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+Using RCU to Protect Read-Mostly Linked Lists
+
+
+One of the best applications of RCU is to protect read-mostly linked lists
+("struct list_head" in list.h).  One big advantage of this approach
+is that all of the required memory barriers are included for you in
+the list macros.  This document describes several applications of RCU,
+with the best fits first.
+
+
+Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates
+
+The best applications are cases where, if reader-writer locking were
+used, the read-side lock would be dropped before taking any action
+based on the results of the search.  The most celebrated example is
+the routing table.  Because the routing table is tracking the state of
+equipment outside of the computer, it will at times contain stale data.
+Therefore, once the route has been computed, there is no need to hold
+the routing table static during transmission of the packet.  After all,
+you can hold the routing table static all you want, but that won't keep
+the external Internet from changing, and it is the state of the external
+Internet that really matters.  In addition, routing entries are typically
+added or deleted, rather than being modified in place.
+
+A straightforward example of this use of RCU may be found in the
+system-call auditing support.  For example, a reader-writer locked
+implementation of audit_filter_task() might be as follows:
+
+	static enum audit_state audit_filter_task(struct task_struct *tsk)
+	{
+		struct audit_entry *e;
+		enum audit_state   state;
+
+		read_lock(&auditsc_lock);
+		/* Note: audit_netlink_sem held by caller. */
+		list_for_each_entry(e, &audit_tsklist, list) {
+			if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
+				read_unlock(&auditsc_lock);
+				return state;
+			}
+		}
+		read_unlock(&auditsc_lock);
+		return AUDIT_BUILD_CONTEXT;
+	}
+
+Here the list is searched under the lock, but the lock is dropped before
+the corresponding value is returned.  By the time that this value is acted
+on, the list may well have been modified.  This makes sense, since if
+you are turning auditing off, it is OK to audit a few extra system calls.
+
+This means that RCU can be easily applied to the read side, as follows:
+
+	static enum audit_state audit_filter_task(struct task_struct *tsk)
+	{
+		struct audit_entry *e;
+		enum audit_state   state;
+
+		rcu_read_lock();
+		/* Note: audit_netlink_sem held by caller. */
+		list_for_each_entry_rcu(e, &audit_tsklist, list) {
+			if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
+				rcu_read_unlock();
+				return state;
+			}
+		}
+		rcu_read_unlock();
+		return AUDIT_BUILD_CONTEXT;
+	}
+
+The read_lock() and read_unlock() calls have become rcu_read_lock()
+and rcu_read_unlock(), respectively, and the list_for_each_entry() has
+become list_for_each_entry_rcu().  The _rcu() list-traversal primitives
+insert the read-side memory barriers that are required on DEC Alpha CPUs.
+
+The changes to the update side are also straightforward.  A reader-writer
+lock might be used as follows for deletion and insertion:
+
+	static inline int audit_del_rule(struct audit_rule *rule,
+					 struct list_head *list)
+	{
+		struct audit_entry  *e;
+
+		write_lock(&auditsc_lock);
+		list_for_each_entry(e, list, list) {
+			if (!audit_compare_rule(rule, &e->rule)) {
+				list_del(&e->list);
+				write_unlock(&auditsc_lock);
+				return 0;
+			}
+		}
+		write_unlock(&auditsc_lock);
+		return -EFAULT;		/* No matching rule */
+	}
+
+	static inline int audit_add_rule(struct audit_entry *entry,
+					 struct list_head *list)
+	{
+		write_lock(&auditsc_lock);
+		if (entry->rule.flags & AUDIT_PREPEND) {
+			entry->rule.flags &= ~AUDIT_PREPEND;
+			list_add(&entry->list, list);
+		} else {
+			list_add_tail(&entry->list, list);
+		}
+		write_unlock(&auditsc_lock);
+		return 0;
+	}
+
+Following are the RCU equivalents for these two functions:
+
+	static inline int audit_del_rule(struct audit_rule *rule,
+					 struct list_head *list)
+	{
+		struct audit_entry  *e;
+
+		/* Do not use the _rcu iterator here, since this is the only
+		 * deletion routine. */
+		list_for_each_entry(e, list, list) {
+			if (!audit_compare_rule(rule, &e->rule)) {
+				list_del_rcu(&e->list);
+				call_rcu(&e->rcu, audit_free_rule);
+				return 0;
+			}
+		}
+		return -EFAULT;		/* No matching rule */
+	}
+
+	static inline int audit_add_rule(struct audit_entry *entry,
+					 struct list_head *list)
+	{
+		if (entry->rule.flags & AUDIT_PREPEND) {
+			entry->rule.flags &= ~AUDIT_PREPEND;
+			list_add_rcu(&entry->list, list);
+		} else {
+			list_add_tail_rcu(&entry->list, list);
+		}
+		return 0;
+	}
+
+Normally, the write_lock() and write_unlock() would be replaced by
+a spin_lock() and a spin_unlock(), but in this case, all callers hold
+audit_netlink_sem, so no additional locking is required.  The auditsc_lock
+can therefore be eliminated, since use of RCU eliminates the need for
+writers to exclude readers.  Normally, the write_lock() calls would
+be converted into spin_lock() calls.
+
+The list_del(), list_add(), and list_add_tail() primitives have been
+replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu().
+The _rcu() list-manipulation primitives add memory barriers that are
+needed on weakly ordered CPUs (most of them!).  The list_del_rcu()
+primitive omits the pointer poisoning debug-assist code that would
+otherwise cause concurrent readers to fail spectacularly.
+
+So, when readers can tolerate stale data and when entries are either added
+or deleted, without in-place modification, it is very easy to use RCU!
+
+
+Example 2: Handling In-Place Updates
+
+The system-call auditing code does not update auditing rules in place.
+However, if it did, reader-writer-locked code to do so might look as
+follows (presumably, the field_count is only permitted to decrease,
+otherwise, the added fields would need to be filled in):
+
+	static inline int audit_upd_rule(struct audit_rule *rule,
+					 struct list_head *list,
+					 __u32 newaction,
+					 __u32 newfield_count)
+	{
+		struct audit_entry  *e;
+		struct audit_newentry *ne;
+
+		write_lock(&auditsc_lock);
+		/* Note: audit_netlink_sem held by caller. */
+		list_for_each_entry(e, list, list) {
+			if (!audit_compare_rule(rule, &e->rule)) {
+				e->rule.action = newaction;
+				e->rule.file_count = newfield_count;
+				write_unlock(&auditsc_lock);
+				return 0;
+			}
+		}
+		write_unlock(&auditsc_lock);
+		return -EFAULT;		/* No matching rule */
+	}
+
+The RCU version creates a copy, updates the copy, then replaces the old
+entry with the newly updated entry.  This sequence of actions, allowing
+concurrent reads while doing a copy to perform an update, is what gives
+RCU ("read-copy update") its name.  The RCU code is as follows:
+
+	static inline int audit_upd_rule(struct audit_rule *rule,
+					 struct list_head *list,
+					 __u32 newaction,
+					 __u32 newfield_count)
+	{
+		struct audit_entry  *e;
+		struct audit_newentry *ne;
+
+		list_for_each_entry(e, list, list) {
+			if (!audit_compare_rule(rule, &e->rule)) {
+				ne = kmalloc(sizeof(*entry), GFP_ATOMIC);
+				if (ne == NULL)
+					return -ENOMEM;
+				audit_copy_rule(&ne->rule, &e->rule);
+				ne->rule.action = newaction;
+				ne->rule.file_count = newfield_count;
+				list_replace_rcu(&e->list, &ne->list);
+				call_rcu(&e->rcu, audit_free_rule);
+				return 0;
+			}
+		}
+		return -EFAULT;		/* No matching rule */
+	}
+
+Again, this assumes that the caller holds audit_netlink_sem.  Normally,
+the reader-writer lock would become a spinlock in this sort of code.
+
+
+Example 3: Eliminating Stale Data
+
+The auditing examples above tolerate stale data, as do most algorithms
+that are tracking external state.  Because there is a delay from the
+time the external state changes before Linux becomes aware of the change,
+additional RCU-induced staleness is normally not a problem.
+
+However, there are many examples where stale data cannot be tolerated.
+One example in the Linux kernel is the System V IPC (see the ipc_lock()
+function in ipc/util.c).  This code checks a "deleted" flag under a
+per-entry spinlock, and, if the "deleted" flag is set, pretends that the
+entry does not exist.  For this to be helpful, the search function must
+return holding the per-entry spinlock, as ipc_lock() does in fact do.
+
+Quick Quiz:  Why does the search function need to return holding the
+	per-entry lock for this deleted-flag technique to be helpful?
+
+If the system-call audit module were to ever need to reject stale data,
+one way to accomplish this would be to add a "deleted" flag and a "lock"
+spinlock to the audit_entry structure, and modify audit_filter_task()
+as follows:
+
+	static enum audit_state audit_filter_task(struct task_struct *tsk)
+	{
+		struct audit_entry *e;
+		enum audit_state   state;
+
+		rcu_read_lock();
+		list_for_each_entry_rcu(e, &audit_tsklist, list) {
+			if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
+				spin_lock(&e->lock);
+				if (e->deleted) {
+					spin_unlock(&e->lock);
+					rcu_read_unlock();
+					return AUDIT_BUILD_CONTEXT;
+				}
+				rcu_read_unlock();
+				return state;
+			}
+		}
+		rcu_read_unlock();
+		return AUDIT_BUILD_CONTEXT;
+	}
+
+Note that this example assumes that entries are only added and deleted.
+Additional mechanism is required to deal correctly with the
+update-in-place performed by audit_upd_rule().  For one thing,
+audit_upd_rule() would need additional memory barriers to ensure
+that the list_add_rcu() was really executed before the list_del_rcu().
+
+The audit_del_rule() function would need to set the "deleted"
+flag under the spinlock as follows:
+
+	static inline int audit_del_rule(struct audit_rule *rule,
+					 struct list_head *list)
+	{
+		struct audit_entry  *e;
+
+		/* Do not need to use the _rcu iterator here, since this
+		 * is the only deletion routine. */
+		list_for_each_entry(e, list, list) {
+			if (!audit_compare_rule(rule, &e->rule)) {
+				spin_lock(&e->lock);
+				list_del_rcu(&e->list);
+				e->deleted = 1;
+				spin_unlock(&e->lock);
+				call_rcu(&e->rcu, audit_free_rule);
+				return 0;
+			}
+		}
+		return -EFAULT;		/* No matching rule */
+	}
+
+
+Summary
+
+Read-mostly list-based data structures that can tolerate stale data are
+the most amenable to use of RCU.  The simplest case is where entries are
+either added or deleted from the data structure (or atomically modified
+in place), but non-atomic in-place modifications can be handled by making
+a copy, updating the copy, then replacing the original with the copy.
+If stale data cannot be tolerated, then a "deleted" flag may be used
+in conjunction with a per-entry spinlock in order to allow the search
+function to reject newly deleted data.
+
+
+Answer to Quick Quiz
+	Why does the search function need to return holding the per-entry
+	lock for this deleted-flag technique to be helpful?
+
+	If the search function drops the per-entry lock before returning,
+	then the caller will be processing stale data in any case.  If it
+	is really OK to be processing stale data, then you don't need a
+	"deleted" flag.  If processing stale data really is a problem,
+	then you need to hold the per-entry lock across all of the code
+	that uses the value that was returned.