linux/fs/notify/mark.c
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   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 mark->refcnt tells how many "things" in the kernel currently are
  24 * referencing this object.  The object typically will live inside the kernel
  25 * with a refcnt of 2, one for each list it is on (i_list, g_list).  Any task
  26 * which can find this object holding the appropriete locks, can take a reference
  27 * and the object itself is guaranteed to survive until the reference is dropped.
  28 *
  29 * LOCKING:
  30 * There are 3 spinlocks involved with fsnotify inode marks and they MUST
  31 * be taken in order as follows:
  32 *
  33 * mark->lock
  34 * group->mark_lock
  35 * inode->i_lock
  36 *
  37 * mark->lock protects 2 things, mark->group and mark->inode.  You must hold
  38 * that lock to dereference either of these things (they could be NULL even with
  39 * the lock)
  40 *
  41 * group->mark_lock protects the marks_list anchored inside a given group
  42 * and each mark is hooked via the g_list.  It also sorta protects the
  43 * free_g_list, which when used is anchored by a private list on the stack of the
  44 * task which held the group->mark_lock.
  45 *
  46 * inode->i_lock protects the i_fsnotify_marks list anchored inside a
  47 * given inode and each mark is hooked via the i_list. (and sorta the
  48 * free_i_list)
  49 *
  50 *
  51 * LIFETIME:
  52 * Inode marks survive between when they are added to an inode and when their
  53 * refcnt==0.
  54 *
  55 * The inode mark can be cleared for a number of different reasons including:
  56 * - The inode is unlinked for the last time.  (fsnotify_inode_remove)
  57 * - The inode is being evicted from cache. (fsnotify_inode_delete)
  58 * - The fs the inode is on is unmounted.  (fsnotify_inode_delete/fsnotify_unmount_inodes)
  59 * - Something explicitly requests that it be removed.  (fsnotify_destroy_mark)
  60 * - The fsnotify_group associated with the mark is going away and all such marks
  61 *   need to be cleaned up. (fsnotify_clear_marks_by_group)
  62 *
  63 * Worst case we are given an inode and need to clean up all the marks on that
  64 * inode.  We take i_lock and walk the i_fsnotify_marks safely.  For each
  65 * mark on the list we take a reference (so the mark can't disappear under us).
  66 * We remove that mark form the inode's list of marks and we add this mark to a
  67 * private list anchored on the stack using i_free_list;  At this point we no
  68 * longer fear anything finding the mark using the inode's list of marks.
  69 *
  70 * We can safely and locklessly run the private list on the stack of everything
  71 * we just unattached from the original inode.  For each mark on the private list
  72 * we grab the mark-> and can thus dereference mark->group and mark->inode.  If
  73 * we see the group and inode are not NULL we take those locks.  Now holding all
  74 * 3 locks we can completely remove the mark from other tasks finding it in the
  75 * future.  Remember, 10 things might already be referencing this mark, but they
  76 * better be holding a ref.  We drop our reference we took before we unhooked it
  77 * from the inode.  When the ref hits 0 we can free the mark.
  78 *
  79 * Very similarly for freeing by group, except we use free_g_list.
  80 *
  81 * This has the very interesting property of being able to run concurrently with
  82 * any (or all) other directions.
  83 */
  84
  85#include <linux/fs.h>
  86#include <linux/init.h>
  87#include <linux/kernel.h>
  88#include <linux/kthread.h>
  89#include <linux/module.h>
  90#include <linux/mutex.h>
  91#include <linux/slab.h>
  92#include <linux/spinlock.h>
  93#include <linux/srcu.h>
  94
  95#include <linux/atomic.h>
  96
  97#include <linux/fsnotify_backend.h>
  98#include "fsnotify.h"
  99
 100struct srcu_struct fsnotify_mark_srcu;
 101static DEFINE_SPINLOCK(destroy_lock);
 102static LIST_HEAD(destroy_list);
 103static DECLARE_WAIT_QUEUE_HEAD(destroy_waitq);
 104
 105void fsnotify_get_mark(struct fsnotify_mark *mark)
 106{
 107        atomic_inc(&mark->refcnt);
 108}
 109
 110void fsnotify_put_mark(struct fsnotify_mark *mark)
 111{
 112        if (atomic_dec_and_test(&mark->refcnt))
 113                mark->free_mark(mark);
 114}
 115
 116/*
 117 * Any time a mark is getting freed we end up here.
 118 * The caller had better be holding a reference to this mark so we don't actually
 119 * do the final put under the mark->lock
 120 */
 121void fsnotify_destroy_mark(struct fsnotify_mark *mark)
 122{
 123        struct fsnotify_group *group;
 124        struct inode *inode = NULL;
 125
 126        spin_lock(&mark->lock);
 127
 128        group = mark->group;
 129
 130        /* something else already called this function on this mark */
 131        if (!(mark->flags & FSNOTIFY_MARK_FLAG_ALIVE)) {
 132                spin_unlock(&mark->lock);
 133                return;
 134        }
 135
 136        mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
 137
 138        spin_lock(&group->mark_lock);
 139
 140        if (mark->flags & FSNOTIFY_MARK_FLAG_INODE) {
 141                inode = mark->i.inode;
 142                fsnotify_destroy_inode_mark(mark);
 143        } else if (mark->flags & FSNOTIFY_MARK_FLAG_VFSMOUNT)
 144                fsnotify_destroy_vfsmount_mark(mark);
 145        else
 146                BUG();
 147
 148        list_del_init(&mark->g_list);
 149
 150        spin_unlock(&group->mark_lock);
 151        spin_unlock(&mark->lock);
 152
 153        spin_lock(&destroy_lock);
 154        list_add(&mark->destroy_list, &destroy_list);
 155        spin_unlock(&destroy_lock);
 156        wake_up(&destroy_waitq);
 157
 158        /*
 159         * Some groups like to know that marks are being freed.  This is a
 160         * callback to the group function to let it know that this mark
 161         * is being freed.
 162         */
 163        if (group->ops->freeing_mark)
 164                group->ops->freeing_mark(mark, group);
 165
 166        /*
 167         * __fsnotify_update_child_dentry_flags(inode);
 168         *
 169         * I really want to call that, but we can't, we have no idea if the inode
 170         * still exists the second we drop the mark->lock.
 171         *
 172         * The next time an event arrive to this inode from one of it's children
 173         * __fsnotify_parent will see that the inode doesn't care about it's
 174         * children and will update all of these flags then.  So really this
 175         * is just a lazy update (and could be a perf win...)
 176         */
 177
 178        if (inode && (mark->flags & FSNOTIFY_MARK_FLAG_OBJECT_PINNED))
 179                iput(inode);
 180
 181        /*
 182         * We don't necessarily have a ref on mark from caller so the above iput
 183         * may have already destroyed it.  Don't touch from now on.
 184         */
 185
 186        /*
 187         * it's possible that this group tried to destroy itself, but this
 188         * this mark was simultaneously being freed by inode.  If that's the
 189         * case, we finish freeing the group here.
 190         */
 191        if (unlikely(atomic_dec_and_test(&group->num_marks)))
 192                fsnotify_final_destroy_group(group);
 193}
 194
 195void 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
 205void 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 */
 217int fsnotify_add_mark(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
 226        /*
 227         * LOCKING ORDER!!!!
 228         * mark->lock
 229         * group->mark_lock
 230         * inode->i_lock
 231         */
 232        spin_lock(&mark->lock);
 233        spin_lock(&group->mark_lock);
 234
 235        mark->flags |= FSNOTIFY_MARK_FLAG_ALIVE;
 236
 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        spin_unlock(&group->mark_lock);
 255
 256        /* this will pin the object if appropriate */
 257        fsnotify_set_mark_mask_locked(mark, mark->mask);
 258
 259        spin_unlock(&mark->lock);
 260
 261        if (inode)
 262                __fsnotify_update_child_dentry_flags(inode);
 263
 264        return ret;
 265err:
 266        mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
 267        list_del_init(&mark->g_list);
 268        mark->group = NULL;
 269        atomic_dec(&group->num_marks);
 270
 271        spin_unlock(&group->mark_lock);
 272        spin_unlock(&mark->lock);
 273
 274        spin_lock(&destroy_lock);
 275        list_add(&mark->destroy_list, &destroy_list);
 276        spin_unlock(&destroy_lock);
 277        wake_up(&destroy_waitq);
 278
 279        return ret;
 280}
 281
 282/*
 283 * clear any marks in a group in which mark->flags & flags is true
 284 */
 285void fsnotify_clear_marks_by_group_flags(struct fsnotify_group *group,
 286                                         unsigned int flags)
 287{
 288        struct fsnotify_mark *lmark, *mark;
 289        LIST_HEAD(free_list);
 290
 291        spin_lock(&group->mark_lock);
 292        list_for_each_entry_safe(mark, lmark, &group->marks_list, g_list) {
 293                if (mark->flags & flags) {
 294                        list_add(&mark->free_g_list, &free_list);
 295                        list_del_init(&mark->g_list);
 296                        fsnotify_get_mark(mark);
 297                }
 298        }
 299        spin_unlock(&group->mark_lock);
 300
 301        list_for_each_entry_safe(mark, lmark, &free_list, free_g_list) {
 302                fsnotify_destroy_mark(mark);
 303                fsnotify_put_mark(mark);
 304        }
 305}
 306
 307/*
 308 * Given a group, destroy all of the marks associated with that group.
 309 */
 310void fsnotify_clear_marks_by_group(struct fsnotify_group *group)
 311{
 312        fsnotify_clear_marks_by_group_flags(group, (unsigned int)-1);
 313}
 314
 315void fsnotify_duplicate_mark(struct fsnotify_mark *new, struct fsnotify_mark *old)
 316{
 317        assert_spin_locked(&old->lock);
 318        new->i.inode = old->i.inode;
 319        new->m.mnt = old->m.mnt;
 320        new->group = old->group;
 321        new->mask = old->mask;
 322        new->free_mark = old->free_mark;
 323}
 324
 325/*
 326 * Nothing fancy, just initialize lists and locks and counters.
 327 */
 328void fsnotify_init_mark(struct fsnotify_mark *mark,
 329                        void (*free_mark)(struct fsnotify_mark *mark))
 330{
 331        memset(mark, 0, sizeof(*mark));
 332        spin_lock_init(&mark->lock);
 333        atomic_set(&mark->refcnt, 1);
 334        mark->free_mark = free_mark;
 335}
 336
 337static int fsnotify_mark_destroy(void *ignored)
 338{
 339        struct fsnotify_mark *mark, *next;
 340        LIST_HEAD(private_destroy_list);
 341
 342        for (;;) {
 343                spin_lock(&destroy_lock);
 344                /* exchange the list head */
 345                list_replace_init(&destroy_list, &private_destroy_list);
 346                spin_unlock(&destroy_lock);
 347
 348                synchronize_srcu(&fsnotify_mark_srcu);
 349
 350                list_for_each_entry_safe(mark, next, &private_destroy_list, destroy_list) {
 351                        list_del_init(&mark->destroy_list);
 352                        fsnotify_put_mark(mark);
 353                }
 354
 355                wait_event_interruptible(destroy_waitq, !list_empty(&destroy_list));
 356        }
 357
 358        return 0;
 359}
 360
 361static int __init fsnotify_mark_init(void)
 362{
 363        struct task_struct *thread;
 364
 365        thread = kthread_run(fsnotify_mark_destroy, NULL,
 366                             "fsnotify_mark");
 367        if (IS_ERR(thread))
 368                panic("unable to start fsnotify mark destruction thread.");
 369
 370        return 0;
 371}
 372device_initcall(fsnotify_mark_init);
 373
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