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 #ifdef CONFIG_CLASSIC_RCU 56 #include <linux/rcuclassic.h> 57 #else /* #ifdef CONFIG_CLASSIC_RCU */ 58 #include <linux/rcupreempt.h> 59 #endif /* #else #ifdef CONFIG_CLASSIC_RCU */ 60 61 #define RCU_HEAD_INIT { .next = NULL, .func = NULL } 62 #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT 63 #define INIT_RCU_HEAD(ptr) do { \ 64 (ptr)->next = NULL; (ptr)->func = NULL; \ 65 } while (0) 66 67 /** 68 * rcu_read_lock - mark the beginning of an RCU read-side critical section. 69 * 70 * When synchronize_rcu() is invoked on one CPU while other CPUs 71 * are within RCU read-side critical sections, then the 72 * synchronize_rcu() is guaranteed to block until after all the other 73 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked 74 * on one CPU while other CPUs are within RCU read-side critical 75 * sections, invocation of the corresponding RCU callback is deferred 76 * until after the all the other CPUs exit their critical sections. 77 * 78 * Note, however, that RCU callbacks are permitted to run concurrently 79 * with RCU read-side critical sections. One way that this can happen 80 * is via the following sequence of events: (1) CPU 0 enters an RCU 81 * read-side critical section, (2) CPU 1 invokes call_rcu() to register 82 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section, 83 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU 84 * callback is invoked. This is legal, because the RCU read-side critical 85 * section that was running concurrently with the call_rcu() (and which 86 * therefore might be referencing something that the corresponding RCU 87 * callback would free up) has completed before the corresponding 88 * RCU callback is invoked. 89 * 90 * RCU read-side critical sections may be nested. Any deferred actions 91 * will be deferred until the outermost RCU read-side critical section 92 * completes. 93 * 94 * It is illegal to block while in an RCU read-side critical section. 95 */ 96 #define rcu_read_lock() __rcu_read_lock() 97 98 /** 99 * rcu_read_unlock - marks the end of an RCU read-side critical section. 100 * 101 * See rcu_read_lock() for more information. 102 */ 103 104 /* 105 * So where is rcu_write_lock()? It does not exist, as there is no 106 * way for writers to lock out RCU readers. This is a feature, not 107 * a bug -- this property is what provides RCU's performance benefits. 108 * Of course, writers must coordinate with each other. The normal 109 * spinlock primitives work well for this, but any other technique may be 110 * used as well. RCU does not care how the writers keep out of each 111 * others' way, as long as they do so. 112 */ 113 #define rcu_read_unlock() __rcu_read_unlock() 114 115 /** 116 * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section 117 * 118 * This is equivalent of rcu_read_lock(), but to be used when updates 119 * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks 120 * consider completion of a softirq handler to be a quiescent state, 121 * a process in RCU read-side critical section must be protected by 122 * disabling softirqs. Read-side critical sections in interrupt context 123 * can use just rcu_read_lock(). 124 * 125 */ 126 #define rcu_read_lock_bh() __rcu_read_lock_bh() 127 128 /* 129 * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section 130 * 131 * See rcu_read_lock_bh() for more information. 132 */ 133 #define rcu_read_unlock_bh() __rcu_read_unlock_bh() 134 135 /** 136 * rcu_dereference - fetch an RCU-protected pointer in an 137 * RCU read-side critical section. This pointer may later 138 * be safely dereferenced. 139 * 140 * Inserts memory barriers on architectures that require them 141 * (currently only the Alpha), and, more importantly, documents 142 * exactly which pointers are protected by RCU. 143 */ 144 145 #define rcu_dereference(p) ({ \ 146 typeof(p) _________p1 = ACCESS_ONCE(p); \ 147 smp_read_barrier_depends(); \ 148 (_________p1); \ 149 }) 150 151 /** 152 * rcu_assign_pointer - assign (publicize) a pointer to a newly 153 * initialized structure that will be dereferenced by RCU read-side 154 * critical sections. Returns the value assigned. 155 * 156 * Inserts memory barriers on architectures that require them 157 * (pretty much all of them other than x86), and also prevents 158 * the compiler from reordering the code that initializes the 159 * structure after the pointer assignment. More importantly, this 160 * call documents which pointers will be dereferenced by RCU read-side 161 * code. 162 */ 163 164 #define rcu_assign_pointer(p, v) \ 165 ({ \ 166 if (!__builtin_constant_p(v) || \ 167 ((v) != NULL)) \ 168 smp_wmb(); \ 169 (p) = (v); \ 170 }) 171 172 /* Infrastructure to implement the synchronize_() primitives. */ 173 174 struct rcu_synchronize { 175 struct rcu_head head; 176 struct completion completion; 177 }; 178 179 extern void wakeme_after_rcu(struct rcu_head *head); 180 181 #define synchronize_rcu_xxx(name, func) \ 182 void name(void) \ 183 { \ 184 struct rcu_synchronize rcu; \ 185 \ 186 init_completion(&rcu.completion); \ 187 /* Will wake me after RCU finished. */ \ 188 func(&rcu.head, wakeme_after_rcu); \ 189 /* Wait for it. */ \ 190 wait_for_completion(&rcu.completion); \ 191 } 192 193 /** 194 * synchronize_sched - block until all CPUs have exited any non-preemptive 195 * kernel code sequences. 196 * 197 * This means that all preempt_disable code sequences, including NMI and 198 * hardware-interrupt handlers, in progress on entry will have completed 199 * before this primitive returns. However, this does not guarantee that 200 * softirq handlers will have completed, since in some kernels, these 201 * handlers can run in process context, and can block. 202 * 203 * This primitive provides the guarantees made by the (now removed) 204 * synchronize_kernel() API. In contrast, synchronize_rcu() only 205 * guarantees that rcu_read_lock() sections will have completed. 206 * In "classic RCU", these two guarantees happen to be one and 207 * the same, but can differ in realtime RCU implementations. 208 */ 209 #define synchronize_sched() __synchronize_sched() 210 211 /** 212 * call_rcu - Queue an RCU callback for invocation after a grace period. 213 * @head: structure to be used for queueing the RCU updates. 214 * @func: actual update function to be invoked after the grace period 215 * 216 * The update function will be invoked some time after a full grace 217 * period elapses, in other words after all currently executing RCU 218 * read-side critical sections have completed. RCU read-side critical 219 * sections are delimited by rcu_read_lock() and rcu_read_unlock(), 220 * and may be nested. 221 */ 222 extern void call_rcu(struct rcu_head *head, 223 void (*func)(struct rcu_head *head)); 224 225 /** 226 * call_rcu_bh - Queue an RCU for invocation after a quicker 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. call_rcu_bh() assumes 233 * that the read-side critical sections end on completion of a softirq 234 * handler. This means that read-side critical sections in process 235 * context must not be interrupted by softirqs. This interface is to be 236 * used when most of the read-side critical sections are in softirq context. 237 * RCU read-side critical sections are delimited by : 238 * - rcu_read_lock() and rcu_read_unlock(), if in interrupt context. 239 * OR 240 * - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context. 241 * These may be nested. 242 */ 243 extern void call_rcu_bh(struct rcu_head *head, 244 void (*func)(struct rcu_head *head)); 245 246 /* Exported common interfaces */ 247 extern void synchronize_rcu(void); 248 extern void rcu_barrier(void); 249 extern void rcu_barrier_bh(void); 250 extern void rcu_barrier_sched(void); 251 252 /* Internal to kernel */ 253 extern void rcu_init(void); 254 extern int rcu_needs_cpu(int cpu); 255 256 #endif /* __LINUX_RCUPDATE_H */ 257