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/*
* Copyright (c) 2014, 2015, 2016 Nicira, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at:
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef OVS_RCU_H
#define OVS_RCU_H 1
/* Read-Copy-Update (RCU)
* ======================
*
* Introduction
* ------------
*
* Atomic pointer access makes it pretty easy to implement lock-free
* algorithms. There is one big problem, though: when a writer updates a
* pointer to point to a new data structure, some thread might be reading the
* old version, and there's no convenient way to free the old version when all
* threads are done with the old version.
*
* The function ovsrcu_postpone() solves that problem. The function pointer
* passed in as its argument is called only after all threads are done with old
* versions of data structures. The function callback frees an old version of
* data no longer in use. This technique is called "read-copy-update", or RCU
* for short.
*
*
* Details
* -------
*
* A "quiescent state" is a time at which a thread holds no pointers to memory
* that is managed by RCU; that is, when the thread is known not to reference
* memory that might be an old version of some object freed via RCU. For
* example, poll_block() includes a quiescent state.
*
* The following functions manage the recognition of quiescent states:
*
* void ovsrcu_quiesce(void)
*
* Recognizes a momentary quiescent state in the current thread.
*
* void ovsrcu_quiesce_start(void)
* void ovsrcu_quiesce_end(void)
*
* Brackets a time period during which the current thread is quiescent.
*
* A newly created thread is initially active, not quiescent. When a process
* becomes multithreaded, the main thread becomes active, not quiescent.
*
* When a quiescient state has occurred in every thread, we say that a "grace
* period" has occurred. Following a grace period, all of the callbacks
* postponed before the start of the grace period MAY be invoked. OVS takes
* care of this automatically through the RCU mechanism: while a process still
* has only a single thread, it invokes the postponed callbacks directly from
* ovsrcu_quiesce() and ovsrcu_quiesce_start(); after additional threads have
* been created, it creates an extra helper thread to invoke callbacks.
*
* Please note that while a postponed function call is guaranteed to happen
* after the next time all participating threads have quiesced at least once,
* there is no quarantee that all postponed functions are called as early as
* possible, or that the functions postponed by different threads would be
* called in the order the registrations took place. In particular, even if
* two threads provably postpone a function each in a specific order, the
* postponed functions may still be called in the opposite order, depending on
* the timing of when the threads call ovsrcu_quiesce(), how many functions
* they postpone, and when the ovs-rcu thread happens to grab the functions to
* be called.
*
* All functions postponed by a single thread are guaranteed to execute in the
* order they were postponed, however.
*
* Usage
* -----
*
* Use OVSRCU_TYPE(TYPE) to declare a pointer to RCU-protected data, e.g. the
* following declares an RCU-protected "struct flow *" named flowp:
*
* OVSRCU_TYPE(struct flow *) flowp;
*
* Use ovsrcu_get(TYPE, VAR) to read an RCU-protected pointer, e.g. to read the
* pointer variable declared above:
*
* struct flow *flow = ovsrcu_get(struct flow *, &flowp);
*
* If the pointer variable is currently protected against change (because
* the current thread holds a mutex that protects it), ovsrcu_get_protected()
* may be used instead. Only on the Alpha architecture is this likely to
* generate different code, but it may be useful documentation.
*
* (With GNU C or Clang, you get a compiler error if TYPE is wrong; other
* compilers will merrily carry along accepting the wrong type.)
*
* Use ovsrcu_set() to write an RCU-protected pointer and ovsrcu_postpone() to
* free the previous data. ovsrcu_set_hidden() can be used on RCU protected
* data not visible to any readers yet, but will be made visible by a later
* ovsrcu_set(). ovsrcu_init() can be used to initialize RCU pointers when
* no readers are yet executing. If more than one thread can write the
* pointer, then some form of external synchronization, e.g. a mutex, is
* needed to prevent writers from interfering with one another. For example,
* to write the pointer variable declared above while safely freeing the old
* value:
*
* static struct ovs_mutex mutex = OVS_MUTEX_INITIALIZER;
*
* OVSRCU_TYPE(struct flow *) flowp;
*
* void
* change_flow(struct flow *new_flow)
* {
* ovs_mutex_lock(&mutex);
* ovsrcu_postpone(free,
* ovsrcu_get_protected(struct flow *, &flowp));
* ovsrcu_set(&flowp, new_flow);
* ovs_mutex_unlock(&mutex);
* }
*
* In some rare cases an object may not be addressable with a pointer, but only
* through an array index (e.g. because it's provided by another library). It
* is still possible to have RCU semantics by using the ovsrcu_index type.
*
* static struct ovs_mutex mutex = OVS_MUTEX_INITIALIZER;
*
* ovsrcu_index port_id;
*
* void tx()
* {
* int id = ovsrcu_index_get(&port_id);
* if (id == -1) {
* return;
* }
* port_tx(id);
* }
*
* void delete()
* {
* int id;
*
* ovs_mutex_lock(&mutex);
* id = ovsrcu_index_get_protected(&port_id);
* ovsrcu_index_set(&port_id, -1);
* ovs_mutex_unlock(&mutex);
*
* ovsrcu_synchronize();
* port_delete(id);
* }
*
*/
#include "compiler.h"
#include "ovs-atomic.h"
#if __GNUC__
#define OVSRCU_TYPE(TYPE) struct { ATOMIC(TYPE) p; }
#define OVSRCU_INITIALIZER(VALUE) { ATOMIC_VAR_INIT(VALUE) }
#define ovsrcu_get__(TYPE, VAR, ORDER) \
({ \
TYPE value__; \
typeof(VAR) ovsrcu_var = (VAR); \
\
atomic_read_explicit(CONST_CAST(ATOMIC(TYPE) *, &ovsrcu_var->p), \
&value__, ORDER); \
\
value__; \
})
#define ovsrcu_get(TYPE, VAR) \
ovsrcu_get__(TYPE, VAR, memory_order_consume)
#define ovsrcu_get_protected(TYPE, VAR) \
ovsrcu_get__(TYPE, VAR, memory_order_relaxed)
/* 'VALUE' may be an atomic operation, which must be evaluated before
* any of the body of the atomic_store_explicit. Since the type of
* 'VAR' is not fixed, we cannot use an inline function to get
* function semantics for this. */
#define ovsrcu_set__(VAR, VALUE, ORDER) \
({ \
typeof(VAR) ovsrcu_var = (VAR); \
typeof(VALUE) ovsrcu_value = (VALUE); \
memory_order ovsrcu_order = (ORDER); \
\
atomic_store_explicit(&ovsrcu_var->p, ovsrcu_value, ovsrcu_order); \
(void *) 0; \
})
#else /* not GNU C */
struct ovsrcu_pointer { ATOMIC(void *) p; };
#define OVSRCU_TYPE(TYPE) struct ovsrcu_pointer
#define OVSRCU_INITIALIZER(VALUE) { ATOMIC_VAR_INIT(VALUE) }
static inline void *
ovsrcu_get__(const struct ovsrcu_pointer *pointer, memory_order order)
{
void *value;
atomic_read_explicit(&CONST_CAST(struct ovsrcu_pointer *, pointer)->p,
&value, order);
return value;
}
#define ovsrcu_get(TYPE, VAR) \
CONST_CAST(TYPE, ovsrcu_get__(VAR, memory_order_consume))
#define ovsrcu_get_protected(TYPE, VAR) \
CONST_CAST(TYPE, ovsrcu_get__(VAR, memory_order_relaxed))
static inline void ovsrcu_set__(struct ovsrcu_pointer *pointer,
const void *value,
memory_order order)
{
atomic_store_explicit(&pointer->p, CONST_CAST(void *, value), order);
}
#endif
/* Writes VALUE to the RCU-protected pointer whose address is VAR.
*
* Users require external synchronization (e.g. a mutex). See "Usage" above
* for an example. */
#define ovsrcu_set(VAR, VALUE) \
ovsrcu_set__(VAR, VALUE, memory_order_release)
/* This can be used for initializing RCU pointers before any readers can
* see them. A later ovsrcu_set() needs to make the bigger structure this
* is part of visible to the readers. */
#define ovsrcu_set_hidden(VAR, VALUE) \
ovsrcu_set__(VAR, VALUE, memory_order_relaxed)
/* This can be used for initializing RCU pointers before any readers are
* executing. */
#define ovsrcu_init(VAR, VALUE) atomic_init(&(VAR)->p, VALUE)
/* Calls FUNCTION passing ARG as its pointer-type argument following the next
* grace period. See "Usage" above for an example. */
void ovsrcu_postpone__(void (*function)(void *aux), void *aux);
#define ovsrcu_postpone(FUNCTION, ARG) \
(/* Verify that ARG is appropriate for FUNCTION. */ \
(void) sizeof((FUNCTION)(ARG), 1), \
/* Verify that ARG is a pointer type. */ \
(void) sizeof(*(ARG)), \
ovsrcu_postpone__((void (*)(void *))(FUNCTION), ARG))
/* An array index protected by RCU semantics. This is an easier alternative to
* an RCU protected pointer to a malloc'd int. */
typedef struct { atomic_int v; } ovsrcu_index;
static inline int ovsrcu_index_get__(const ovsrcu_index *i, memory_order order)
{
int ret;
atomic_read_explicit(CONST_CAST(atomic_int *, &i->v), &ret, order);
return ret;
}
/* Returns the index contained in 'i'. The returned value can be used until
* the next grace period. */
static inline int ovsrcu_index_get(const ovsrcu_index *i)
{
return ovsrcu_index_get__(i, memory_order_consume);
}
/* Returns the index contained in 'i'. This is an alternative to
* ovsrcu_index_get() that can be used when there's no possible concurrent
* writer. */
static inline int ovsrcu_index_get_protected(const ovsrcu_index *i)
{
return ovsrcu_index_get__(i, memory_order_relaxed);
}
static inline void ovsrcu_index_set__(ovsrcu_index *i, int value,
memory_order order)
{
atomic_store_explicit(&i->v, value, order);
}
/* Writes the index 'value' in 'i'. The previous value of 'i' may still be
* used by readers until the next grace period. */
static inline void ovsrcu_index_set(ovsrcu_index *i, int value)
{
ovsrcu_index_set__(i, value, memory_order_release);
}
/* Writes the index 'value' in 'i'. This is an alternative to
* ovsrcu_index_set() that can be used when there's no possible concurrent
* reader. */
static inline void ovsrcu_index_set_hidden(ovsrcu_index *i, int value)
{
ovsrcu_index_set__(i, value, memory_order_relaxed);
}
/* Initializes 'i' with 'value'. This is safe to call as long as there are no
* concurrent readers. */
static inline void ovsrcu_index_init(ovsrcu_index *i, int value)
{
atomic_init(&i->v, value);
}
/* Quiescent states. */
void ovsrcu_quiesce_start(void);
void ovsrcu_quiesce_end(void);
void ovsrcu_quiesce(void);
int ovsrcu_try_quiesce(void);
bool ovsrcu_is_quiescent(void);
/* Synchronization. Waits for all non-quiescent threads to quiesce at least
* once. This can block for a relatively long time. */
void ovsrcu_synchronize(void);
void ovsrcu_exit(void);
#endif /* ovs-rcu.h */