fs/notify/notification.c

   1 /*
   2  *  Copyright (C) 2008 Red Hat, Inc., Eric Paris <eparis@redhat.com>
   3  *
   4  *  This program is free software; you can redistribute it and/or modify
   5  *  it under the terms of the GNU General Public License as published by
   6  *  the Free Software Foundation; either version 2, or (at your option)
   7  *  any later version.
   8  *
   9  *  This program is distributed in the hope that it will be useful,
  10  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  *  GNU General Public License for more details.
  13  *
  14  *  You should have received a copy of the GNU General Public License
  15  *  along with this program; see the file COPYING.  If not, write to
  16  *  the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
  17  */
  18
  19 /*
  20  * Basic idea behind the notification queue: An fsnotify group (like inotify)
  21  * sends the userspace notification about events asyncronously some time after
  22  * the event happened.  When inotify gets an event it will need to add that
  23  * event to the group notify queue.  Since a single event might need to be on
  24  * multiple group's notification queues we can't add the event directly to each
  25  * queue and instead add a small "event_holder" to each queue.  This event_holder
  26  * has a pointer back to the original event.  Since the majority of events are
  27  * going to end up on one, and only one, notification queue we embed one
  28  * event_holder into each event.  This means we have a single allocation instead
  29  * of always needing two.  If the embedded event_holder is already in use by
  30  * another group a new event_holder (from fsnotify_event_holder_cachep) will be
  31  * allocated and used.
  32  */
  33
  34 #include <linux/fs.h>
  35 #include <linux/init.h>
  36 #include <linux/kernel.h>
  37 #include <linux/list.h>
  38 #include <linux/module.h>
  39 #include <linux/mount.h>
  40 #include <linux/mutex.h>
  41 #include <linux/namei.h>
  42 #include <linux/path.h>
  43 #include <linux/slab.h>
  44 #include <linux/spinlock.h>
  45
  46 #include <asm/atomic.h>
  47
  48 #include <linux/fsnotify_backend.h>
  49 #include "fsnotify.h"
  50
  51 static struct kmem_cache *fsnotify_event_cachep;
  52 static struct kmem_cache *fsnotify_event_holder_cachep;
  53 /*
  54  * This is a magic event we send when the q is too full.  Since it doesn't
  55  * hold real event information we just keep one system wide and use it any time
  56  * it is needed.  It's refcnt is set 1 at kernel init time and will never
  57  * get set to 0 so it will never get 'freed'
  58  */
  59 static struct fsnotify_event q_overflow_event;
  60 static atomic_t fsnotify_sync_cookie = ATOMIC_INIT(0);
  61
  62 /**
  63  * fsnotify_get_cookie - return a unique cookie for use in synchronizing events.
  64  * Called from fsnotify_move, which is inlined into filesystem modules.
  65  */
  66 u32 fsnotify_get_cookie(void)
  67 {
  68         return atomic_inc_return(&fsnotify_sync_cookie);
  69 }
  70 EXPORT_SYMBOL_GPL(fsnotify_get_cookie);
  71
  72 /* return true if the notify queue is empty, false otherwise */
  73 bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group)
  74 {
  75         BUG_ON(!mutex_is_locked(&group->notification_mutex));
  76         return list_empty(&group->notification_list) ? true : false;
  77 }
  78
  79 void fsnotify_get_event(struct fsnotify_event *event)
  80 {
  81         atomic_inc(&event->refcnt);
  82 }
  83
  84 void fsnotify_put_event(struct fsnotify_event *event)
  85 {
  86         if (!event)
  87                 return;
  88
  89         if (atomic_dec_and_test(&event->refcnt)) {
  90                 if (event->data_type == FSNOTIFY_EVENT_PATH)
  91                         path_put(&event->path);
  92
  93                 BUG_ON(!list_empty(&event->private_data_list));
  94
  95                 kfree(event->file_name);
  96                 kmem_cache_free(fsnotify_event_cachep, event);
  97         }
  98 }
  99
 100 struct fsnotify_event_holder *fsnotify_alloc_event_holder(void)
 101 {
 102         return kmem_cache_alloc(fsnotify_event_holder_cachep, GFP_KERNEL);
 103 }
 104
 105 void fsnotify_destroy_event_holder(struct fsnotify_event_holder *holder)
 106 {
 107         kmem_cache_free(fsnotify_event_holder_cachep, holder);
 108 }
 109
 110 /*
 111  * Find the private data that the group previously attached to this event when
 112  * the group added the event to the notification queue (fsnotify_add_notify_event)
 113  */
 114 struct fsnotify_event_private_data *fsnotify_remove_priv_from_event(struct fsnotify_group *group, struct fsnotify_event *event)
 115 {
 116         struct fsnotify_event_private_data *lpriv;
 117         struct fsnotify_event_private_data *priv = NULL;
 118
 119         assert_spin_locked(&event->lock);
 120
 121         list_for_each_entry(lpriv, &event->private_data_list, event_list) {
 122                 if (lpriv->group == group) {
 123                         priv = lpriv;
 124                         list_del(&priv->event_list);
 125                         break;
 126                 }
 127         }
 128         return priv;
 129 }
 130
 131 /*
 132  * Check if 2 events contain the same information.  We do not compare private data
 133  * but at this moment that isn't a problem for any know fsnotify listeners.
 134  */
 135 static bool event_compare(struct fsnotify_event *old, struct fsnotify_event *new)
 136 {
 137         if ((old->mask == new->mask) &&
 138             (old->to_tell == new->to_tell) &&
 139             (old->data_type == new->data_type)) {
 140                 switch (old->data_type) {
 141                 case (FSNOTIFY_EVENT_INODE):
 142                         if (old->inode == new->inode)
 143                                 return true;
 144                         break;
 145                 case (FSNOTIFY_EVENT_PATH):
 146                         if ((old->path.mnt == new->path.mnt) &&
 147                             (old->path.dentry == new->path.dentry))
 148                                 return true;
 149                 case (FSNOTIFY_EVENT_NONE):
 150                         return true;
 151                 };
 152         }
 153         return false;
 154 }
 155
 156 /*
 157  * Add an event to the group notification queue.  The group can later pull this
 158  * event off the queue to deal with.  If the event is successfully added to the
 159  * group's notification queue, a reference is taken on event.
 160  */
 161 int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_event *event,
 162                               struct fsnotify_event_private_data *priv)
 163 {
 164         struct fsnotify_event_holder *holder = NULL;
 165         struct list_head *list = &group->notification_list;
 166         struct fsnotify_event_holder *last_holder;
 167         struct fsnotify_event *last_event;
 168
 169         /* easy to tell if priv was attached to the event */
 170         INIT_LIST_HEAD(&priv->event_list);
 171
 172         /*
 173          * There is one fsnotify_event_holder embedded inside each fsnotify_event.
 174          * Check if we expect to be able to use that holder.  If not alloc a new
 175          * holder.
 176          * For the overflow event it's possible that something will use the in
 177          * event holder before we get the lock so we may need to jump back and
 178          * alloc a new holder, this can't happen for most events...
 179          */
 180         if (!list_empty(&event->holder.event_list)) {
 181 alloc_holder:
 182                 holder = fsnotify_alloc_event_holder();
 183                 if (!holder)
 184                         return -ENOMEM;
 185         }
 186
 187         mutex_lock(&group->notification_mutex);
 188
 189         if (group->q_len >= group->max_events) {
 190                 event = &q_overflow_event;
 191                 /* sorry, no private data on the overflow event */
 192                 priv = NULL;
 193         }
 194
 195         spin_lock(&event->lock);
 196
 197         if (list_empty(&event->holder.event_list)) {
 198                 if (unlikely(holder))
 199                         fsnotify_destroy_event_holder(holder);
 200                 holder = &event->holder;
 201         } else if (unlikely(!holder)) {
 202                 /* between the time we checked above and got the lock the in
 203                  * event holder was used, go back and get a new one */
 204                 spin_unlock(&event->lock);
 205                 mutex_unlock(&group->notification_mutex);
 206                 goto alloc_holder;
 207         }
 208
 209         if (!list_empty(list)) {
 210                 last_holder = list_entry(list->prev, struct fsnotify_event_holder, event_list);
 211                 last_event = last_holder->event;
 212                 if (event_compare(last_event, event)) {
 213                         spin_unlock(&event->lock);
 214                         mutex_unlock(&group->notification_mutex);
 215                         if (holder != &event->holder)
 216                                 fsnotify_destroy_event_holder(holder);
 217                         return -EEXIST;
 218                 }
 219         }
 220
 221         group->q_len++;
 222         holder->event = event;
 223
 224         fsnotify_get_event(event);
 225         list_add_tail(&holder->event_list, list);
 226         if (priv)
 227                 list_add_tail(&priv->event_list, &event->private_data_list);
 228         spin_unlock(&event->lock);
 229         mutex_unlock(&group->notification_mutex);
 230
 231         wake_up(&group->notification_waitq);
 232         return 0;
 233 }
 234
 235 /*
 236  * Remove and return the first event from the notification list.  There is a
 237  * reference held on this event since it was on the list.  It is the responsibility
 238  * of the caller to drop this reference.
 239  */
 240 struct fsnotify_event *fsnotify_remove_notify_event(struct fsnotify_group *group)
 241 {
 242         struct fsnotify_event *event;
 243         struct fsnotify_event_holder *holder;
 244
 245         BUG_ON(!mutex_is_locked(&group->notification_mutex));
 246
 247         holder = list_first_entry(&group->notification_list, struct fsnotify_event_holder, event_list);
 248
 249         event = holder->event;
 250
 251         spin_lock(&event->lock);
 252         holder->event = NULL;
 253         list_del_init(&holder->event_list);
 254         spin_unlock(&event->lock);
 255
 256         /* event == holder means we are referenced through the in event holder */
 257         if (holder != &event->holder)
 258                 fsnotify_destroy_event_holder(holder);
 259
 260         group->q_len--;
 261
 262         return event;
 263 }
 264
 265 /*
 266  * This will not remove the event, that must be done with fsnotify_remove_notify_event()
 267  */
 268 struct fsnotify_event *fsnotify_peek_notify_event(struct fsnotify_group *group)
 269 {
 270         struct fsnotify_event *event;
 271         struct fsnotify_event_holder *holder;
 272
 273         BUG_ON(!mutex_is_locked(&group->notification_mutex));
 274
 275         holder = list_first_entry(&group->notification_list, struct fsnotify_event_holder, event_list);
 276         event = holder->event;
 277
 278         return event;
 279 }
 280
 281 /*
 282  * Called when a group is being torn down to clean up any outstanding
 283  * event notifications.
 284  */
 285 void fsnotify_flush_notify(struct fsnotify_group *group)
 286 {
 287         struct fsnotify_event *event;
 288         struct fsnotify_event_private_data *priv;
 289
 290         mutex_lock(&group->notification_mutex);
 291         while (!fsnotify_notify_queue_is_empty(group)) {
 292                 event = fsnotify_remove_notify_event(group);
 293                 /* if they don't implement free_event_priv they better not have attached any */
 294                 if (group->ops->free_event_priv) {
 295                         spin_lock(&event->lock);
 296                         priv = fsnotify_remove_priv_from_event(group, event);
 297                         spin_unlock(&event->lock);
 298                         if (priv)
 299                                 group->ops->free_event_priv(priv);
 300                 }
 301                 fsnotify_put_event(event); /* matches fsnotify_add_notify_event */
 302         }
 303         mutex_unlock(&group->notification_mutex);
 304 }
 305
 306 static void initialize_event(struct fsnotify_event *event)
 307 {
 308         event->holder.event = NULL;
 309         INIT_LIST_HEAD(&event->holder.event_list);
 310         atomic_set(&event->refcnt, 1);
 311
 312         spin_lock_init(&event->lock);
 313
 314         event->path.dentry = NULL;
 315         event->path.mnt = NULL;
 316         event->inode = NULL;
 317         event->data_type = FSNOTIFY_EVENT_NONE;
 318
 319         INIT_LIST_HEAD(&event->private_data_list);
 320
 321         event->to_tell = NULL;
 322
 323         event->file_name = NULL;
 324         event->name_len = 0;
 325
 326         event->sync_cookie = 0;
 327 }
 328
 329 /*
 330  * fsnotify_create_event - Allocate a new event which will be sent to each
 331  * group's handle_event function if the group was interested in this
 332  * particular event.
 333  *
 334  * @to_tell the inode which is supposed to receive the event (sometimes a
 335  *      parent of the inode to which the event happened.
 336  * @mask what actually happened.
 337  * @data pointer to the object which was actually affected
 338  * @data_type flag indication if the data is a file, path, inode, nothing...
 339  * @name the filename, if available
 340  */
 341 struct fsnotify_event *fsnotify_create_event(struct inode *to_tell, __u32 mask, void *data,
 342                                              int data_type, const char *name, u32 cookie)
 343 {
 344         struct fsnotify_event *event;
 345
 346         event = kmem_cache_alloc(fsnotify_event_cachep, GFP_KERNEL);
 347         if (!event)
 348                 return NULL;
 349
 350         initialize_event(event);
 351
 352         if (name) {
 353                 event->file_name = kstrdup(name, GFP_KERNEL);
 354                 if (!event->file_name) {
 355                         kmem_cache_free(fsnotify_event_cachep, event);
 356                         return NULL;
 357                 }
 358                 event->name_len = strlen(event->file_name);
 359         }
 360
 361         event->sync_cookie = cookie;
 362         event->to_tell = to_tell;
 363
 364         switch (data_type) {
 365         case FSNOTIFY_EVENT_FILE: {
 366                 struct file *file = data;
 367                 struct path *path = &file->f_path;
 368                 event->path.dentry = path->dentry;
 369                 event->path.mnt = path->mnt;
 370                 path_get(&event->path);
 371                 event->data_type = FSNOTIFY_EVENT_PATH;
 372                 break;
 373         }
 374         case FSNOTIFY_EVENT_PATH: {
 375                 struct path *path = data;
 376                 event->path.dentry = path->dentry;
 377                 event->path.mnt = path->mnt;
 378                 path_get(&event->path);
 379                 event->data_type = FSNOTIFY_EVENT_PATH;
 380                 break;
 381         }
 382         case FSNOTIFY_EVENT_INODE:
 383                 event->inode = data;
 384                 event->data_type = FSNOTIFY_EVENT_INODE;
 385                 break;
 386         case FSNOTIFY_EVENT_NONE:
 387                 event->inode = NULL;
 388                 event->path.dentry = NULL;
 389                 event->path.mnt = NULL;
 390                 break;
 391         default:
 392                 BUG();
 393         }
 394
 395         event->mask = mask;
 396
 397         return event;
 398 }
 399
 400 __init int fsnotify_notification_init(void)
 401 {
 402         fsnotify_event_cachep = KMEM_CACHE(fsnotify_event, SLAB_PANIC);
 403         fsnotify_event_holder_cachep = KMEM_CACHE(fsnotify_event_holder, SLAB_PANIC);
 404
 405         initialize_event(&q_overflow_event);
 406         q_overflow_event.mask = FS_Q_OVERFLOW;
 407
 408         return 0;
 409 }
 410 subsys_initcall(fsnotify_notification_init);
 411