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" 99 100 struct srcu_struct fsnotify_mark_srcu; 101 static DEFINE_SPINLOCK(destroy_lock); 102 static LIST_HEAD(destroy_list); 103 static DECLARE_WAIT_QUEUE_HEAD(destroy_waitq); 104 105 void fsnotify_get_mark(struct fsnotify_mark *mark) 106 { 107 atomic_inc(&mark->refcnt); 108 } 109 110 void 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 */ 121 void 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 } 194 195 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(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; 265 err: 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 */ 285 void 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 */ 310 void fsnotify_clear_marks_by_group(struct fsnotify_group *group) 311 { 312 fsnotify_clear_marks_by_group_flags(group, (unsigned int)-1); 313 } 314 315 void 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 */ 328 void 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 } 336 337 static 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 } 360 361 static 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 } 372 device_initcall(fsnotify_mark_init); 373