pointer to the object which was actually affected
 * @data_type flag indication if the data is a file, path, inode, nothing...
 * @name the filename, if available
 */
struct fsnotify_event *fsnotify_create_event(struct inode *to_tell, __u32 mask, void *data,
                                             int data_type, const char *name, u32 cookie,
                                             gfp_t gfp)
{
        struct fsnotify_event *event;

        event = kmem_cache_alloc(fsnotify_event_cachep, gfp);
        if (!event)
                return NULL;

        initialize_event(event);

        if (name) {
                event->file_name = kstrdup(name, gfp);
                if (!event->file_name) {
                        kmem_cache_free(fsnotify_event_cachep, event);
                        return NULL;
                }
                event->name_len = strlen(event->file_name);
        }

        event->sync_cookie = cookie;
        event->to_tell = to_tell;

        switch (data_type) {
        case FSNOTIFY_EVENT_FILE: {
                struct file *file = data;
                struct path *path = &file->f_path;
                event->path.dentry = path->dentry;
                event->path.mnt = path->mnt;
                path_get(&event->path);
                event->data_type = FSNOTIFY_EVENT_PATH;
                break;
        }
        case FSNOTIFY_EVENT_PATH: {
                struct path *path = data;
                event->path.dentry = path->dentry;
                event->path.mnt = path->mnt;
                path_get(&event->path);
                event->data_type = FSNOTIFY_EVENT_PATH;
                break;
        }
        case FSNOTIFY_EVENT_INODE:
                event->inode = data;
                event->data_type = FSNOTIFY_EVENT_INODE;
                break;
        case FSNOTIFY_EVENT_NONE:
                event->inode = NULL;
                event->path.dentry = NULL;
                event->path.mnt = NULL;
                break;
        default:
                BUG();
        }

        event->mask = mask;

        return event;
}

__init int fsnotify_notification_init(void)
{
        fsnotify_event_cachep = KMEM_CACHE(fsnotify_event, SLAB_PANIC);
        fsnotify_event_holder_cachep = KMEM_CACHE(fsnotify_event_holder, SLAB_PANIC);

        initialize_event(&q_overflow_event);
        q_overflow_event.mask = FS_Q_OVERFLOW;

        return 0;
}
subsys_initcall(fsnotify_notification_init);