1 /* 2 * Read-Copy Update mechanism for mutual exclusion 3 * 4 * This program is free software; you can redistribute it and/or modify 5 * it under the terms of the GNU General Public License as published by 6 * the Free Software Foundation; either version 2 of the License, or 7 * (at your option) any later version. 8 * 9 * This program is distributed in the hope that it will be useful, 10 * but WITHOUT ANY WARRANTY; without even the implied warranty of 11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 12 * GNU General Public License for more details. 13 * 14 * You should have received a copy of the GNU General Public License 15 * along with this program; if not, write to the Free Software 16 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. 17 * 18 * Copyright IBM Corporation, 2001 19 * 20 * Author: Dipankar Sarma <[email protected]> 21 * 22 * Based on the original work by Paul McKenney <[email protected]> 23 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen. 24 * Papers: 25 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf 26 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001) 27 * 28 * For detailed explanation of Read-Copy Update mechanism see - 29 * http://lse.sourceforge.net/locking/rcupdate.html 30 * 31 */ 32 33 #ifndef __LINUX_RCUPDATE_H 34 #define __LINUX_RCUPDATE_H 35 36 #include <linux/cache.h> 37 #include <linux/spinlock.h> 38 #include <linux/threads.h> 39 #include <linux/percpu.h> 40 #include <linux/cpumask.h> 41 #include <linux/seqlock.h> 42 #include <linux/lockdep.h> 43 #include <linux/completion.h> 44 45 /** 46 * struct rcu_head - callback structure for use with RCU 47 * @next: next update requests in a list 48 * @func: actual update function to call after the grace period. 49 */ 50 struct rcu_head { 51 struct rcu_head *next; 52 void (*func)(struct rcu_head *head); 53 }; 54 55 #if defined(CONFIG_CLASSIC_RCU) 56 #include <linux/rcuclassic.h> 57 #elif defined(CONFIG_TREE_RCU) 58 #include <linux/rcutree.h> 59 #elif defined(CONFIG_PREEMPT_RCU) 60 #include <linux/rcupreempt.h> 61 #else 62 #error "Unknown RCU implementation specified to kernel configuration" 63 #endif /* #else #if defined(CONFIG_CLASSIC_RCU) */ 64 65 #define RCU_HEAD_INIT { .next = NULL, .func = NULL } 66 #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT 67 #define INIT_RCU_HEAD(ptr) do { \ 68 (ptr)->next = NULL; (ptr)->func = NULL; \ 69 } while (0) 70 71 /** 72 * rcu_read_lock - mark the beginning of an RCU read-side critical section. 73 * 74 * When synchronize_rcu() is invoked on one CPU while other CPUs 75 * are within RCU read-side critical sections, then the 76 * synchronize_rcu() is guaranteed to block until after all the other 77 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked 78 * on one CPU while other CPUs are within RCU read-side critical 79 * sections, invocation of the corresponding RCU callback is deferred 80 * until after the all the other CPUs exit their critical sections. 81 * 82 * Note, however, that RCU callbacks are permitted to run concurrently 83 * with RCU read-side critical sections. One way that this can happen 84 * is via the following sequence of events: (1) CPU 0 enters an RCU 85 * read-side critical section, (2) CPU 1 invokes call_rcu() to register 86 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section, 87 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU 88 * callback is invoked. This is legal, because the RCU read-side critical 89 * section that was running concurrently with the call_rcu() (and which 90 * therefore might be referencing something that the corresponding RCU 91 * callback would free up) has completed before the corresponding 92 * RCU callback is invoked. 93 * 94 * RCU read-side critical sections may be nested. Any deferred actions 95 * will be deferred until the outermost RCU read-side critical section 96 * completes. 97 * 98 * It is illegal to block while in an RCU read-side critical section. 99 */ 100 #define rcu_read_lock() __rcu_read_lock() 101 102 /** 103 * rcu_read_unlock - marks the end of an RCU read-side critical section. 104 * 105 * See rcu_read_lock() for more information. 106 */ 107 108 /* 109 * So where is rcu_write_lock()? It does not exist, as there is no 110 * way for writers to lock out RCU readers. This is a feature, not 111 * a bug -- this property is what provides RCU's performance benefits. 112 * Of course, writers must coordinate with each other. The normal 113 * spinlock primitives work well for this, but any other technique may be 114 * used as well. RCU does not care how the writers keep out of each 115 * others' way, as long as they do so. 116 */ 117 #define rcu_read_unlock() __rcu_read_unlock() 118 119 /** 120 * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section 121 * 122 * This is equivalent of rcu_read_lock(), but to be used when updates 123 * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks 124 * consider completion of a softirq handler to be a quiescent state, 125 * a process in RCU read-side critical section must be protected by 126 * disabling softirqs. Read-side critical sections in interrupt context 127 * can use just rcu_read_lock(). 128 * 129 */ 130 #define rcu_read_lock_bh() __rcu_read_lock_bh() 131 132 /* 133 * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section 134 * 135 * See rcu_read_lock_bh() for more information. 136 */ 137 #define rcu_read_unlock_bh() __rcu_read_unlock_bh() 138 139 /** 140 * rcu_read_lock_sched - mark the beginning of a RCU-classic critical section 141 * 142 * Should be used with either 143 * - synchronize_sched() 144 * or 145 * - call_rcu_sched() and rcu_barrier_sched() 146 * on the write-side to insure proper synchronization. 147 */ 148 #define rcu_read_lock_sched() preempt_disable() 149 #define rcu_read_lock_sched_notrace() preempt_disable_notrace() 150 151 /* 152 * rcu_read_unlock_sched - marks the end of a RCU-classic critical section 153 * 154 * See rcu_read_lock_sched for more information. 155 */ 156 #define rcu_read_unlock_sched() preempt_enable() 157 #define rcu_read_unlock_sched_notrace() preempt_enable_notrace() 158 159 160 161 /** 162 * rcu_dereference - fetch an RCU-protected pointer in an 163 * RCU read-side critical section. This pointer may later 164 * be safely dereferenced. 165 * 166 * Inserts memory barriers on architectures that require them 167 * (currently only the Alpha), and, more importantly, documents 168 * exactly which pointers are protected by RCU. 169 */ 170 171 #define rcu_dereference(p) ({ \ 172 typeof(p) _________p1 = ACCESS_ONCE(p); \ 173 smp_read_barrier_depends(); \ 174 (_________p1); \ 175 }) 176 177 /** 178 * rcu_assign_pointer - assign (publicize) a pointer to a newly 179 * initialized structure that will be dereferenced by RCU read-side 180 * critical sections. Returns the value assigned. 181 * 182 * Inserts memory barriers on architectures that require them 183 * (pretty much all of them other than x86), and also prevents 184 * the compiler from reordering the code that initializes the 185 * structure after the pointer assignment. More importantly, this 186 * call documents which pointers will be dereferenced by RCU read-side 187 * code. 188 */ 189 190 #define rcu_assign_pointer(p, v) \ 191 ({ \ 192 if (!__builtin_constant_p(v) || \ 193 ((v) != NULL)) \ 194 smp_wmb(); \ 195 (p) = (v); \ 196 }) 197 198 /* Infrastructure to implement the synchronize_() primitives. */ 199 200 struct rcu_synchronize { 201 struct rcu_head head; 202 struct completion completion; 203 }; 204 205 extern void wakeme_after_rcu(struct rcu_head *head); 206 207 /** 208 * synchronize_sched - block until all CPUs have exited any non-preemptive 209 * kernel code sequences. 210 * 211 * This means that all preempt_disable code sequences, including NMI and 212 * hardware-interrupt handlers, in progress on entry will have completed 213 * before this primitive returns. However, this does not guarantee that 214 * softirq handlers will have completed, since in some kernels, these 215 * handlers can run in process context, and can block. 216 * 217 * This primitive provides the guarantees made by the (now removed) 218 * synchronize_kernel() API. In contrast, synchronize_rcu() only 219 * guarantees that rcu_read_lock() sections will have completed. 220 * In "classic RCU", these two guarantees happen to be one and 221 * the same, but can differ in realtime RCU implementations. 222 */ 223 #define synchronize_sched() __synchronize_sched() 224 225 /** 226 * call_rcu - Queue an RCU callback for invocation after a grace period. 227 * @head: structure to be used for queueing the RCU updates. 228 * @func: actual update function to be invoked after the grace period 229 * 230 * The update function will be invoked some time after a full grace 231 * period elapses, in other words after all currently executing RCU 232 * read-side critical sections have completed. RCU read-side critical 233 * sections are delimited by rcu_read_lock() and rcu_read_unlock(), 234 * and may be nested. 235 */ 236 extern void call_rcu(struct rcu_head *head, 237 void (*func)(struct rcu_head *head)); 238 239 /** 240 * call_rcu_bh - Queue an RCU for invocation after a quicker grace period. 241 * @head: structure to be used for queueing the RCU updates. 242 * @func: actual update function to be invoked after the grace period 243 * 244 * The update function will be invoked some time after a full grace 245 * period elapses, in other words after all currently executing RCU 246 * read-side critical sections have completed. call_rcu_bh() assumes 247 * that the read-side critical sections end on completion of a softirq 248 * handler. This means that read-side critical sections in process 249 * context must not be interrupted by softirqs. This interface is to be 250 * used when most of the read-side critical sections are in softirq context. 251 * RCU read-side critical sections are delimited by : 252 * - rcu_read_lock() and rcu_read_unlock(), if in interrupt context. 253 * OR 254 * - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context. 255 * These may be nested. 256 */ 257 extern void call_rcu_bh(struct rcu_head *head, 258 void (*func)(struct rcu_head *head)); 259 260 /* Exported common interfaces */ 261 extern void synchronize_rcu(void); 262 extern void rcu_barrier(void); 263 extern void rcu_barrier_bh(void); 264 extern void rcu_barrier_sched(void); 265 266 /* Internal to kernel */ 267 extern void rcu_init(void); 268 extern int rcu_needs_cpu(int cpu); 269 270 #endif /* __LINUX_RCUPDATE_H */ 271