What is RCU? -- "Read, Copy, Update"

Please note that the "What is RCU?" LWN series is an excellent place

to start learning about RCU:

1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/

2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/

3. RCU part 3: the RCU API http://lwn.net/Articles/264090/

4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/

2010 Big API Table http://lwn.net/Articles/419086/

5. The RCU API, 2014 Edition http://lwn.net/Articles/609904/

2014 Big API Table http://lwn.net/Articles/609973/

What is RCU?

RCU is a synchronization mechanism that was added to the Linux kernel

during the 2.5 development effort that is optimized for read-mostly

situations. Although RCU is actually quite simple once you understand it,

getting there can sometimes be a challenge. Part of the problem is that

most of the past descriptions of RCU have been written with the mistaken

assumption that there is "one true way" to describe RCU. Instead,

the experience has been that different people must take different paths

to arrive at an understanding of RCU. This document provides several

different paths, as follows:

1. RCU OVERVIEW

2. WHAT IS RCU'S CORE API?

3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?

4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?

5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?

6. ANALOGY WITH READER-WRITER LOCKING

7. FULL LIST OF RCU APIs

8. ANSWERS TO QUICK QUIZZES

People who prefer starting with a conceptual overview should focus on

Section 1, though most readers will profit by reading this section at

some point. People who prefer to start with an API that they can then

experiment with should focus on Section 2. People who prefer to start

with example uses should focus on Sections 3 and 4. People who need to

understand the RCU implementation should focus on Section 5, then dive

into the kernel source code. People who reason best by analogy should

focus on Section 6. Section 7 serves as an index to the docbook API

documentation, and Section 8 is the traditional answer key.

So, start with the section that makes the most sense to you and your

preferred method of learning. If you need to know everything about

everything, feel free to read the whole thing -- but if you are really

that type of person, you have perused the source code and will therefore

never need this document anyway. ;-)

1. RCU OVERVIEW

The basic idea behind RCU is to split updates into "removal" and

"reclamation" phases. The removal phase removes references to data items

within a data structure (possibly by replacing them with references to

new versions of these data items), and can run concurrently with readers.

The reason that it is safe to run the removal phase concurrently with

readers is the semantics of modern CPUs guarantee that readers will see

either the old or the new version of the data structure rather than a

partially updated reference. The reclamation phase does the work of reclaiming

(e.g., freeing) the data items removed from the data structure during the

removal phase. Because reclaiming data items can disrupt any readers

concurrently referencing those data items, the reclamation phase must

not start until readers no longer hold references to those data items.

Splitting the update into removal and reclamation phases permits the

updater to perform the removal phase immediately, and to defer the

reclamation phase until all readers active during the removal phase have

completed, either by blocking until they finish or by registering a

callback that is invoked after they finish. Only readers that are active

during the removal phase need be considered, because any reader starting

after the removal phase will be unable to gain a reference to the removed

data items, and therefore cannot be disrupted by the reclamation phase.

So the typical RCU update sequence goes something like the following:

a. Remove pointers to a data structure, so that subsequent

readers cannot gain a reference to it.

b. Wait for all previous readers to complete their RCU read-side

critical sections.

c. At this point, there cannot be any readers who hold references

to the data structure, so it now may safely be reclaimed

(e.g., kfree()d).

Step (b) above is the key idea underlying RCU's deferred destruction.

The ability to wait until all readers are done allows RCU readers to

use much lighter-weight synchronization, in some cases, absolutely no

synchronization at all. In contrast, in more conventional lock-based

schemes, readers must use heavy-weight synchronization in order to

prevent an updater from deleting the data structure out from under them.

This is because lock-based updaters typically update data items in place,

and must therefore exclude readers. In contrast, RCU-based updaters

typically take advantage of the fact that writes to single aligned

pointers are atomic on modern CPUs, allowing atomic insertion, removal,

and replacement of data items in a linked structure without disrupting

readers. Concurrent RCU readers can then continue accessing the old

versions, and can dispense with the atomic operations, memory barriers,

and communications cache misses that are so expensive on present-day

SMP computer systems, even in absence of lock contention.

In the three-step procedure shown above, the updater is performing both

the removal and the reclamation step, but it is often helpful for an

entirely different thread to do the reclamation, as is in fact the case

in the Linux kernel's directory-entry cache (dcache). Even if the same

thread performs both the update step (step (a) above) and the reclamation

step (step (c) above), it is often helpful to think of them separately.

For example, RCU readers and updaters need not communicate at all,

but RCU provides implicit low-overhead communication between readers

and reclaimers, namely, in step (b) above.

So how the heck can a reclaimer tell when a reader is done, given

that readers are not doing any sort of synchronization operations???

Read on to learn about how RCU's API makes this easy.

2. WHAT IS RCU'S CORE API?

The core RCU API is quite small:

a. rcu_read_lock()

b. rcu_read_unlock()

c. synchronize_rcu() / call_rcu()

d. rcu_assign_pointer()

e. rcu_dereference()

There are many other members of the RCU API, but the rest can be

expressed in terms of these five, though most implementations instead

express synchronize_rcu() in terms of the call_rcu() callback API.

The five core RCU APIs are described below, the other 18 will be enumerated

later. See the kernel docbook documentation for more info, or look directly

at the function header comments.

rcu_read_lock()

void rcu_read_lock(void);

Used by a reader to inform the reclaimer that the reader is

entering an RCU read-side critical section. It is illegal

to block while in an RCU read-side critical section, though

kernels built with CONFIG_PREEMPT_RCU can preempt RCU

read-side critical sections. Any RCU-protected data structure

accessed during an RCU read-side critical section is guaranteed to

remain unreclaimed for the full duration of that critical section.

Reference counts may be used in conjunction with RCU to maintain

longer-term references to data structures.

rcu_read_unlock()

void rcu_read_unlock(void);

Used by a reader to inform the reclaimer that the reader is

exiting an RCU read-side critical section. Note that RCU

read-side critical sections may be nested and/or overlapping.

synchronize_rcu()

void synchronize_rcu(void);

Marks the end of updater code and the beginning of reclaimer

code. It does this by blocking until all pre-existing RCU

read-side critical sections on all CPUs have completed.

Note that synchronize_rcu() will -not- necessarily wait for

any subsequent RCU read-side critical sections to complete.

For example, consider the following sequence of events:

CPU 0 CPU 1 CPU 2

----------------- ------------------------- ---------------

1. rcu_read_lock()

2. enters synchronize_rcu()

3. rcu_read_lock()

4. rcu_read_unlock()

5. exits synchronize_rcu()

6. rcu_read_unlock()

To reiterate, synchronize_rcu() waits only for ongoing RCU

read-side critical sections to complete, not necessarily for

any that begin after synchronize_rcu() is invoked.

Of course, synchronize_rcu() does not necessarily return

-immediately- after the last pre-existing RCU read-side critical

section completes. For one thing, there might well be scheduling

delays. For another thing, many RCU implementations process

requests in batches in order to improve efficiencies, which can

further delay synchronize_rcu().

Since synchronize_rcu() is the API that must figure out when

readers are done, its implementation is key to RCU. For RCU

to be useful in all but the most read-intensive situations,

synchronize_rcu()'s overhead must also be quite small.

The call_rcu() API is a callback form of synchronize_rcu(),

and is described in more detail in a later section. Instead of

blocking, it registers a function and argument which are invoked

after all ongoing RCU read-side critical sections have completed.

This callback variant is particularly useful in situations where

it is illegal to block or where update-side performance is

critically important.

However, the call_rcu() API should not be used lightly, as use

of the synchronize_rcu() API generally results in simpler code.

In addition, the synchronize_rcu() API has the nice property

of automatically limiting update rate should grace periods

be delayed. This property results in system resilience in face

of denial-of-service attacks. Code using call_rcu() should limit

update rate in order to gain this same sort of resilience. See

checklist.txt for some approaches to limiting the update rate.

rcu_assign_pointer()

typeof(p) rcu_assign_pointer(p, typeof(p) v);

Yes, rcu_assign_pointer() -is- implemented as a macro, though it

would be cool to be able to declare a function in this manner.

(Compiler experts will no doubt disagree.)

The updater uses this function to assign a new value to an

RCU-protected pointer, in order to safely communicate the change

in value from the updater to the reader. This function returns

the new value, and also executes any memory-barrier instructions

required for a given CPU architecture.

Perhaps just as important, it serves to document (1) which

pointers are protected by RCU and (2) the point at which a

given structure becomes accessible to other CPUs. That said,

rcu_assign_pointer() is most frequently used indirectly, via

the _rcu list-manipulation primitives such as list_add_rcu().

rcu_dereference()

typeof(p) rcu_dereference(p);

Like rcu_assign_pointer(), rcu_dereference() must be implemented

as a macro.

The reader uses rcu_dereference() to fetch an RCU-protected

pointer, which returns a value that may then be safely

dereferenced. Note that rcu_dereference() does not actually

dereference the pointer, instead, it protects the pointer for

later dereferencing. It also executes any needed memory-barrier

instructions for a given CPU architecture. Currently, only Alpha

needs memory barriers within rcu_dereference() -- on other CPUs,

it compiles to nothing, not even a compiler directive.

Common coding practice uses rcu_dereference() to copy an

RCU-protected pointer to a local variable, then dereferences

this local variable, for example as follows:

p = rcu_dereference(head.next);

return p->data;

However, in this case, one could just as easily combine these

into one statement:

return rcu_dereference(head.next)->data;

If you are going to be fetching multiple fields from the

RCU-protected structure, using the local variable is of

course preferred. Repeated rcu_dereference() calls look

ugly, do not guarantee that the same pointer will be returned

if an update happened while in the critical section, and incur

unnecessary overhead on Alpha CPUs.

Note that the value returned by rcu_dereference() is valid

only within the enclosing RCU read-side critical section.

For example, the following is -not- legal:

rcu_read_lock();

p = rcu_dereference(head.next);

rcu_read_unlock();

x = p->address; /* BUG!!! */

rcu_read_lock();

y = p->data; /* BUG!!! */

rcu_read_unlock();

Holding a reference from one RCU read-side critical section

to another is just as illegal as holding a reference from

one lock-based critical section to another! Similarly,

using a reference outside of the critical section in which

it was acquired is just as illegal as doing so with normal

locking.

As with rcu_assign_pointer(), an important function of

rcu_dereference() is to document which pointers are protected by

RCU, in particular, flagging a pointer that is subject to changing

at any time, including immediately after the rcu_dereference().

And, again like rcu_assign_pointer(), rcu_dereference() is

typically used indirectly, via the _rcu list-manipulation

primitives, such as list_for_each_entry_rcu().

The following diagram shows how each API communicates among the

reader, updater, and reclaimer.

rcu_assign_pointer()

+--------+

+---------------------->| reader |---------+

| +--------+ |

| | |

| | | Protect:

| | | rcu_read_lock()

| | | rcu_read_unlock()

| rcu_dereference() | |

+---------+ | |

| updater |<---------------------+ |

+---------+ V

| +-----------+

+----------------------------------->| reclaimer |

+-----------+

Defer:

synchronize_rcu() & call_rcu()

The RCU infrastructure observes the time sequence of rcu_read_lock(),

rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in

order to determine when (1) synchronize_rcu() invocations may return

to their callers and (2) call_rcu() callbacks may be invoked. Efficient

implementations of the RCU infrastructure make heavy use of batching in

order to amortize their overhead over many uses of the corresponding APIs.

There are no fewer than three RCU mechanisms in the Linux kernel; the

diagram above shows the first one, which is by far the most commonly used.

The rcu_dereference() and rcu_assign_pointer() primitives are used for

all three mechanisms, but different defer and protect primitives are

used as follows:

Defer Protect

a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()

call_rcu() rcu_dereference()

b. synchronize_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()

call_rcu_bh() rcu_dereference_bh()

c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched()

call_rcu_sched() preempt_disable() / preempt_enable()

local_irq_save() / local_irq_restore()

hardirq enter / hardirq exit

NMI enter / NMI exit

rcu_dereference_sched()

These three mechanisms are used as follows:

a. RCU applied to normal data structures.

b. RCU applied to networking data structures that may be subjected

to remote denial-of-service attacks.

c. RCU applied to scheduler and interrupt/NMI-handler tasks.

Again, most uses will be of (a). The (b) and (c) cases are important

for specialized uses, but are relatively uncommon.

3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?

This section shows a simple use of the core RCU API to protect a

global pointer to a dynamically allocated structure. More-typical

uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.

struct foo {

int a;

char b;

long c;

};

DEFINE_SPINLOCK(foo_mutex);

struct foo __rcu *gbl_foo;

/*

* Create a new struct foo that is the same as the one currently

* pointed to by gbl_foo, except that field "a" is replaced

* with "new_a". Points gbl_foo to the new structure, and

* frees up the old structure after a grace period.

*

* Uses rcu_assign_pointer() to ensure that concurrent readers

* see the initialized version of the new structure.

*

* Uses synchronize_rcu() to ensure that any readers that might

* have references to the old structure complete before freeing

* the old structure.

*/

void foo_update_a(int new_a)

{

struct foo *new_fp;

struct foo *old_fp;

new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);

spin_lock(&foo_mutex);

old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));

*new_fp = *old_fp;

new_fp->a = new_a;

rcu_assign_pointer(gbl_foo, new_fp);

spin_unlock(&foo_mutex);

synchronize_rcu();

kfree(old_fp);

}

/*

* Return the value of field "a" of the current gbl_foo

* structure. Use rcu_read_lock() and rcu_read_unlock()

* to ensure that the structure does not get deleted out

* from under us, and use rcu_dereference() to ensure that

* we see the initialized version of the structure (important

* for DEC Alpha and for people reading the code).

*/

int foo_get_a(void)

{

int retval;

rcu_read_lock();

retval = rcu_dereference(gbl_foo)->a;

rcu_read_unlock();

return retval;

}

So, to sum up:

o Use rcu_read_lock() and rcu_read_unlock() to guard RCU

read-side critical sections.

o Within an RCU read-side critical section, use rcu_dereference()

to dereference RCU-protected pointers.

o Use some solid scheme (such as locks or semaphores) to

keep concurrent updates from interfering with each other.

o Use rcu_assign_pointer() to update an RCU-protected pointer.

This primitive protects concurrent readers from the updater,

-not- concurrent updates from each other! You therefore still

need to use locking (or something similar) to keep concurrent

rcu_assign_pointer() primitives from interfering with each other.

o Use synchronize_rcu() -after- removing a data element from an

RCU-protected data structure, but -before- reclaiming/freeing

the data element, in order to wait for the completion of all

RCU read-side critical sections that might be referencing that

data item.

See checklist.txt for additional rules to follow when using RCU.

And again, more-typical uses of RCU may be found in listRCU.txt,

arrayRCU.txt, and NMI-RCU.txt.

4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?

In the example above, foo_update_a() blocks until a grace period elapses.

This is quite simple, but in some cases one cannot afford to wait so

long -- there might be other high-priority work to be done.

In such cases, one uses call_rcu() rather than synchronize_rcu().

The call_rcu() API is as follows:

void call_rcu(struct rcu_head * head,

void (*func)(struct rcu_head *head));

This function invokes func(head) after a grace period has elapsed.

This invocation might happen from either softirq or process context,

so the function is not permitted to block. The foo struct needs to

have an rcu_head structure added, perhaps as follows:

struct foo {

int a;

char b;

long c;

struct rcu_head rcu;

};

The foo_update_a() function might then be written as follows:

/*

* Create a new struct foo that is the same as the one currently

* pointed to by gbl_foo, except that field "a" is replaced

* with "new_a". Points gbl_foo to the new structure, and

* frees up the old structure after a grace period.

*

* Uses rcu_assign_pointer() to ensure that concurrent readers

* see the initialized version of the new structure.

*

* Uses call_rcu() to ensure that any readers that might have

* references to the old structure complete before freeing the

* old structure.

*/

void foo_update_a(int new_a)

{

struct foo *new_fp;

struct foo *old_fp;

new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);

spin_lock(&foo_mutex);

old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));

*new_fp = *old_fp;

new_fp->a = new_a;

rcu_assign_pointer(gbl_foo, new_fp);

spin_unlock(&foo_mutex);

call_rcu(&old_fp->rcu, foo_reclaim);

}

The foo_reclaim() function might appear as follows:

void foo_reclaim(struct rcu_head *rp)

{

struct foo *fp = container_of(rp, struct foo, rcu);

foo_cleanup(fp->a);

kfree(fp);

}

The container_of() primitive is a macro that, given a pointer into a

struct, the type of the struct, and the pointed-to field within the

struct, returns a pointer to the beginning of the struct.

The use of call_rcu() permits the caller of foo_update_a() to

immediately regain control, without needing to worry further about the

old version of the newly updated element. It also clearly shows the

RCU distinction between updater, namely foo_update_a(), and reclaimer,

namely foo_reclaim().

The summary of advice is the same as for the previous section, except

that we are now using call_rcu() rather than synchronize_rcu():

o Use call_rcu() -after- removing a data element from an

RCU-protected data structure in order to register a callback

function that will be invoked after the completion of all RCU

read-side critical sections that might be referencing that

data item.

If the callback for call_rcu() is not doing anything more than calling

kfree() on the structure, you can use kfree_rcu() instead of call_rcu()

to avoid having to write your own callback:

kfree_rcu(old_fp, rcu);

Again, see checklist.txt for additional rules governing the use of RCU.

5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?

One of the nice things about RCU is that it has extremely simple "toy"

implementations that are a good first step towards understanding the

production-quality implementations in the Linux kernel. This section

presents two such "toy" implementations of RCU, one that is implemented

in terms of familiar locking primitives, and another that more closely

resembles "classic" RCU. Both are way too simple for real-world use,

lacking both functionality and performance. However, they are useful

in getting a feel for how RCU works. See kernel/rcupdate.c for a

production-quality implementation, and see:

http://www.rdrop.com/users/paulmck/RCU

for papers describing the Linux kernel RCU implementation. The OLS'01

and OLS'02 papers are a good introduction, and the dissertation provides

more details on the current implementation as of early 2004.

5A. "TOY" IMPLEMENTATION #1: LOCKING

This section presents a "toy" RCU implementation that is based on

familiar locking primitives. Its overhead makes it a non-starter for

real-life use, as does its lack of scalability. It is also unsuitable

for realtime use, since it allows scheduling latency to "bleed" from

one read-side critical section to another. It also assumes recursive

reader-writer locks: If you try this with non-recursive locks, and

you allow nested rcu_read_lock() calls, you can deadlock.

However, it is probably the easiest implementation to relate to, so is

a good starting point.

It is extremely simple:

static DEFINE_RWLOCK(rcu_gp_mutex);

void rcu_read_lock(void)

{

read_lock(&rcu_gp_mutex);

}

void rcu_read_unlock(void)

{

read_unlock(&rcu_gp_mutex);

}

void synchronize_rcu(void)

{

write_lock(&rcu_gp_mutex);

smp_mb__after_spinlock();

write_unlock(&rcu_gp_mutex);

}

[You can ignore rcu_assign_pointer() and rcu_dereference() without missing

much. But here are simplified versions anyway. And whatever you do,

don't forget about them when submitting patches making use of RCU!]

#define rcu_assign_pointer(p, v) \

({ \

smp_store_release(&(p), (v)); \

})

#define rcu_dereference(p) \

({ \

typeof(p) _________p1 = READ_ONCE(p); \

(_________p1); \

})

The rcu_read_lock() and rcu_read_unlock() primitive read-acquire

and release a global reader-writer lock. The synchronize_rcu()

primitive write-acquires this same lock, then releases it. This means

that once synchronize_rcu() exits, all RCU read-side critical sections

that were in progress before synchronize_rcu() was called are guaranteed

to have completed -- there is no way that synchronize_rcu() would have

been able to write-acquire the lock otherwise. The smp_mb__after_spinlock()

promotes synchronize_rcu() to a full memory barrier in compliance with

the "Memory-Barrier Guarantees" listed in:

Documentation/RCU/Design/Requirements/Requirements.html.

It is possible to nest rcu_read_lock(), since reader-writer locks may

be recursively acquired. Note also that rcu_read_lock() is immune

from deadlock (an important property of RCU). The reason for this is

that the only thing that can block rcu_read_lock() is a synchronize_rcu().

But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,

so there can be no deadlock cycle.

Quick Quiz #1: Why is this argument naive? How could a deadlock

occur when using this algorithm in a real-world Linux

kernel? How could this deadlock be avoided?

5B. "TOY" EXAMPLE #2: CLASSIC RCU

This section presents a "toy" RCU implementation that is based on

"classic RCU". It is also short on performance (but only for updates) and

on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT

kernels. The definitions of rcu_dereference() and rcu_assign_pointer()

are the same as those shown in the preceding section, so they are omitted.

void rcu_read_lock(void) { }

void rcu_read_unlock(void) { }

void synchronize_rcu(void)

{

int cpu;

for_each_possible_cpu(cpu)

run_on(cpu);

}

Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.

This is the great strength of classic RCU in a non-preemptive kernel:

read-side overhead is precisely zero, at least on non-Alpha CPUs.

And there is absolutely no way that rcu_read_lock() can possibly

participate in a deadlock cycle!

The implementation of synchronize_rcu() simply schedules itself on each

CPU in turn. The run_on() primitive can be implemented straightforwardly

in terms of the sched_setaffinity() primitive. Of course, a somewhat less

"toy" implementation would restore the affinity upon completion rather

than just leaving all tasks running on the last CPU, but when I said

"toy", I meant -toy-!

So how the heck is this supposed to work???

Remember that it is illegal to block while in an RCU read-side critical

section. Therefore, if a given CPU executes a context switch, we know

that it must have completed all preceding RCU read-side critical sections.

Once -all- CPUs have executed a context switch, then -all- preceding

RCU read-side critical sections will have completed.

So, suppose that we remove a data item from its structure and then invoke

synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed

that there are no RCU read-side critical sections holding a reference

to that data item, so we can safely reclaim it.

Quick Quiz #2: Give an example where Classic RCU's read-side

overhead is -negative-.

Quick Quiz #3: If it is illegal to block in an RCU read-side

critical section, what the heck do you do in

PREEMPT_RT, where normal spinlocks can block???

6. ANALOGY WITH READER-WRITER LOCKING

Although RCU can be used in many different ways, a very common use of

RCU is analogous to reader-writer locking. The following unified

diff shows how closely related RCU and reader-writer locking can be.

@@ -5,5 +5,5 @@ struct el {

int data;

/* Other data fields */

};

-rwlock_t listmutex;

+spinlock_t listmutex;

struct el head;

@@ -13,15 +14,15 @@

struct list_head *lp;

struct el *p;

- read_lock(&listmutex);

- list_for_each_entry(p, head, lp) {

+ rcu_read_lock();

+ list_for_each_entry_rcu(p, head, lp) {

if (p->key == key) {

*result = p->data;

- read_unlock(&listmutex);

+ rcu_read_unlock();

return 1;

}

}

- read_unlock(&listmutex);

+ rcu_read_unlock();

return 0;

}

@@ -29,15 +30,16 @@

{

struct el *p;

- write_lock(&listmutex);

+ spin_lock(&listmutex);

list_for_each_entry(p, head, lp) {

if (p->key == key) {

- list_del(&p->list);

- write_unlock(&listmutex);

+ list_del_rcu(&p->list);

+ spin_unlock(&listmutex);

+ synchronize_rcu();

kfree(p);

return 1;

}

}

- write_unlock(&listmutex);

+ spin_unlock(&listmutex);

return 0;

}

Or, for those who prefer a side-by-side listing:

1 struct el { 1 struct el {

2 struct list_head list; 2 struct list_head list;

3 long key; 3 long key;

4 spinlock_t mutex; 4 spinlock_t mutex;

5 int data; 5 int data;

6 /* Other data fields */ 6 /* Other data fields */

7 }; 7 };

8 rwlock_t listmutex; 8 spinlock_t listmutex;

9 struct el head; 9 struct el head;

1 int search(long key, int *result) 1 int search(long key, int *result)

2 { 2 {

3 struct list_head *lp; 3 struct list_head *lp;

4 struct el *p; 4 struct el *p;

5 5

6 read_lock(&listmutex); 6 rcu_read_lock();

7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {

8 if (p->key == key) { 8 if (p->key == key) {

9 *result = p->data; 9 *result = p->data;

10 read_unlock(&listmutex); 10 rcu_read_unlock();

11 return 1; 11 return 1;

12 } 12 }

13 } 13 }

14 read_unlock(&listmutex); 14 rcu_read_unlock();

15 return 0; 15 return 0;

16 } 16 }

1 int delete(long key) 1 int delete(long key)

2 { 2 {

3 struct el *p; 3 struct el *p;

4 4

5 write_lock(&listmutex); 5 spin_lock(&listmutex);

6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {

7 if (p->key == key) { 7 if (p->key == key) {

8 list_del(&p->list); 8 list_del_rcu(&p->list);

9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);

10 synchronize_rcu();

10 kfree(p); 11 kfree(p);

11 return 1; 12 return 1;

12 } 13 }

13 } 14 }

14 write_unlock(&listmutex); 15 spin_unlock(&listmutex);

15 return 0; 16 return 0;

16 } 17 }

Either way, the differences are quite small. Read-side locking moves

to rcu_read_lock() and rcu_read_unlock, update-side locking moves from

a reader-writer lock to a simple spinlock, and a synchronize_rcu()

precedes the kfree().

However, there is one potential catch: the read-side and update-side

critical sections can now run concurrently. In many cases, this will

not be a problem, but it is necessary to check carefully regardless.

For example, if multiple independent list updates must be seen as

a single atomic update, converting to RCU will require special care.

Also, the presence of synchronize_rcu() means that the RCU version of

delete() can now block. If this is a problem, there is a callback-based

mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can

be used in place of synchronize_rcu().

7. FULL LIST OF RCU APIs

The RCU APIs are documented in docbook-format header comments in the

Linux-kernel source code, but it helps to have a full list of the

APIs, since there does not appear to be a way to categorize them

in docbook. Here is the list, by category.

RCU list traversal:

list_entry_rcu

list_first_entry_rcu

list_next_rcu

list_for_each_entry_rcu

list_for_each_entry_continue_rcu

list_for_each_entry_from_rcu

hlist_first_rcu

hlist_next_rcu

hlist_pprev_rcu

hlist_for_each_entry_rcu

hlist_for_each_entry_rcu_bh

hlist_for_each_entry_from_rcu

hlist_for_each_entry_continue_rcu

hlist_for_each_entry_continue_rcu_bh

hlist_nulls_first_rcu

hlist_nulls_for_each_entry_rcu

hlist_bl_first_rcu

hlist_bl_for_each_entry_rcu

RCU pointer/list update:

rcu_assign_pointer

list_add_rcu

list_add_tail_rcu

list_del_rcu

list_replace_rcu

hlist_add_behind_rcu

hlist_add_before_rcu

hlist_add_head_rcu

hlist_del_rcu

hlist_del_init_rcu

hlist_replace_rcu

list_splice_init_rcu()

hlist_nulls_del_init_rcu

hlist_nulls_del_rcu

hlist_nulls_add_head_rcu

hlist_bl_add_head_rcu

hlist_bl_del_init_rcu

hlist_bl_del_rcu

hlist_bl_set_first_rcu

RCU: Critical sections Grace period Barrier

rcu_read_lock synchronize_net rcu_barrier

rcu_read_unlock synchronize_rcu

rcu_dereference synchronize_rcu_expedited

rcu_read_lock_held call_rcu

rcu_dereference_check kfree_rcu

rcu_dereference_protected

bh: Critical sections Grace period Barrier

rcu_read_lock_bh call_rcu_bh rcu_barrier_bh

rcu_read_unlock_bh synchronize_rcu_bh

rcu_dereference_bh synchronize_rcu_bh_expedited

rcu_dereference_bh_check

rcu_dereference_bh_protected

rcu_read_lock_bh_held

sched: Critical sections Grace period Barrier

rcu_read_lock_sched synchronize_sched rcu_barrier_sched

rcu_read_unlock_sched call_rcu_sched

[preempt_disable] synchronize_sched_expedited

[and friends]

rcu_read_lock_sched_notrace

rcu_read_unlock_sched_notrace

rcu_dereference_sched

rcu_dereference_sched_check

rcu_dereference_sched_protected

rcu_read_lock_sched_held

SRCU: Critical sections Grace period Barrier

srcu_read_lock synchronize_srcu srcu_barrier

srcu_read_unlock call_srcu

srcu_dereference synchronize_srcu_expedited

srcu_dereference_check

srcu_read_lock_held

SRCU: Initialization/cleanup

DEFINE_SRCU

DEFINE_STATIC_SRCU

init_srcu_struct

cleanup_srcu_struct

All: lockdep-checked RCU-protected pointer access

rcu_access_pointer

rcu_dereference_raw

RCU_LOCKDEP_WARN

rcu_sleep_check

RCU_NONIDLE

See the comment headers in the source code (or the docbook generated

from them) for more information.

However, given that there are no fewer than four families of RCU APIs

in the Linux kernel, how do you choose which one to use? The following

list can be helpful:

a. Will readers need to block? If so, you need SRCU.

b. What about the -rt patchset? If readers would need to block

in an non-rt kernel, you need SRCU. If readers would block

in a -rt kernel, but not in a non-rt kernel, SRCU is not

necessary. (The -rt patchset turns spinlocks into sleeplocks,

hence this distinction.)

c. Do you need to treat NMI handlers, hardirq handlers,

and code segments with preemption disabled (whether

via preempt_disable(), local_irq_save(), local_bh_disable(),

or some other mechanism) as if they were explicit RCU readers?

If so, RCU-sched is the only choice that will work for you.

d. Do you need RCU grace periods to complete even in the face

of softirq monopolization of one or more of the CPUs? For

example, is your code subject to network-based denial-of-service

attacks? If so, you need RCU-bh.

e. Is your workload too update-intensive for normal use of

RCU, but inappropriate for other synchronization mechanisms?

If so, consider SLAB_TYPESAFE_BY_RCU (which was originally

named SLAB_DESTROY_BY_RCU). But please be careful!

f. Do you need read-side critical sections that are respected

even though they are in the middle of the idle loop, during

user-mode execution, or on an offlined CPU? If so, SRCU is the

only choice that will work for you.

g. Otherwise, use RCU.

Of course, this all assumes that you have determined that RCU is in fact

the right tool for your job.

8. ANSWERS TO QUICK QUIZZES

Quick Quiz #1: Why is this argument naive? How could a deadlock

occur when using this algorithm in a real-world Linux

kernel? [Referring to the lock-based "toy" RCU

algorithm.]

Answer: Consider the following sequence of events:

1. CPU 0 acquires some unrelated lock, call it

"problematic_lock", disabling irq via

spin_lock_irqsave().

2. CPU 1 enters synchronize_rcu(), write-acquiring

rcu_gp_mutex.

3. CPU 0 enters rcu_read_lock(), but must wait

because CPU 1 holds rcu_gp_mutex.

4. CPU 1 is interrupted, and the irq handler

attempts to acquire problematic_lock.

The system is now deadlocked.

One way to avoid this deadlock is to use an approach like

that of CONFIG_PREEMPT_RT, where all normal spinlocks

become blocking locks, and all irq handlers execute in

the context of special tasks. In this case, in step 4

above, the irq handler would block, allowing CPU 1 to

release rcu_gp_mutex, avoiding the deadlock.

Even in the absence of deadlock, this RCU implementation

allows latency to "bleed" from readers to other

readers through synchronize_rcu(). To see this,

consider task A in an RCU read-side critical section

(thus read-holding rcu_gp_mutex), task B blocked

attempting to write-acquire rcu_gp_mutex, and

task C blocked in rcu_read_lock() attempting to

read_acquire rcu_gp_mutex. Task A's RCU read-side

latency is holding up task C, albeit indirectly via

task B.

Realtime RCU implementations therefore use a counter-based

approach where tasks in RCU read-side critical sections

cannot be blocked by tasks executing synchronize_rcu().