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 (C) 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 #ifdef __KERNEL__ 37 38 #include <linux/cache.h> 39 #include <linux/spinlock.h> 40 #include <linux/threads.h> 41 #include <linux/percpu.h> 42 #include <linux/cpumask.h> 43 #include <linux/seqlock.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 #define RCU_HEAD_INIT(head) { .next = NULL, .func = NULL } 56 #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT(head) 57 #define INIT_RCU_HEAD(ptr) do { \ 58 (ptr)->next = NULL; (ptr)->func = NULL; \ 59 } while (0) 60 61 62 63 /* Global control variables for rcupdate callback mechanism. */ 64 struct rcu_ctrlblk { 65 long cur; /* Current batch number. */ 66 long completed; /* Number of the last completed batch */ 67 int next_pending; /* Is the next batch already waiting? */ 68 } ____cacheline_maxaligned_in_smp; 69 70 /* Is batch a before batch b ? */ 71 static inline int rcu_batch_before(long a, long b) 72 { 73 return (a - b) < 0; 74 } 75 76 /* Is batch a after batch b ? */ 77 static inline int rcu_batch_after(long a, long b) 78 { 79 return (a - b) > 0; 80 } 81 82 /* 83 * Per-CPU data for Read-Copy UPdate. 84 * nxtlist - new callbacks are added here 85 * curlist - current batch for which quiescent cycle started if any 86 */ 87 struct rcu_data { 88 /* 1) quiescent state handling : */ 89 long quiescbatch; /* Batch # for grace period */ 90 int passed_quiesc; /* User-mode/idle loop etc. */ 91 int qs_pending; /* core waits for quiesc state */ 92 93 /* 2) batch handling */ 94 long batch; /* Batch # for current RCU batch */ 95 struct rcu_head *nxtlist; 96 struct rcu_head **nxttail; 97 struct rcu_head *curlist; 98 struct rcu_head **curtail; 99 struct rcu_head *donelist; 100 struct rcu_head **donetail; 101 int cpu; 102 }; 103 104 DECLARE_PER_CPU(struct rcu_data, rcu_data); 105 DECLARE_PER_CPU(struct rcu_data, rcu_bh_data); 106 extern struct rcu_ctrlblk rcu_ctrlblk; 107 extern struct rcu_ctrlblk rcu_bh_ctrlblk; 108 109 /* 110 * Increment the quiescent state counter. 111 * The counter is a bit degenerated: We do not need to know 112 * how many quiescent states passed, just if there was at least 113 * one since the start of the grace period. Thus just a flag. 114 */ 115 static inline void rcu_qsctr_inc(int cpu) 116 { 117 struct rcu_data *rdp = &per_cpu(rcu_data, cpu); 118 rdp->passed_quiesc = 1; 119 } 120 static inline void rcu_bh_qsctr_inc(int cpu) 121 { 122 struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu); 123 rdp->passed_quiesc = 1; 124 } 125 126 static inline int __rcu_pending(struct rcu_ctrlblk *rcp, 127 struct rcu_data *rdp) 128 { 129 /* This cpu has pending rcu entries and the grace period 130 * for them has completed. 131 */ 132 if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch)) 133 return 1; 134 135 /* This cpu has no pending entries, but there are new entries */ 136 if (!rdp->curlist && rdp->nxtlist) 137 return 1; 138 139 /* This cpu has finished callbacks to invoke */ 140 if (rdp->donelist) 141 return 1; 142 143 /* The rcu core waits for a quiescent state from the cpu */ 144 if (rdp->quiescbatch != rcp->cur || rdp->qs_pending) 145 return 1; 146 147 /* nothing to do */ 148 return 0; 149 } 150 151 static inline int rcu_pending(int cpu) 152 { 153 return __rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)) || 154 __rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu)); 155 } 156 157 /** 158 * rcu_read_lock - mark the beginning of an RCU read-side critical section. 159 * 160 * When synchronize_rcu() is invoked on one CPU while other CPUs 161 * are within RCU read-side critical sections, then the 162 * synchronize_rcu() is guaranteed to block until after all the other 163 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked 164 * on one CPU while other CPUs are within RCU read-side critical 165 * sections, invocation of the corresponding RCU callback is deferred 166 * until after the all the other CPUs exit their critical sections. 167 * 168 * Note, however, that RCU callbacks are permitted to run concurrently 169 * with RCU read-side critical sections. One way that this can happen 170 * is via the following sequence of events: (1) CPU 0 enters an RCU 171 * read-side critical section, (2) CPU 1 invokes call_rcu() to register 172 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section, 173 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU 174 * callback is invoked. This is legal, because the RCU read-side critical 175 * section that was running concurrently with the call_rcu() (and which 176 * therefore might be referencing something that the corresponding RCU 177 * callback would free up) has completed before the corresponding 178 * RCU callback is invoked. 179 * 180 * RCU read-side critical sections may be nested. Any deferred actions 181 * will be deferred until the outermost RCU read-side critical section 182 * completes. 183 * 184 * It is illegal to block while in an RCU read-side critical section. 185 */ 186 #define rcu_read_lock() preempt_disable() 187 188 /** 189 * rcu_read_unlock - marks the end of an RCU read-side critical section. 190 * 191 * See rcu_read_lock() for more information. 192 */ 193 #define rcu_read_unlock() preempt_enable() 194 195 /* 196 * So where is rcu_write_lock()? It does not exist, as there is no 197 * way for writers to lock out RCU readers. This is a feature, not 198 * a bug -- this property is what provides RCU's performance benefits. 199 * Of course, writers must coordinate with each other. The normal 200 * spinlock primitives work well for this, but any other technique may be 201 * used as well. RCU does not care how the writers keep out of each 202 * others' way, as long as they do so. 203 */ 204 205 /** 206 * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section 207 * 208 * This is equivalent of rcu_read_lock(), but to be used when updates 209 * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks 210 * consider completion of a softirq handler to be a quiescent state, 211 * a process in RCU read-side critical section must be protected by 212 * disabling softirqs. Read-side critical sections in interrupt context 213 * can use just rcu_read_lock(). 214 * 215 */ 216 #define rcu_read_lock_bh() local_bh_disable() 217 218 /* 219 * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section 220 * 221 * See rcu_read_lock_bh() for more information. 222 */ 223 #define rcu_read_unlock_bh() local_bh_enable() 224 225 /** 226 * rcu_dereference - fetch an RCU-protected pointer in an 227 * RCU read-side critical section. This pointer may later 228 * be safely dereferenced. 229 * 230 * Inserts memory barriers on architectures that require them 231 * (currently only the Alpha), and, more importantly, documents 232 * exactly which pointers are protected by RCU. 233 */ 234 235 #define rcu_dereference(p) ({ \ 236 typeof(p) _________p1 = p; \ 237 smp_read_barrier_depends(); \ 238 (_________p1); \ 239 }) 240 241 /** 242 * rcu_assign_pointer - assign (publicize) a pointer to a newly 243 * initialized structure that will be dereferenced by RCU read-side 244 * critical sections. Returns the value assigned. 245 * 246 * Inserts memory barriers on architectures that require them 247 * (pretty much all of them other than x86), and also prevents 248 * the compiler from reordering the code that initializes the 249 * structure after the pointer assignment. More importantly, this 250 * call documents which pointers will be dereferenced by RCU read-side 251 * code. 252 */ 253 254 #define rcu_assign_pointer(p, v) ({ \ 255 smp_wmb(); \ 256 (p) = (v); \ 257 }) 258 259 /** 260 * synchronize_sched - block until all CPUs have exited any non-preemptive 261 * kernel code sequences. 262 * 263 * This means that all preempt_disable code sequences, including NMI and 264 * hardware-interrupt handlers, in progress on entry will have completed 265 * before this primitive returns. However, this does not guarantee that 266 * softirq handlers will have completed, since in some kernels 267 * 268 * This primitive provides the guarantees made by the (deprecated) 269 * synchronize_kernel() API. In contrast, synchronize_rcu() only 270 * guarantees that rcu_read_lock() sections will have completed. 271 */ 272 #define synchronize_sched() synchronize_rcu() 273 274 extern void rcu_init(void); 275 extern void rcu_check_callbacks(int cpu, int user); 276 extern void rcu_restart_cpu(int cpu); 277 278 /* Exported interfaces */ 279 extern void FASTCALL(call_rcu(struct rcu_head *head, 280 void (*func)(struct rcu_head *head))); 281 extern void FASTCALL(call_rcu_bh(struct rcu_head *head, 282 void (*func)(struct rcu_head *head))); 283 extern __deprecated_for_modules void synchronize_kernel(void); 284 extern void synchronize_rcu(void); 285 void synchronize_idle(void); 286 287 #endif /* __KERNEL__ */ 288 #endif /* __LINUX_RCUPDATE_H */ 289