fs/notify/mark.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  * fsnotify inode mark locking/lifetime/and refcnting
  21  *
  22  * REFCNT:
  23  * The group->recnt and mark->refcnt tell how many "things" in the kernel
  24  * currently are referencing the objects. Both kind of objects typically will
  25  * live inside the kernel with a refcnt of 2, one for its creation and one for
  26  * the reference a group and a mark hold to each other.
  27  * If you are holding the appropriate locks, you can take a reference and the
  28  * object itself is guaranteed to survive until the reference is dropped.
  29  *
  30  * LOCKING:
  31  * There are 3 locks involved with fsnotify inode marks and they MUST be taken
  32  * in order as follows:
  33  *
  34  * group->mark_mutex
  35  * mark->lock
  36  * inode->i_lock
  37  *
  38  * group->mark_mutex protects the marks_list anchored inside a given group and
  39  * each mark is hooked via the g_list.  It also protects the groups private
  40  * data (i.e group limits).
  41
  42  * mark->lock protects the marks attributes like its masks and flags.
  43  * Furthermore it protects the access to a reference of the group that the mark
  44  * is assigned to as well as the access to a reference of the inode/vfsmount
  45  * that is being watched by the mark.
  46  *
  47  * inode->i_lock protects the i_fsnotify_marks list anchored inside a
  48  * given inode and each mark is hooked via the i_list. (and sorta the
  49  * free_i_list)
  50  *
  51  *
  52  * LIFETIME:
  53  * Inode marks survive between when they are added to an inode and when their
  54  * refcnt==0.
  55  *
  56  * The inode mark can be cleared for a number of different reasons including:
  57  * - The inode is unlinked for the last time.  (fsnotify_inode_remove)
  58  * - The inode is being evicted from cache. (fsnotify_inode_delete)
  59  * - The fs the inode is on is unmounted.  (fsnotify_inode_delete/fsnotify_unmount_inodes)
  60  * - Something explicitly requests that it be removed.  (fsnotify_destroy_mark)
  61  * - The fsnotify_group associated with the mark is going away and all such marks
  62  *   need to be cleaned up. (fsnotify_clear_marks_by_group)
  63  *
  64  * Worst case we are given an inode and need to clean up all the marks on that
  65  * inode.  We take i_lock and walk the i_fsnotify_marks safely.  For each
  66  * mark on the list we take a reference (so the mark can't disappear under us).
  67  * We remove that mark form the inode's list of marks and we add this mark to a
  68  * private list anchored on the stack using i_free_list; we walk i_free_list
  69  * and before we destroy the mark we make sure that we dont race with a
  70  * concurrent destroy_group by getting a ref to the marks group and taking the
  71  * groups mutex.
  72
  73  * Very similarly for freeing by group, except we use free_g_list.
  74  *
  75  * This has the very interesting property of being able to run concurrently with
  76  * any (or all) other directions.
  77  */
  78
  79 #include <linux/fs.h>
  80 #include <linux/init.h>
  81 #include <linux/kernel.h>
  82 #include <linux/kthread.h>
  83 #include <linux/module.h>
  84 #include <linux/mutex.h>
  85 #include <linux/slab.h>
  86 #include <linux/spinlock.h>
  87 #include <linux/srcu.h>
  88
  89 #include <linux/atomic.h>
  90
  91 #include <linux/fsnotify_backend.h>
  92 #include "fsnotify.h"
  93
  94 struct srcu_struct fsnotify_mark_srcu;
  95 static DEFINE_SPINLOCK(destroy_lock);
  96 static LIST_HEAD(destroy_list);
  97 static DECLARE_WAIT_QUEUE_HEAD(destroy_waitq);
  98
  99 void fsnotify_get_mark(struct fsnotify_mark *mark)
 100 {
 101         atomic_inc(&mark->refcnt);
 102 }
 103
 104 void fsnotify_put_mark(struct fsnotify_mark *mark)
 105 {
 106         if (atomic_dec_and_test(&mark->refcnt)) {
 107                 if (mark->group)
 108                         fsnotify_put_group(mark->group);
 109                 mark->free_mark(mark);
 110         }
 111 }
 112
 113 /*
 114  * Any time a mark is getting freed we end up here.
 115  * The caller had better be holding a reference to this mark so we don't actually
 116  * do the final put under the mark->lock
 117  */
 118 void fsnotify_destroy_mark_locked(struct fsnotify_mark *mark,
 119                                   struct fsnotify_group *group)
 120 {
 121         struct inode *inode = NULL;
 122
 123         BUG_ON(!mutex_is_locked(&group->mark_mutex));
 124
 125         spin_lock(&mark->lock);
 126
 127         /* something else already called this function on this mark */
 128         if (!(mark->flags & FSNOTIFY_MARK_FLAG_ALIVE)) {
 129                 spin_unlock(&mark->lock);
 130                 return;
 131         }
 132
 133         mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
 134
 135         if (mark->flags & FSNOTIFY_MARK_FLAG_INODE) {
 136                 inode = mark->i.inode;
 137                 fsnotify_destroy_inode_mark(mark);
 138         } else if (mark->flags & FSNOTIFY_MARK_FLAG_VFSMOUNT)
 139                 fsnotify_destroy_vfsmount_mark(mark);
 140         else
 141                 BUG();
 142
 143         list_del_init(&mark->g_list);
 144
 145         spin_unlock(&mark->lock);
 146
 147         if (inode && (mark->flags & FSNOTIFY_MARK_FLAG_OBJECT_PINNED))
 148                 iput(inode);
 149         /* release lock temporarily */
 150         mutex_unlock(&group->mark_mutex);
 151
 152         spin_lock(&destroy_lock);
 153         list_add(&mark->destroy_list, &destroy_list);
 154         spin_unlock(&destroy_lock);
 155         wake_up(&destroy_waitq);
 156         /*
 157          * We don't necessarily have a ref on mark from caller so the above destroy
 158          * may have actually freed it, unless this group provides a 'freeing_mark'
 159          * function which must be holding a reference.
 160          */
 161
 162         /*
 163          * Some groups like to know that marks are being freed.  This is a
 164          * callback to the group function to let it know that this mark
 165          * is being freed.
 166          */
 167         if (group->ops->freeing_mark)
 168                 group->ops->freeing_mark(mark, group);
 169
 170         /*
 171          * __fsnotify_update_child_dentry_flags(inode);
 172          *
 173          * I really want to call that, but we can't, we have no idea if the inode
 174          * still exists the second we drop the mark->lock.
 175          *
 176          * The next time an event arrive to this inode from one of it's children
 177          * __fsnotify_parent will see that the inode doesn't care about it's
 178          * children and will update all of these flags then.  So really this
 179          * is just a lazy update (and could be a perf win...)
 180          */
 181
 182         atomic_dec(&group->num_marks);
 183
 184         mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
 185 }
 186
 187 void fsnotify_destroy_mark(struct fsnotify_mark *mark,
 188                            struct fsnotify_group *group)
 189 {
 190         mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
 191         fsnotify_destroy_mark_locked(mark, group);
 192         mutex_unlock(&group->mark_mutex);
 193 }
 194
 195 void fsnotify_set_mark_mask_locked(struct fsnotify_mark *mark, __u32 mask)
 196 {
 197         assert_spin_locked(&mark->lock);
 198
 199         mark->mask = mask;
 200
 201         if (mark->flags & FSNOTIFY_MARK_FLAG_INODE)
 202                 fsnotify_set_inode_mark_mask_locked(mark, mask);
 203 }
 204
 205 void fsnotify_set_mark_ignored_mask_locked(struct fsnotify_mark *mark, __u32 mask)
 206 {
 207         assert_spin_locked(&mark->lock);
 208
 209         mark->ignored_mask = mask;
 210 }
 211
 212 /*
 213  * Attach an initialized mark to a given group and fs object.
 214  * These marks may be used for the fsnotify backend to determine which
 215  * event types should be delivered to which group.
 216  */
 217 int fsnotify_add_mark_locked(struct fsnotify_mark *mark,
 218                              struct fsnotify_group *group, struct inode *inode,
 219                              struct vfsmount *mnt, int allow_dups)
 220 {
 221         int ret = 0;
 222
 223         BUG_ON(inode && mnt);
 224         BUG_ON(!inode && !mnt);
 225         BUG_ON(!mutex_is_locked(&group->mark_mutex));
 226
 227         /*
 228          * LOCKING ORDER!!!!
 229          * group->mark_mutex
 230          * mark->lock
 231          * inode->i_lock
 232          */
 233         spin_lock(&mark->lock);
 234         mark->flags |= FSNOTIFY_MARK_FLAG_ALIVE;
 235
 236         fsnotify_get_group(group);
 237         mark->group = group;
 238         list_add(&mark->g_list, &group->marks_list);
 239         atomic_inc(&group->num_marks);
 240         fsnotify_get_mark(mark); /* for i_list and g_list */
 241
 242         if (inode) {
 243                 ret = fsnotify_add_inode_mark(mark, group, inode, allow_dups);
 244                 if (ret)
 245                         goto err;
 246         } else if (mnt) {
 247                 ret = fsnotify_add_vfsmount_mark(mark, group, mnt, allow_dups);
 248                 if (ret)
 249                         goto err;
 250         } else {
 251                 BUG();
 252         }
 253
 254         /* this will pin the object if appropriate */
 255         fsnotify_set_mark_mask_locked(mark, mark->mask);
 256         spin_unlock(&mark->lock);
 257
 258         if (inode)
 259                 __fsnotify_update_child_dentry_flags(inode);
 260
 261         return ret;
 262 err:
 263         mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
 264         list_del_init(&mark->g_list);
 265         fsnotify_put_group(group);
 266         mark->group = NULL;
 267         atomic_dec(&group->num_marks);
 268
 269         spin_unlock(&mark->lock);
 270
 271         spin_lock(&destroy_lock);
 272         list_add(&mark->destroy_list, &destroy_list);
 273         spin_unlock(&destroy_lock);
 274         wake_up(&destroy_waitq);
 275
 276         return ret;
 277 }
 278
 279 int fsnotify_add_mark(struct fsnotify_mark *mark, struct fsnotify_group *group,
 280                       struct inode *inode, struct vfsmount *mnt, int allow_dups)
 281 {
 282         int ret;
 283         mutex_lock(&group->mark_mutex);
 284         ret = fsnotify_add_mark_locked(mark, group, inode, mnt, allow_dups);
 285         mutex_unlock(&group->mark_mutex);
 286         return ret;
 287 }
 288
 289 /*
 290  * clear any marks in a group in which mark->flags & flags is true
 291  */
 292 void fsnotify_clear_marks_by_group_flags(struct fsnotify_group *group,
 293                                          unsigned int flags)
 294 {
 295         struct fsnotify_mark *lmark, *mark;
 296
 297         mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
 298         list_for_each_entry_safe(mark, lmark, &group->marks_list, g_list) {
 299                 if (mark->flags & flags) {
 300                         fsnotify_get_mark(mark);
 301                         fsnotify_destroy_mark_locked(mark, group);
 302                         fsnotify_put_mark(mark);
 303                 }
 304         }
 305         mutex_unlock(&group->mark_mutex);
 306 }
 307
 308 /*
 309  * Given a group, destroy all of the marks associated with that group.
 310  */
 311 void fsnotify_clear_marks_by_group(struct fsnotify_group *group)
 312 {
 313         fsnotify_clear_marks_by_group_flags(group, (unsigned int)-1);
 314 }
 315
 316 void fsnotify_duplicate_mark(struct fsnotify_mark *new, struct fsnotify_mark *old)
 317 {
 318         assert_spin_locked(&old->lock);
 319         new->i.inode = old->i.inode;
 320         new->m.mnt = old->m.mnt;
 321         if (old->group)
 322                 fsnotify_get_group(old->group);
 323         new->group = old->group;
 324         new->mask = old->mask;
 325         new->free_mark = old->free_mark;
 326 }
 327
 328 /*
 329  * Nothing fancy, just initialize lists and locks and counters.
 330  */
 331 void fsnotify_init_mark(struct fsnotify_mark *mark,
 332                         void (*free_mark)(struct fsnotify_mark *mark))
 333 {
 334         memset(mark, 0, sizeof(*mark));
 335         spin_lock_init(&mark->lock);
 336         atomic_set(&mark->refcnt, 1);
 337         mark->free_mark = free_mark;
 338 }
 339
 340 static int fsnotify_mark_destroy(void *ignored)
 341 {
 342         struct fsnotify_mark *mark, *next;
 343         LIST_HEAD(private_destroy_list);
 344
 345         for (;;) {
 346                 spin_lock(&destroy_lock);
 347                 /* exchange the list head */
 348                 list_replace_init(&destroy_list, &private_destroy_list);
 349                 spin_unlock(&destroy_lock);
 350
 351                 synchronize_srcu(&fsnotify_mark_srcu);
 352
 353                 list_for_each_entry_safe(mark, next, &private_destroy_list, destroy_list) {
 354                         list_del_init(&mark->destroy_list);
 355                         fsnotify_put_mark(mark);
 356                 }
 357
 358                 wait_event_interruptible(destroy_waitq, !list_empty(&destroy_list));
 359         }
 360
 361         return 0;
 362 }
 363
 364 static int __init fsnotify_mark_init(void)
 365 {
 366         struct task_struct *thread;
 367
 368         thread = kthread_run(fsnotify_mark_destroy, NULL,
 369                              "fsnotify_mark");
 370         if (IS_ERR(thread))
 371                 panic("unable to start fsnotify mark destruction thread.");
 372
 373         return 0;
 374 }
 375 device_initcall(fsnotify_mark_init);