Using RCU to Protect Read-Mostly Linked Lists¶
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.
When iterating a list while holding the rcu_read_lock(), writers may
modify the list. The reader is guaranteed to see all of the elements
which were added to the list before they acquired the rcu_read_lock()
and are still on the list when they drop the rcu_read_unlock().
Elements which are added to, or removed from the list may or may not
be seen. If the writer calls list_replace_rcu(), the reader may see
either the old element or the new element; they will not see both,
nor will they see neither.
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 and heavily inlined 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)
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¶
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, char **key)
{
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)) {
if (state == AUDIT_STATE_RECORD)
*key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
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, char **key)
{
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)) {
if (state == AUDIT_STATE_RECORD)
*key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
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 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 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)
{
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. 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 version of audit_upd_rule() 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.
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
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. 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() 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();
if (state == AUDIT_STATE_RECORD)
*key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
return state;
}
}
rcu_read_unlock();
return AUDIT_BUILD_CONTEXT;
}
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.
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 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 ||
ctx->clockid == CLOCK_REALTIME_ALARM) &&
(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);
}
} 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, as shown in this simplified and inlined
version of timerfd_release():
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);
if (isalarm(ctx))
alarm_cancel(&ctx->t.alarm);
else
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)
{
ktime_t moffs = ktime_mono_to_real(0);
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 != moffs) {
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 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¶
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.