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doc: Update listRCU.rst 

This commit updates listRCU.txt to reflect RCU additions and changes
over the past few years.

[ paulmck: Apply kernel test robot feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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Paul E. McKenney 2022-09-11 02:57:47 -07:00
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172
Documentation/RCU/listRCU.rst
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@ -3,11 +3,10 @@
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
@ -35,7 +34,8 @@ The code traversing the list of all processes typically looks like::
}
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)
{
@ -45,39 +45,48 @@ The simplified code for removing a process from a task list is::
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.
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 use of RCU may be found in the
system-call auditing support. For example, a reader-writer locked
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;
@ -86,6 +95,8 @@ implementation of ``audit_filter_task()`` might be as follows::
/* 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;
}
@ -101,7 +112,7 @@ 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)
static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
{
struct audit_entry *e;
enum audit_state state;
@ -110,6 +121,8 @@ This means that RCU can be easily applied to the read side, as follows::
/* 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;
}
@ -118,13 +131,15 @@ This means that RCU can be easily applied to the read side, as follows::
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)
@ -188,16 +203,16 @@ Following are the RCU equivalents for these two functions::
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.
@ -238,7 +253,9 @@ need to be filled in)::
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,
@ -267,6 +284,9 @@ RCU (*read-copy update*) its name. The RCU code is as follows::
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
@ -281,9 +301,10 @@ 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.
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()
@ -302,7 +323,7 @@ Quick Quiz:
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)
{
@ -319,6 +340,8 @@ audit_entry structure, and modify ``audit_filter_task()`` as follows::
return AUDIT_BUILD_CONTEXT;
}
rcu_read_unlock();
if (state == AUDIT_STATE_RECORD)
*key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
return state;
}
}
@ -326,12 +349,6 @@ audit_entry structure, and modify ``audit_filter_task()`` as follows::
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::
@ -357,24 +374,32 @@ 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;
@ -382,13 +407,16 @@ to an RCU-managed ``cancel_list`` when they are setup in
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)
{
@ -403,7 +431,10 @@ destroyed::
}
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;
}
@ -416,6 +447,7 @@ objects::
void timerfd_clock_was_set(void)
{
ktime_t moffs = ktime_mono_to_real(0);
struct timerfd_ctx *ctx;
unsigned long flags;
@ -424,7 +456,7 @@ objects::
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);
@ -434,10 +466,10 @@ objects::
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