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:
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 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.
- 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.