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.
+One of the most common uses of RCU is protecting read-mostly linked lists
+(``struct list_head`` in list.h). One big advantage of this approach is
+that all of the required memory ordering is provided by the list macros.
+This document describes several list-based RCU use cases.
Example 1: Read-mostly list: Deferred Destruction
}
rcu_read_unlock();
-The simplified code for removing a process from a task list is::
+The simplified and heavily inlined code for removing a process from a
+task list is::
void release_task(struct task_struct *p)
{
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.
+When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)``
+via __exit_signal() and __unhash_process() under ``tasklist_lock``
+writer lock protection. The list_del_rcu() invocation removes
+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(), which is invoked via
+put_task_struct_rcu_user(). 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 refrains
+from invoking the delayed_put_task_struct() callback function until
+all existing readers finish, which guarantees that the ``task_struct``
+object in question will remain in existence until after the completion
+of all RCU readers that might possibly have a reference to that object.
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
+Some reader-writer locking use cases compute a value while holding
+the read-side lock, but continue to use that value after that lock is
+released. These use cases are often good candidates for conversion
+to RCU. One prominent example involves network packet routing.
+Because the packet-routing data tracks 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. This is a rare example
+of the finite speed of light and the non-zero size of atoms actually
+helping make synchronization be lighter weight.
+
+A straightforward example of this type of RCU use case 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)
+ static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
{
struct audit_entry *e;
enum audit_state state;
/* Note: audit_filter_mutex held by caller. */
list_for_each_entry(e, &audit_tsklist, list) {
if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
+ if (state == AUDIT_STATE_RECORD)
+ *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
read_unlock(&auditsc_lock);
return state;
}
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)
+ static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
{
struct audit_entry *e;
enum audit_state state;
/* 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)) {
+ if (state == AUDIT_STATE_RECORD)
+ *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
rcu_read_unlock();
return state;
}
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 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 add READ_ONCE() and diagnostic checks for incorrect use
+outside of an RCU read-side critical section.
The changes to the update side are also straightforward. A reader-writer lock
-might be used as follows for deletion and insertion::
+might be used as follows for deletion and insertion in these simplified
+versions of audit_del_rule() and audit_add_rule()::
static inline int audit_del_rule(struct audit_rule *rule,
struct list_head *list)
return 0;
}
-Normally, the ``write_lock()`` and ``write_unlock()`` would be replaced by a
+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
+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
+The **_rcu()** list-manipulation primitives add memory barriers that are
+needed on weakly ordered CPUs. The list_del_rcu() primitive omits the
pointer poisoning debug-assist code that would otherwise cause concurrent
readers to fail spectacularly.
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::
+RCU (*read-copy update*) its name.
+
+The RCU version of audit_upd_rule() is as follows::
static inline int audit_upd_rule(struct audit_rule *rule,
struct list_head *list,
Again, this assumes that the caller holds ``audit_filter_mutex``. Normally, the
writer lock would become a spinlock in this sort of code.
+The update_lsm_rule() does something very similar, for those who would
+prefer to look at real Linux-kernel 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
---------------------------------
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.
+that are tracking external state. After all, given there is a delay
+from the time the external state changes before Linux becomes aware
+of the change, and so as noted earlier, a small quantity of 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()
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::
+``audit_entry`` structure, and modify audit_filter_task() as follows::
static enum audit_state audit_filter_task(struct task_struct *tsk)
{
return AUDIT_BUILD_CONTEXT;
}
rcu_read_unlock();
+ if (state == AUDIT_STATE_RECORD)
+ *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
return state;
}
}
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::
This too assumes that the caller holds ``audit_filter_mutex``.
+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 to hold the locks of both the old ``audit_entry`` and its replacement
+while executing the list_replace_rcu().
+
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
+For some use cases, reader performance can be improved by skipping
+stale objects during read-side list traversal, where stale objects
+are those that will be removed and destroyed 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 are awakened 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 &&
+ if ((ctx->clockid == CLOCK_REALTIME ||
+ ctx->clockid == CLOCK_REALTIME_ALARM) &&
(flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) {
if (!ctx->might_cancel) {
ctx->might_cancel = true;
list_add_rcu(&ctx->clist, &cancel_list);
spin_unlock(&cancel_lock);
}
+ } else {
+ __timerfd_remove_cancel(ctx);
}
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::
+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, as shown in this simplified and inlined
+version of timerfd_release()::
int timerfd_release(struct inode *inode, struct file *file)
{
}
spin_unlock(&ctx->cancel_lock);
- hrtimer_cancel(&ctx->t.tmr);
+ if (isalarm(ctx))
+ alarm_cancel(&ctx->t.alarm);
+ else
+ hrtimer_cancel(&ctx->t.tmr);
kfree_rcu(ctx, rcu);
return 0;
}
void timerfd_clock_was_set(void)
{
+ ktime_t moffs = ktime_mono_to_real(0);
struct timerfd_ctx *ctx;
unsigned long flags;
if (!ctx->might_cancel)
continue;
spin_lock_irqsave(&ctx->wqh.lock, flags);
- if (ctx->moffs != ktime_mono_to_real(0)) {
+ if (ctx->moffs != moffs) {
ctx->moffs = KTIME_MAX;
ctx->ticks++;
wake_up_locked_poll(&ctx->wqh, EPOLLIN);
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.
+The key point is that because RCU-protected traversal of the
+``cancel_list`` happens concurrently with object addition and removal,
+sometimes the traversal can access an object that has been removed from
+the list. In this example, a flag is used to skip such objects.
Summary
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