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+.. _list_rcu_doc:
+
+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-mostly list: Deferred Destruction
+-------------------------------------------------
+
+A widely used usecase for RCU lists in the kernel is lockless iteration over
+all processes in the system. ``task_struct::tasks`` represents the list node that
+links all the processes. The list can be traversed in parallel to any list
+additions or removals.
+
+The traversal of the list is done using ``for_each_process()`` which is defined
+by the 2 macros::
+
+ #define next_task(p) \
+ list_entry_rcu((p)->tasks.next, struct task_struct, tasks)
+
+ #define for_each_process(p) \
+ for (p = &init_task ; (p = next_task(p)) != &init_task ; )
+
+The code traversing the list of all processes typically looks like::
+
+ rcu_read_lock();
+ for_each_process(p) {
+ /* Do something with p */
+ }
+ rcu_read_unlock();
+
+The simplified code for removing a process from a task list is::
+
+ void release_task(struct task_struct *p)
+ {
+ write_lock(&tasklist_lock);
+ list_del_rcu(&p->tasks);
+ write_unlock(&tasklist_lock);
+ call_rcu(&p->rcu, delayed_put_task_struct);
+ }
+
+When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)`` under
+``tasklist_lock`` writer lock protection, to remove the task from the list of
+all tasks. The ``tasklist_lock`` prevents concurrent list additions/removals
+from corrupting the list. Readers using ``for_each_process()`` are not protected
+with the ``tasklist_lock``. To prevent readers from noticing changes in the list
+pointers, the ``task_struct`` object is freed only after one or more grace
+periods elapse (with the help of call_rcu()). This deferring of destruction
+ensures that any readers traversing the list will see valid ``p->tasks.next``
+pointers and deletion/freeing can happen in parallel with traversal of the list.
+This pattern is also called an **existence lock**, since RCU pins the object in
+memory until all existing readers finish.
+
+
+Example 2: 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_filter_mutex 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_filter_mutex 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;
+
+ /* No 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)) {
+ 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_filter_mutex``, 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.
+
+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 3: Handling In-Place Updates
+------------------------------------
+
+The system-call auditing code does not update auditing rules in place. However,
+if it did, the reader-writer-locked code to do so might look as follows
+(assuming only ``field_count`` is updated, 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_entry *ne;
+
+ write_lock(&auditsc_lock);
+ /* Note: audit_filter_mutex held by caller. */
+ list_for_each_entry(e, list, list) {
+ if (!audit_compare_rule(rule, &e->rule)) {
+ e->rule.action = newaction;
+ e->rule.field_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 making 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_entry *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.field_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_filter_mutex``. Normally, the
+writer lock would become a spinlock in this sort of code.
+
+Another use of this pattern can be found in the openswitch driver's *connection
+tracking table* code in ``ct_limit_set()``. The table holds connection tracking
+entries and has a limit on the maximum entries. There is one such table
+per-zone and hence one *limit* per zone. The zones are mapped to their limits
+through a hashtable using an RCU-managed hlist for the hash chains. When a new
+limit is set, a new limit object is allocated and ``ct_limit_set()`` is called
+to replace the old limit object with the new one using list_replace_rcu().
+The old limit object is then freed after a grace period using kfree_rcu().
+
+
+Example 4: Eliminating Stale Data
+---------------------------------
+
+The auditing example above tolerates 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 generally 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 shm_lock()
+function in ipc/shm.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 shm_lock() does in fact do.
+
+.. _quick_quiz:
+
+Quick Quiz:
+ For the deleted-flag technique to be helpful, why is it necessary
+ to hold the per-entry lock while returning from the search function?
+
+:ref:`Answer to Quick Quiz <quick_quiz_answer>`
+
+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;
+
+ /* No 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 */
+ }
+
+This too assumes that the caller holds ``audit_filter_mutex``.
+
+
+Example 5: Skipping Stale Objects
+---------------------------------
+
+For some usecases, reader performance can be improved by skipping stale objects
+during read-side list traversal if the object in concern is pending destruction
+after one or more grace periods. One such example can be found in the timerfd
+subsystem. When a ``CLOCK_REALTIME`` clock is reprogrammed - for example due to
+setting of the system time, then all programmed timerfds that depend on this
+clock get triggered and processes waiting on them to expire are woken up in
+advance of their scheduled expiry. To facilitate this, all such timers are added
+to an RCU-managed ``cancel_list`` when they are setup in
+``timerfd_setup_cancel()``::
+
+ static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
+ {
+ spin_lock(&ctx->cancel_lock);
+ if ((ctx->clockid == CLOCK_REALTIME &&
+ (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) {
+ if (!ctx->might_cancel) {
+ ctx->might_cancel = true;
+ spin_lock(&cancel_lock);
+ list_add_rcu(&ctx->clist, &cancel_list);
+ spin_unlock(&cancel_lock);
+ }
+ }
+ spin_unlock(&ctx->cancel_lock);
+ }
+
+When a timerfd is freed (fd is closed), then the ``might_cancel`` flag of the
+timerfd object is cleared, the object removed from the ``cancel_list`` and
+destroyed::
+
+ int timerfd_release(struct inode *inode, struct file *file)
+ {
+ struct timerfd_ctx *ctx = file->private_data;
+
+ spin_lock(&ctx->cancel_lock);
+ if (ctx->might_cancel) {
+ ctx->might_cancel = false;
+ spin_lock(&cancel_lock);
+ list_del_rcu(&ctx->clist);
+ spin_unlock(&cancel_lock);
+ }
+ spin_unlock(&ctx->cancel_lock);
+
+ hrtimer_cancel(&ctx->t.tmr);
+ kfree_rcu(ctx, rcu);
+ return 0;
+ }
+
+If the ``CLOCK_REALTIME`` clock is set, for example by a time server, the
+hrtimer framework calls ``timerfd_clock_was_set()`` which walks the
+``cancel_list`` and wakes up processes waiting on the timerfd. While iterating
+the ``cancel_list``, the ``might_cancel`` flag is consulted to skip stale
+objects::
+
+ void timerfd_clock_was_set(void)
+ {
+ struct timerfd_ctx *ctx;
+ unsigned long flags;
+
+ rcu_read_lock();
+ list_for_each_entry_rcu(ctx, &cancel_list, clist) {
+ if (!ctx->might_cancel)
+ continue;
+ spin_lock_irqsave(&ctx->wqh.lock, flags);
+ if (ctx->moffs != ktime_mono_to_real(0)) {
+ ctx->moffs = KTIME_MAX;
+ ctx->ticks++;
+ wake_up_locked_poll(&ctx->wqh, EPOLLIN);
+ }
+ spin_unlock_irqrestore(&ctx->wqh.lock, flags);
+ }
+ rcu_read_unlock();
+ }
+
+The key point here is, because RCU-traversal of the ``cancel_list`` happens
+while objects are being added and removed to the list, sometimes the traversal
+can step on an object that has been removed from the list. In this example, it
+is seen that it is better to skip such objects using a flag.
+
+
+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.
+
+.. _quick_quiz_answer:
+
+Answer to Quick Quiz:
+ For the deleted-flag technique to be helpful, why is it necessary
+ to hold the per-entry lock while returning from the search function?
+
+ 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.
+
+:ref:`Back to Quick Quiz <quick_quiz>`
generated by cgit v1.2.3 (git 2.39.1) at 2024-07-17 09:23:51 +0000