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 { .next = NULL, .func = NULL } 56 #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT 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 69 int signaled; 70 71 spinlock_t lock ____cacheline_internodealigned_in_smp; 72 cpumask_t cpumask; /* CPUs that need to switch in order */ 73 /* for current batch to proceed. */ 74 } ____cacheline_internodealigned_in_smp; 75 76 /* Is batch a before batch b ? */ 77 static inline int rcu_batch_before(long a, long b) 78 { 79 return (a - b) < 0; 80 } 81 82 /* Is batch a after batch b ? */ 83 static inline int rcu_batch_after(long a, long b) 84 { 85 return (a - b) > 0; 86 } 87 88 /* 89 * Per-CPU data for Read-Copy UPdate. 90 * nxtlist - new callbacks are added here 91 * curlist - current batch for which quiescent cycle started if any 92 */ 93 struct rcu_data { 94 /* 1) quiescent state handling : */ 95 long quiescbatch; /* Batch # for grace period */ 96 int passed_quiesc; /* User-mode/idle loop etc. */ 97 int qs_pending; /* core waits for quiesc state */ 98 99 /* 2) batch handling */ 100 long batch; /* Batch # for current RCU batch */ 101 struct rcu_head *nxtlist; 102 struct rcu_head **nxttail; 103 long qlen; /* # of queued callbacks */ 104 struct rcu_head *curlist; 105 struct rcu_head **curtail; 106 struct rcu_head *donelist; 107 struct rcu_head **donetail; 108 long blimit; /* Upper limit on a processed batch */ 109 int cpu; 110 struct rcu_head barrier; 111 }; 112 113 DECLARE_PER_CPU(struct rcu_data, rcu_data); 114 DECLARE_PER_CPU(struct rcu_data, rcu_bh_data); 115 116 /* 117 * Increment the quiescent state counter. 118 * The counter is a bit degenerated: We do not need to know 119 * how many quiescent states passed, just if there was at least 120 * one since the start of the grace period. Thus just a flag. 121 */ 122 static inline void rcu_qsctr_inc(int cpu) 123 { 124 struct rcu_data *rdp = &per_cpu(rcu_data, cpu); 125 rdp->passed_quiesc = 1; 126 } 127 static inline void rcu_bh_qsctr_inc(int cpu) 128 { 129 struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu); 130 rdp->passed_quiesc = 1; 131 } 132 133 extern int rcu_pending(int cpu); 134 extern int rcu_needs_cpu(int cpu); 135 136 /** 137 * rcu_read_lock - mark the beginning of an RCU read-side critical section. 138 * 139 * When synchronize_rcu() is invoked on one CPU while other CPUs 140 * are within RCU read-side critical sections, then the 141 * synchronize_rcu() is guaranteed to block until after all the other 142 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked 143 * on one CPU while other CPUs are within RCU read-side critical 144 * sections, invocation of the corresponding RCU callback is deferred 145 * until after the all the other CPUs exit their critical sections. 146 * 147 * Note, however, that RCU callbacks are permitted to run concurrently 148 * with RCU read-side critical sections. One way that this can happen 149 * is via the following sequence of events: (1) CPU 0 enters an RCU 150 * read-side critical section, (2) CPU 1 invokes call_rcu() to register 151 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section, 152 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU 153 * callback is invoked. This is legal, because the RCU read-side critical 154 * section that was running concurrently with the call_rcu() (and which 155 * therefore might be referencing something that the corresponding RCU 156 * callback would free up) has completed before the corresponding 157 * RCU callback is invoked. 158 * 159 * RCU read-side critical sections may be nested. Any deferred actions 160 * will be deferred until the outermost RCU read-side critical section 161 * completes. 162 * 163 * It is illegal to block while in an RCU read-side critical section. 164 */ 165 #define rcu_read_lock() \ 166 do { \ 167 preempt_disable(); \ 168 __acquire(RCU); \ 169 } while(0) 170 171 /** 172 * rcu_read_unlock - marks the end of an RCU read-side critical section. 173 * 174 * See rcu_read_lock() for more information. 175 */ 176 #define rcu_read_unlock() \ 177 do { \ 178 __release(RCU); \ 179 preempt_enable(); \ 180 } while(0) 181 182 /* 183 * So where is rcu_write_lock()? It does not exist, as there is no 184 * way for writers to lock out RCU readers. This is a feature, not 185 * a bug -- this property is what provides RCU's performance benefits. 186 * Of course, writers must coordinate with each other. The normal 187 * spinlock primitives work well for this, but any other technique may be 188 * used as well. RCU does not care how the writers keep out of each 189 * others' way, as long as they do so. 190 */ 191 192 /** 193 * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section 194 * 195 * This is equivalent of rcu_read_lock(), but to be used when updates 196 * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks 197 * consider completion of a softirq handler to be a quiescent state, 198 * a process in RCU read-side critical section must be protected by 199 * disabling softirqs. Read-side critical sections in interrupt context 200 * can use just rcu_read_lock(). 201 * 202 */ 203 #define rcu_read_lock_bh() \ 204 do { \ 205 local_bh_disable(); \ 206 __acquire(RCU_BH); \ 207 } while(0) 208 209 /* 210 * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section 211 * 212 * See rcu_read_lock_bh() for more information. 213 */ 214 #define rcu_read_unlock_bh() \ 215 do { \ 216 __release(RCU_BH); \ 217 local_bh_enable(); \ 218 } while(0) 219 220 /** 221 * rcu_dereference - fetch an RCU-protected pointer in an 222 * RCU read-side critical section. This pointer may later 223 * be safely dereferenced. 224 * 225 * Inserts memory barriers on architectures that require them 226 * (currently only the Alpha), and, more importantly, documents 227 * exactly which pointers are protected by RCU. 228 */ 229 230 #define rcu_dereference(p) ({ \ 231 typeof(p) _________p1 = p; \ 232 smp_read_barrier_depends(); \ 233 (_________p1); \ 234 }) 235 236 /** 237 * rcu_assign_pointer - assign (publicize) a pointer to a newly 238 * initialized structure that will be dereferenced by RCU read-side 239 * critical sections. Returns the value assigned. 240 * 241 * Inserts memory barriers on architectures that require them 242 * (pretty much all of them other than x86), and also prevents 243 * the compiler from reordering the code that initializes the 244 * structure after the pointer assignment. More importantly, this 245 * call documents which pointers will be dereferenced by RCU read-side 246 * code. 247 */ 248 249 #define rcu_assign_pointer(p, v) ({ \ 250 smp_wmb(); \ 251 (p) = (v); \ 252 }) 253 254 /** 255 * synchronize_sched - block until all CPUs have exited any non-preemptive 256 * kernel code sequences. 257 * 258 * This means that all preempt_disable code sequences, including NMI and 259 * hardware-interrupt handlers, in progress on entry will have completed 260 * before this primitive returns. However, this does not guarantee that 261 * softirq handlers will have completed, since in some kernels, these 262 * handlers can run in process context, and can block. 263 * 264 * This primitive provides the guarantees made by the (now removed) 265 * synchronize_kernel() API. In contrast, synchronize_rcu() only 266 * guarantees that rcu_read_lock() sections will have completed. 267 * In "classic RCU", these two guarantees happen to be one and 268 * the same, but can differ in realtime RCU implementations. 269 */ 270 #define synchronize_sched() synchronize_rcu() 271 272 extern void rcu_init(void); 273 extern void rcu_check_callbacks(int cpu, int user); 274 extern void rcu_restart_cpu(int cpu); 275 extern long rcu_batches_completed(void); 276 extern long rcu_batches_completed_bh(void); 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 void synchronize_rcu(void); 284 void synchronize_idle(void); 285 extern void rcu_barrier(void); 286 287 #endif /* __KERNEL__ */ 288 #endif /* __LINUX_RCUPDATE_H */ 289