linux/Documentation/RCU/listRCU.txt
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   1Using RCU to Protect Read-Mostly Linked Lists
   2
   3
   4One of the best applications of RCU is to protect read-mostly linked lists
   5("struct list_head" in list.h).  One big advantage of this approach
   6is that all of the required memory barriers are included for you in
   7the list macros.  This document describes several applications of RCU,
   8with the best fits first.
   9
  10
  11Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates
  12
  13The best applications are cases where, if reader-writer locking were
  14used, the read-side lock would be dropped before taking any action
  15based on the results of the search.  The most celebrated example is
  16the routing table.  Because the routing table is tracking the state of
  17equipment outside of the computer, it will at times contain stale data.
  18Therefore, once the route has been computed, there is no need to hold
  19the routing table static during transmission of the packet.  After all,
  20you can hold the routing table static all you want, but that won't keep
  21the external Internet from changing, and it is the state of the external
  22Internet that really matters.  In addition, routing entries are typically
  23added or deleted, rather than being modified in place.
  24
  25A straightforward example of this use of RCU may be found in the
  26system-call auditing support.  For example, a reader-writer locked
  27implementation of audit_filter_task() might be as follows:
  28
  29        static enum audit_state audit_filter_task(struct task_struct *tsk)
  30        {
  31                struct audit_entry *e;
  32                enum audit_state   state;
  33
  34                read_lock(&auditsc_lock);
  35                /* Note: audit_netlink_sem held by caller. */
  36                list_for_each_entry(e, &audit_tsklist, list) {
  37                        if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
  38                                read_unlock(&auditsc_lock);
  39                                return state;
  40                        }
  41                }
  42                read_unlock(&auditsc_lock);
  43                return AUDIT_BUILD_CONTEXT;
  44        }
  45
  46Here the list is searched under the lock, but the lock is dropped before
  47the corresponding value is returned.  By the time that this value is acted
  48on, the list may well have been modified.  This makes sense, since if
  49you are turning auditing off, it is OK to audit a few extra system calls.
  50
  51This means that RCU can be easily applied to the read side, as follows:
  52
  53        static enum audit_state audit_filter_task(struct task_struct *tsk)
  54        {
  55                struct audit_entry *e;
  56                enum audit_state   state;
  57
  58                rcu_read_lock();
  59                /* Note: audit_netlink_sem held by caller. */
  60                list_for_each_entry_rcu(e, &audit_tsklist, list) {
  61                        if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
  62                                rcu_read_unlock();
  63                                return state;
  64                        }
  65                }
  66                rcu_read_unlock();
  67                return AUDIT_BUILD_CONTEXT;
  68        }
  69
  70The read_lock() and read_unlock() calls have become rcu_read_lock()
  71and rcu_read_unlock(), respectively, and the list_for_each_entry() has
  72become list_for_each_entry_rcu().  The _rcu() list-traversal primitives
  73insert the read-side memory barriers that are required on DEC Alpha CPUs.
  74
  75The changes to the update side are also straightforward.  A reader-writer
  76lock might be used as follows for deletion and insertion:
  77
  78        static inline int audit_del_rule(struct audit_rule *rule,
  79                                         struct list_head *list)
  80        {
  81                struct audit_entry  *e;
  82
  83                write_lock(&auditsc_lock);
  84                list_for_each_entry(e, list, list) {
  85                        if (!audit_compare_rule(rule, &e->rule)) {
  86                                list_del(&e->list);
  87                                write_unlock(&auditsc_lock);
  88                                return 0;
  89                        }
  90                }
  91                write_unlock(&auditsc_lock);
  92                return -EFAULT;         /* No matching rule */
  93        }
  94
  95        static inline int audit_add_rule(struct audit_entry *entry,
  96                                         struct list_head *list)
  97        {
  98                write_lock(&auditsc_lock);
  99                if (entry->rule.flags & AUDIT_PREPEND) {
 100                        entry->rule.flags &= ~AUDIT_PREPEND;
 101                        list_add(&entry->list, list);
 102                } else {
 103                        list_add_tail(&entry->list, list);
 104                }
 105                write_unlock(&auditsc_lock);
 106                return 0;
 107        }
 108
 109Following are the RCU equivalents for these two functions:
 110
 111        static inline int audit_del_rule(struct audit_rule *rule,
 112                                         struct list_head *list)
 113        {
 114                struct audit_entry  *e;
 115
 116                /* Do not use the _rcu iterator here, since this is the only
 117                 * deletion routine. */
 118                list_for_each_entry(e, list, list) {
 119                        if (!audit_compare_rule(rule, &e->rule)) {
 120                                list_del_rcu(&e->list);
 121                                call_rcu(&e->rcu, audit_free_rule);
 122                                return 0;
 123                        }
 124                }
 125                return -EFAULT;         /* No matching rule */
 126        }
 127
 128        static inline int audit_add_rule(struct audit_entry *entry,
 129                                         struct list_head *list)
 130        {
 131                if (entry->rule.flags & AUDIT_PREPEND) {
 132                        entry->rule.flags &= ~AUDIT_PREPEND;
 133                        list_add_rcu(&entry->list, list);
 134                } else {
 135                        list_add_tail_rcu(&entry->list, list);
 136                }
 137                return 0;
 138        }
 139
 140Normally, the write_lock() and write_unlock() would be replaced by
 141a spin_lock() and a spin_unlock(), but in this case, all callers hold
 142audit_netlink_sem, so no additional locking is required.  The auditsc_lock
 143can therefore be eliminated, since use of RCU eliminates the need for
 144writers to exclude readers.  Normally, the write_lock() calls would
 145be converted into spin_lock() calls.
 146
 147The list_del(), list_add(), and list_add_tail() primitives have been
 148replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu().
 149The _rcu() list-manipulation primitives add memory barriers that are
 150needed on weakly ordered CPUs (most of them!).  The list_del_rcu()
 151primitive omits the pointer poisoning debug-assist code that would
 152otherwise cause concurrent readers to fail spectacularly.
 153
 154So, when readers can tolerate stale data and when entries are either added
 155or deleted, without in-place modification, it is very easy to use RCU!
 156
 157
 158Example 2: Handling In-Place Updates
 159
 160The system-call auditing code does not update auditing rules in place.
 161However, if it did, reader-writer-locked code to do so might look as
 162follows (presumably, the field_count is only permitted to decrease,
 163otherwise, the added fields would need to be filled in):
 164
 165        static inline int audit_upd_rule(struct audit_rule *rule,
 166                                         struct list_head *list,
 167                                         __u32 newaction,
 168                                         __u32 newfield_count)
 169        {
 170                struct audit_entry  *e;
 171                struct audit_newentry *ne;
 172
 173                write_lock(&auditsc_lock);
 174                /* Note: audit_netlink_sem held by caller. */
 175                list_for_each_entry(e, list, list) {
 176                        if (!audit_compare_rule(rule, &e->rule)) {
 177                                e->rule.action = newaction;
 178                                e->rule.file_count = newfield_count;
 179                                write_unlock(&auditsc_lock);
 180                                return 0;
 181                        }
 182                }
 183                write_unlock(&auditsc_lock);
 184                return -EFAULT;         /* No matching rule */
 185        }
 186
 187The RCU version creates a copy, updates the copy, then replaces the old
 188entry with the newly updated entry.  This sequence of actions, allowing
 189concurrent reads while doing a copy to perform an update, is what gives
 190RCU ("read-copy update") its name.  The RCU code is as follows:
 191
 192        static inline int audit_upd_rule(struct audit_rule *rule,
 193                                         struct list_head *list,
 194                                         __u32 newaction,
 195                                         __u32 newfield_count)
 196        {
 197                struct audit_entry  *e;
 198                struct audit_newentry *ne;
 199
 200                list_for_each_entry(e, list, list) {
 201                        if (!audit_compare_rule(rule, &e->rule)) {
 202                                ne = kmalloc(sizeof(*entry), GFP_ATOMIC);
 203                                if (ne == NULL)
 204                                        return -ENOMEM;
 205                                audit_copy_rule(&ne->rule, &e->rule);
 206                                ne->rule.action = newaction;
 207                                ne->rule.file_count = newfield_count;
 208                                list_replace_rcu(e, ne);
 209                                call_rcu(&e->rcu, audit_free_rule);
 210                                return 0;
 211                        }
 212                }
 213                return -EFAULT;         /* No matching rule */
 214        }
 215
 216Again, this assumes that the caller holds audit_netlink_sem.  Normally,
 217the reader-writer lock would become a spinlock in this sort of code.
 218
 219
 220Example 3: Eliminating Stale Data
 221
 222The auditing examples above tolerate stale data, as do most algorithms
 223that are tracking external state.  Because there is a delay from the
 224time the external state changes before Linux becomes aware of the change,
 225additional RCU-induced staleness is normally not a problem.
 226
 227However, there are many examples where stale data cannot be tolerated.
 228One example in the Linux kernel is the System V IPC (see the ipc_lock()
 229function in ipc/util.c).  This code checks a "deleted" flag under a
 230per-entry spinlock, and, if the "deleted" flag is set, pretends that the
 231entry does not exist.  For this to be helpful, the search function must
 232return holding the per-entry spinlock, as ipc_lock() does in fact do.
 233
 234Quick Quiz:  Why does the search function need to return holding the
 235        per-entry lock for this deleted-flag technique to be helpful?
 236
 237If the system-call audit module were to ever need to reject stale data,
 238one way to accomplish this would be to add a "deleted" flag and a "lock"
 239spinlock to the audit_entry structure, and modify audit_filter_task()
 240as follows:
 241
 242        static enum audit_state audit_filter_task(struct task_struct *tsk)
 243        {
 244                struct audit_entry *e;
 245                enum audit_state   state;
 246
 247                rcu_read_lock();
 248                list_for_each_entry_rcu(e, &audit_tsklist, list) {
 249                        if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
 250                                spin_lock(&e->lock);
 251                                if (e->deleted) {
 252                                        spin_unlock(&e->lock);
 253                                        rcu_read_unlock();
 254                                        return AUDIT_BUILD_CONTEXT;
 255                                }
 256                                rcu_read_unlock();
 257                                return state;
 258                        }
 259                }
 260                rcu_read_unlock();
 261                return AUDIT_BUILD_CONTEXT;
 262        }
 263
 264Note that this example assumes that entries are only added and deleted.
 265Additional mechanism is required to deal correctly with the
 266update-in-place performed by audit_upd_rule().  For one thing,
 267audit_upd_rule() would need additional memory barriers to ensure
 268that the list_add_rcu() was really executed before the list_del_rcu().
 269
 270The audit_del_rule() function would need to set the "deleted"
 271flag under the spinlock as follows:
 272
 273        static inline int audit_del_rule(struct audit_rule *rule,
 274                                         struct list_head *list)
 275        {
 276                struct audit_entry  *e;
 277
 278                /* Do not need to use the _rcu iterator here, since this
 279                 * is the only deletion routine. */
 280                list_for_each_entry(e, list, list) {
 281                        if (!audit_compare_rule(rule, &e->rule)) {
 282                                spin_lock(&e->lock);
 283                                list_del_rcu(&e->list);
 284                                e->deleted = 1;
 285                                spin_unlock(&e->lock);
 286                                call_rcu(&e->rcu, audit_free_rule);
 287                                return 0;
 288                        }
 289                }
 290                return -EFAULT;         /* No matching rule */
 291        }
 292
 293
 294Summary
 295
 296Read-mostly list-based data structures that can tolerate stale data are
 297the most amenable to use of RCU.  The simplest case is where entries are
 298either added or deleted from the data structure (or atomically modified
 299in place), but non-atomic in-place modifications can be handled by making
 300a copy, updating the copy, then replacing the original with the copy.
 301If stale data cannot be tolerated, then a "deleted" flag may be used
 302in conjunction with a per-entry spinlock in order to allow the search
 303function to reject newly deleted data.
 304
 305
 306Answer to Quick Quiz
 307        Why does the search function need to return holding the per-entry
 308        lock for this deleted-flag technique to be helpful?
 309
 310        If the search function drops the per-entry lock before returning,
 311        then the caller will be processing stale data in any case.  If it
 312        is really OK to be processing stale data, then you don't need a
 313        "deleted" flag.  If processing stale data really is a problem,
 314        then you need to hold the per-entry lock across all of the code
 315        that uses the value that was returned.
 316
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