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/cpumask.h> 40 #include <linux/seqlock.h> 41 #include <linux/lockdep.h> 42 #include <linux/completion.h> 43 44 /** 45 * struct rcu_head - callback structure for use with RCU 46 * @next: next update requests in a list 47 * @func: actual update function to call after the grace period. 48 */ 49 struct rcu_head { 50 struct rcu_head *next; 51 void (*func)(struct rcu_head *head); 52 }; 53 54 /* Exported common interfaces */ 55 extern void synchronize_rcu(void); 56 extern void synchronize_rcu_bh(void); 57 extern void rcu_barrier(void); 58 extern void rcu_barrier_bh(void); 59 extern void rcu_barrier_sched(void); 60 extern void synchronize_sched_expedited(void); 61 extern int sched_expedited_torture_stats(char *page); 62 63 /* Internal to kernel */ 64 extern void rcu_init(void); 65 extern void rcu_scheduler_starting(void); 66 extern int rcu_needs_cpu(int cpu); 67 extern int rcu_scheduler_active; 68 69 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU) 70 #include <linux/rcutree.h> 71 #else 72 #error "Unknown RCU implementation specified to kernel configuration" 73 #endif 74 75 #define RCU_HEAD_INIT { .next = NULL, .func = NULL } 76 #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT 77 #define INIT_RCU_HEAD(ptr) do { \ 78 (ptr)->next = NULL; (ptr)->func = NULL; \ 79 } while (0) 80 81 #ifdef CONFIG_DEBUG_LOCK_ALLOC 82 extern struct lockdep_map rcu_lock_map; 83 # define rcu_read_acquire() \ 84 lock_acquire(&rcu_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_) 85 # define rcu_read_release() lock_release(&rcu_lock_map, 1, _THIS_IP_) 86 #else 87 # define rcu_read_acquire() do { } while (0) 88 # define rcu_read_release() do { } while (0) 89 #endif 90 91 /** 92 * rcu_read_lock - mark the beginning of an RCU read-side critical section. 93 * 94 * When synchronize_rcu() is invoked on one CPU while other CPUs 95 * are within RCU read-side critical sections, then the 96 * synchronize_rcu() is guaranteed to block until after all the other 97 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked 98 * on one CPU while other CPUs are within RCU read-side critical 99 * sections, invocation of the corresponding RCU callback is deferred 100 * until after the all the other CPUs exit their critical sections. 101 * 102 * Note, however, that RCU callbacks are permitted to run concurrently 103 * with RCU read-side critical sections. One way that this can happen 104 * is via the following sequence of events: (1) CPU 0 enters an RCU 105 * read-side critical section, (2) CPU 1 invokes call_rcu() to register 106 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section, 107 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU 108 * callback is invoked. This is legal, because the RCU read-side critical 109 * section that was running concurrently with the call_rcu() (and which 110 * therefore might be referencing something that the corresponding RCU 111 * callback would free up) has completed before the corresponding 112 * RCU callback is invoked. 113 * 114 * RCU read-side critical sections may be nested. Any deferred actions 115 * will be deferred until the outermost RCU read-side critical section 116 * completes. 117 * 118 * It is illegal to block while in an RCU read-side critical section. 119 */ 120 static inline void rcu_read_lock(void) 121 { 122 __rcu_read_lock(); 123 __acquire(RCU); 124 rcu_read_acquire(); 125 } 126 127 /** 128 * rcu_read_unlock - marks the end of an RCU read-side critical section. 129 * 130 * See rcu_read_lock() for more information. 131 */ 132 133 /* 134 * So where is rcu_write_lock()? It does not exist, as there is no 135 * way for writers to lock out RCU readers. This is a feature, not 136 * a bug -- this property is what provides RCU's performance benefits. 137 * Of course, writers must coordinate with each other. The normal 138 * spinlock primitives work well for this, but any other technique may be 139 * used as well. RCU does not care how the writers keep out of each 140 * others' way, as long as they do so. 141 */ 142 static inline void rcu_read_unlock(void) 143 { 144 rcu_read_release(); 145 __release(RCU); 146 __rcu_read_unlock(); 147 } 148 149 /** 150 * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section 151 * 152 * This is equivalent of rcu_read_lock(), but to be used when updates 153 * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks 154 * consider completion of a softirq handler to be a quiescent state, 155 * a process in RCU read-side critical section must be protected by 156 * disabling softirqs. Read-side critical sections in interrupt context 157 * can use just rcu_read_lock(). 158 * 159 */ 160 static inline void rcu_read_lock_bh(void) 161 { 162 __rcu_read_lock_bh(); 163 __acquire(RCU_BH); 164 rcu_read_acquire(); 165 } 166 167 /* 168 * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section 169 * 170 * See rcu_read_lock_bh() for more information. 171 */ 172 static inline void rcu_read_unlock_bh(void) 173 { 174 rcu_read_release(); 175 __release(RCU_BH); 176 __rcu_read_unlock_bh(); 177 } 178 179 /** 180 * rcu_read_lock_sched - mark the beginning of a RCU-classic critical section 181 * 182 * Should be used with either 183 * - synchronize_sched() 184 * or 185 * - call_rcu_sched() and rcu_barrier_sched() 186 * on the write-side to insure proper synchronization. 187 */ 188 static inline void rcu_read_lock_sched(void) 189 { 190 preempt_disable(); 191 __acquire(RCU_SCHED); 192 rcu_read_acquire(); 193 } 194 static inline notrace void rcu_read_lock_sched_notrace(void) 195 { 196 preempt_disable_notrace(); 197 __acquire(RCU_SCHED); 198 } 199 200 /* 201 * rcu_read_unlock_sched - marks the end of a RCU-classic critical section 202 * 203 * See rcu_read_lock_sched for more information. 204 */ 205 static inline void rcu_read_unlock_sched(void) 206 { 207 rcu_read_release(); 208 __release(RCU_SCHED); 209 preempt_enable(); 210 } 211 static inline notrace void rcu_read_unlock_sched_notrace(void) 212 { 213 __release(RCU_SCHED); 214 preempt_enable_notrace(); 215 } 216 217 218 /** 219 * rcu_dereference - fetch an RCU-protected pointer in an 220 * RCU read-side critical section. This pointer may later 221 * be safely dereferenced. 222 * 223 * Inserts memory barriers on architectures that require them 224 * (currently only the Alpha), and, more importantly, documents 225 * exactly which pointers are protected by RCU. 226 */ 227 228 #define rcu_dereference(p) ({ \ 229 typeof(p) _________p1 = ACCESS_ONCE(p); \ 230 smp_read_barrier_depends(); \ 231 (_________p1); \ 232 }) 233 234 /** 235 * rcu_assign_pointer - assign (publicize) a pointer to a newly 236 * initialized structure that will be dereferenced by RCU read-side 237 * critical sections. Returns the value assigned. 238 * 239 * Inserts memory barriers on architectures that require them 240 * (pretty much all of them other than x86), and also prevents 241 * the compiler from reordering the code that initializes the 242 * structure after the pointer assignment. More importantly, this 243 * call documents which pointers will be dereferenced by RCU read-side 244 * code. 245 */ 246 247 #define rcu_assign_pointer(p, v) \ 248 ({ \ 249 if (!__builtin_constant_p(v) || \ 250 ((v) != NULL)) \ 251 smp_wmb(); \ 252 (p) = (v); \ 253 }) 254 255 /* Infrastructure to implement the synchronize_() primitives. */ 256 257 struct rcu_synchronize { 258 struct rcu_head head; 259 struct completion completion; 260 }; 261 262 extern void wakeme_after_rcu(struct rcu_head *head); 263 264 /** 265 * synchronize_sched - block until all CPUs have exited any non-preemptive 266 * kernel code sequences. 267 * 268 * This means that all preempt_disable code sequences, including NMI and 269 * hardware-interrupt handlers, in progress on entry will have completed 270 * before this primitive returns. However, this does not guarantee that 271 * softirq handlers will have completed, since in some kernels, these 272 * handlers can run in process context, and can block. 273 * 274 * This primitive provides the guarantees made by the (now removed) 275 * synchronize_kernel() API. In contrast, synchronize_rcu() only 276 * guarantees that rcu_read_lock() sections will have completed. 277 * In "classic RCU", these two guarantees happen to be one and 278 * the same, but can differ in realtime RCU implementations. 279 */ 280 #define synchronize_sched() __synchronize_sched() 281 282 /** 283 * call_rcu - Queue an RCU callback for invocation after a grace period. 284 * @head: structure to be used for queueing the RCU updates. 285 * @func: actual update function to be invoked after the grace period 286 * 287 * The update function will be invoked some time after a full grace 288 * period elapses, in other words after all currently executing RCU 289 * read-side critical sections have completed. RCU read-side critical 290 * sections are delimited by rcu_read_lock() and rcu_read_unlock(), 291 * and may be nested. 292 */ 293 extern void call_rcu(struct rcu_head *head, 294 void (*func)(struct rcu_head *head)); 295 296 /** 297 * call_rcu_bh - Queue an RCU for invocation after a quicker grace period. 298 * @head: structure to be used for queueing the RCU updates. 299 * @func: actual update function to be invoked after the grace period 300 * 301 * The update function will be invoked some time after a full grace 302 * period elapses, in other words after all currently executing RCU 303 * read-side critical sections have completed. call_rcu_bh() assumes 304 * that the read-side critical sections end on completion of a softirq 305 * handler. This means that read-side critical sections in process 306 * context must not be interrupted by softirqs. This interface is to be 307 * used when most of the read-side critical sections are in softirq context. 308 * RCU read-side critical sections are delimited by : 309 * - rcu_read_lock() and rcu_read_unlock(), if in interrupt context. 310 * OR 311 * - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context. 312 * These may be nested. 313 */ 314 extern void call_rcu_bh(struct rcu_head *head, 315 void (*func)(struct rcu_head *head)); 316 317 #endif /* __LINUX_RCUPDATE_H */ 318