2 * Copyright (c) 2013, 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.
19 #include "fat-rwlock.h"
25 #include "ovs-thread.h"
28 struct fat_rwlock_slot {
29 /* Membership in rwlock's list of "struct fat_rwlock_slot"s.
31 * fat_rwlock_destroy() sets 'rwlock' to NULL to indicate that this
32 * slot may be destroyed. */
33 struct list list_node; /* In struct rwlock's 'threads' list. */
34 struct fat_rwlock *rwlock; /* Owner. */
38 * A thread holding the read-lock holds its own mutex.
40 * A thread holding the write-lock holds every thread's mutex, plus
42 struct ovs_mutex mutex;
44 /* This thread's locking status for 'rwlock':
46 * - 0: This thread does not have any lock on 'rwlock'. This thread
47 * does not have 'mutex' locked.
49 * - 1: This thread has a read-lock on 'rwlock' and holds 'mutex'.
51 * - 2...UINT_MAX-1: This thread has recursively taken the read-lock on
52 * 'rwlock' to the level of 'depth'. This thread holds 'mutex'.
54 * - UINT_MAX: This thread has the write-lock on 'rwlock' and holds
55 * 'mutex' (plus the 'mutex' of all of 'rwlock''s other slots).
57 * Accessed only by the slot's own thread, so no synchronization is
61 /* To prevent two of these structures from accidentally occupying the same
62 * cache line (causing "false sharing"), we cache-align each of these data
63 * structures. That requires malloc()ing extra space and throwing away
64 * some space at the beginning, which means that the pointer to this struct
65 * isn't necessarily the pointer to the beginning of the block, and so we
66 * need to retain the original pointer to free later.
68 * Accessed only by a single thread, so no synchronization is needed. */
69 void *base; /* Pointer to pass to free() for this block. */
73 free_slot(struct fat_rwlock_slot *slot)
79 list_remove(&slot->list_node);
84 slot_destructor(void *slot_)
86 struct fat_rwlock_slot *slot = slot_;
87 struct fat_rwlock *rwlock = slot->rwlock;
89 ovs_mutex_lock(&rwlock->mutex);
91 ovs_mutex_unlock(&rwlock->mutex);
94 /* Initialize 'rwlock' as a new fat_rwlock. */
96 fat_rwlock_init(struct fat_rwlock *rwlock)
98 ovsthread_key_create(&rwlock->key, slot_destructor);
99 ovs_mutex_init(&rwlock->mutex);
100 ovs_mutex_lock(&rwlock->mutex);
101 list_init(&rwlock->threads);
102 ovs_mutex_unlock(&rwlock->mutex);
105 /* Destroys 'rwlock', which must not be locked or otherwise in use by any
108 fat_rwlock_destroy(struct fat_rwlock *rwlock)
110 struct fat_rwlock_slot *slot, *next;
112 /* Order is important here. By destroying the thread-specific data first,
113 * before we destroy the slots, we ensure that the thread-specific
114 * data destructor can't race with our loop below. */
115 ovsthread_key_delete(rwlock->key);
117 LIST_FOR_EACH_SAFE (slot, next, list_node, &rwlock->threads) {
120 ovs_mutex_destroy(&rwlock->mutex);
123 static struct fat_rwlock_slot *
124 fat_rwlock_get_slot__(struct fat_rwlock *rwlock)
126 struct fat_rwlock_slot *slot;
130 slot = ovsthread_getspecific(rwlock->key);
135 /* Slow path: create a new slot for 'rwlock' in this thread. */
137 /* Allocate room for:
139 * - Up to CACHE_LINE_SIZE - 1 bytes before the per-thread, so that
140 * the start of the slot doesn't potentially share a cache line.
144 * - Space following the slot up to the end of the cache line, so
145 * that the end of the slot doesn't potentially share a cache
147 base = xmalloc((CACHE_LINE_SIZE - 1)
148 + ROUND_UP(sizeof *slot, CACHE_LINE_SIZE));
149 slot = (void *) ROUND_UP((uintptr_t) base, CACHE_LINE_SIZE);
152 slot->rwlock = rwlock;
153 ovs_mutex_init(&slot->mutex);
156 ovs_mutex_lock(&rwlock->mutex);
157 list_push_back(&rwlock->threads, &slot->list_node);
158 ovs_mutex_unlock(&rwlock->mutex);
160 ovsthread_setspecific(rwlock->key, slot);
165 /* Locks 'rwlock' for reading. The read-lock is recursive: it may be acquired
166 * any number of times by a single thread (which must then release it the same
167 * number of times for it to truly be released). */
169 fat_rwlock_rdlock(const struct fat_rwlock *rwlock_)
170 OVS_ACQ_RDLOCK(rwlock_)
171 OVS_NO_THREAD_SAFETY_ANALYSIS
173 struct fat_rwlock *rwlock = CONST_CAST(struct fat_rwlock *, rwlock_);
174 struct fat_rwlock_slot *this = fat_rwlock_get_slot__(rwlock);
176 switch (this->depth) {
178 /* This thread already holds the write-lock. */
182 ovs_mutex_lock(&this->mutex);
190 /* Tries to lock 'rwlock' for reading. If successful, returns 0. If taking
191 * the lock would require blocking, returns EBUSY (without blocking). */
193 fat_rwlock_tryrdlock(const struct fat_rwlock *rwlock_)
194 OVS_TRY_RDLOCK(0, rwlock_)
195 OVS_NO_THREAD_SAFETY_ANALYSIS
197 struct fat_rwlock *rwlock = CONST_CAST(struct fat_rwlock *, rwlock_);
198 struct fat_rwlock_slot *this = fat_rwlock_get_slot__(rwlock);
201 switch (this->depth) {
206 error = ovs_mutex_trylock(&this->mutex);
219 /* Locks 'rwlock' for writing.
221 * The write lock is not recursive. */
223 fat_rwlock_wrlock(const struct fat_rwlock *rwlock_)
224 OVS_ACQ_WRLOCK(rwlock_)
225 OVS_NO_THREAD_SAFETY_ANALYSIS
227 struct fat_rwlock *rwlock = CONST_CAST(struct fat_rwlock *, rwlock_);
228 struct fat_rwlock_slot *this = fat_rwlock_get_slot__(rwlock);
229 struct fat_rwlock_slot *slot;
231 ovs_assert(!this->depth);
232 this->depth = UINT_MAX;
234 ovs_mutex_lock(&rwlock->mutex);
235 LIST_FOR_EACH (slot, list_node, &rwlock->threads) {
236 ovs_mutex_lock(&slot->mutex);
240 /* Unlocks 'rwlock', which the current thread must have locked for reading or
241 * for writing. If the read lock has been taken recursively, it must be
242 * released the same number of times to be truly released. */
244 fat_rwlock_unlock(const struct fat_rwlock *rwlock_)
245 OVS_RELEASES(rwlock_)
246 OVS_NO_THREAD_SAFETY_ANALYSIS
248 struct fat_rwlock *rwlock = CONST_CAST(struct fat_rwlock *, rwlock_);
249 struct fat_rwlock_slot *this = fat_rwlock_get_slot__(rwlock);
250 struct fat_rwlock_slot *slot;
252 switch (this->depth) {
254 LIST_FOR_EACH (slot, list_node, &rwlock->threads) {
255 ovs_mutex_unlock(&slot->mutex);
257 ovs_mutex_unlock(&rwlock->mutex);
262 /* This thread doesn't hold any lock. */
266 ovs_mutex_unlock(&this->mutex);