2 * Copyright (c) 2014 Nicira, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at:
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
20 /* Read-Copy-Update (RCU)
21 * ======================
26 * Atomic pointer access makes it pretty easy to implement lock-free
27 * algorithms. There is one big problem, though: when a writer updates a
28 * pointer to point to a new data structure, some thread might be reading the
29 * old version, and there's no convenient way to free the old version when all
30 * threads are done with the old version.
32 * The function ovsrcu_postpone() solves that problem. The function pointer
33 * passed in as its argument is called only after all threads are done with old
34 * versions of data structures. The function callback frees an old version of
35 * data no longer in use. This technique is called "read-copy-update", or RCU
42 * A "quiescent state" is a time at which a thread holds no pointers to memory
43 * that is managed by RCU; that is, when the thread is known not to reference
44 * memory that might be an old version of some object freed via RCU. For
45 * example, poll_block() includes a quiescent state, as does
46 * ovs_mutex_cond_wait().
48 * The following functions manage the recognition of quiescent states:
50 * void ovsrcu_quiesce(void)
52 * Recognizes a momentary quiescent state in the current thread.
54 * void ovsrcu_quiesce_start(void)
55 * void ovsrcu_quiesce_end(void)
57 * Brackets a time period during which the current thread is quiescent.
59 * A newly created thread is initially active, not quiescent.
61 * When a quiescient state has occurred in every thread, we say that a "grace
62 * period" has occurred. Following a grace period, all of the callbacks
63 * postponed before the start of the grace period may be invoked. OVS takes
64 * care of this automatically through the RCU mechanism: while a process still
65 * has only a single thread, it invokes the postponed callbacks directly from
66 * ovsrcu_quiesce() and ovsrcu_quiesce_start(); after additional threads have
67 * been created, it creates an extra helper thread to invoke callbacks.
73 * Use OVSRCU_TYPE(TYPE) to declare a pointer to RCU-protected data, e.g. the
74 * following declares an RCU-protected "struct flow *" named flowp:
76 * OVSRCU_TYPE(struct flow *) flowp;
78 * Use ovsrcu_get(TYPE, VAR) to read an RCU-protected pointer, e.g. to read the
79 * pointer variable declared above:
81 * struct flow *flow = ovsrcu_get(struct flow *, flowp);
83 * Use ovsrcu_set() to write an RCU-protected pointer and ovsrcu_postpone() to
84 * free the previous data. If more than one thread can write the pointer, then
85 * some form of external synchronization, e.g. a mutex, is needed to prevent
86 * writers from interfering with one another. For example, to write the
87 * pointer variable declared above while safely freeing the old value:
89 * static struct ovs_mutex mutex = OVS_MUTEX_INITIALIZER;
92 * free_flow(struct flow *flow)
98 * change_flow(struct flow *new_flow)
100 * ovs_mutex_lock(&mutex);
101 * ovsrcu_postpone(free_flow,
102 * ovsrcu_get_protected(struct flow *, &flowp));
103 * ovsrcu_set(&flowp, new_flow);
104 * ovs_mutex_unlock(&mutex);
109 #include "compiler.h"
110 #include "ovs-atomic.h"
112 /* Use OVSRCU_TYPE(TYPE) to declare a pointer to RCU-protected data, e.g. the
113 * following declares an RCU-protected "struct flow *" named flowp:
115 * OVSRCU_TYPE(struct flow *) flowp;
117 * Use ovsrcu_get(TYPE, VAR) to read an RCU-protected pointer, e.g. to read the
118 * pointer variable declared above:
120 * struct flow *flow = ovsrcu_get(struct flow *, flowp);
122 * If the pointer variable is currently protected against change (because
123 * the current thread holds a mutex that protects it), ovsrcu_get_protected()
124 * may be used instead. Only on the Alpha architecture is this likely to
125 * generate different code, but it may be useful documentation.
127 * (With GNU C or Clang, you get a compiler error if TYPE is wrong; other
128 * compilers will merrily carry along accepting the wrong type.)
131 #define OVSRCU_TYPE(TYPE) struct { ATOMIC(TYPE) p; }
132 #define ovsrcu_get__(TYPE, VAR, ORDER) \
136 atomic_read_explicit(CONST_CAST(ATOMIC(TYPE) *, &(VAR)->p), \
141 #define ovsrcu_get(TYPE, VAR) \
142 CONST_CAST(TYPE, ovsrcu_get__(TYPE, VAR, memory_order_consume))
143 #define ovsrcu_get_protected(TYPE, VAR) \
144 CONST_CAST(TYPE, ovsrcu_get__(TYPE, VAR, memory_order_relaxed))
145 #else /* not GNU C */
146 typedef struct ovsrcu_pointer { ATOMIC(void *) p; };
147 #define OVSRCU_TYPE(TYPE) struct ovsrcu_pointer
149 ovsrcu_get__(const struct ovsrcu_pointer *pointer, memory_order order)
152 atomic_read_explicit(&CONST_CAST(struct ovsrcu_pointer *, pointer)->p,
156 #define ovsrcu_get(TYPE, VAR) \
157 CONST_CAST(TYPE, ovsrcu_get__(VAR, memory_order_consume))
158 #define ovsrcu_get_protected(TYPE, VAR) \
159 CONST_CAST(TYPE, ovsrcu_get__(VAR, memory_order_relaxed))
162 /* Writes VALUE to the RCU-protected pointer whose address is VAR.
164 * Users require external synchronization (e.g. a mutex). See "Usage" above
166 #define ovsrcu_set(VAR, VALUE) \
167 atomic_store_explicit(&(VAR)->p, VALUE, memory_order_release)
169 /* Calls FUNCTION passing ARG as its pointer-type argument following the next
170 * grace period. See "Usage" above for example. */
171 void ovsrcu_postpone__(void (*function)(void *aux), void *aux);
172 #define ovsrcu_postpone(FUNCTION, ARG) \
173 ((void) sizeof((FUNCTION)(ARG), 1), \
174 (void) sizeof(*(ARG)), \
175 ovsrcu_postpone__((void (*)(void *))(FUNCTION), ARG))
177 /* Quiescent states. */
178 void ovsrcu_quiesce_start(void);
179 void ovsrcu_quiesce_end(void);
180 void ovsrcu_quiesce(void);
182 #endif /* ovs-rcu.h */