include/linux/rcupdate.h

   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 <dipankar@in.ibm.com>
  21  *
  22  * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
  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
  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 #ifdef CONFIG_CLASSIC_RCU
  55 #include <linux/rcuclassic.h>
  56 #else /* #ifdef CONFIG_CLASSIC_RCU */
  57 #include <linux/rcupreempt.h>
  58 #endif /* #else #ifdef CONFIG_CLASSIC_RCU */
  59
  60 #define RCU_HEAD_INIT   { .next = NULL, .func = NULL }
  61 #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
  62 #define INIT_RCU_HEAD(ptr) do { \
  63        (ptr)->next = NULL; (ptr)->func = NULL; \
  64 } while (0)
  65
  66 /**
  67  * rcu_read_lock - mark the beginning of an RCU read-side critical section.
  68  *
  69  * When synchronize_rcu() is invoked on one CPU while other CPUs
  70  * are within RCU read-side critical sections, then the
  71  * synchronize_rcu() is guaranteed to block until after all the other
  72  * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
  73  * on one CPU while other CPUs are within RCU read-side critical
  74  * sections, invocation of the corresponding RCU callback is deferred
  75  * until after the all the other CPUs exit their critical sections.
  76  *
  77  * Note, however, that RCU callbacks are permitted to run concurrently
  78  * with RCU read-side critical sections.  One way that this can happen
  79  * is via the following sequence of events: (1) CPU 0 enters an RCU
  80  * read-side critical section, (2) CPU 1 invokes call_rcu() to register
  81  * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
  82  * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
  83  * callback is invoked.  This is legal, because the RCU read-side critical
  84  * section that was running concurrently with the call_rcu() (and which
  85  * therefore might be referencing something that the corresponding RCU
  86  * callback would free up) has completed before the corresponding
  87  * RCU callback is invoked.
  88  *
  89  * RCU read-side critical sections may be nested.  Any deferred actions
  90  * will be deferred until the outermost RCU read-side critical section
  91  * completes.
  92  *
  93  * It is illegal to block while in an RCU read-side critical section.
  94  */
  95 #define rcu_read_lock() __rcu_read_lock()
  96
  97 /**
  98  * rcu_read_unlock - marks the end of an RCU read-side critical section.
  99  *
 100  * See rcu_read_lock() for more information.
 101  */
 102
 103 /*
 104  * So where is rcu_write_lock()?  It does not exist, as there is no
 105  * way for writers to lock out RCU readers.  This is a feature, not
 106  * a bug -- this property is what provides RCU's performance benefits.
 107  * Of course, writers must coordinate with each other.  The normal
 108  * spinlock primitives work well for this, but any other technique may be
 109  * used as well.  RCU does not care how the writers keep out of each
 110  * others' way, as long as they do so.
 111  */
 112 #define rcu_read_unlock() __rcu_read_unlock()
 113
 114 /**
 115  * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section
 116  *
 117  * This is equivalent of rcu_read_lock(), but to be used when updates
 118  * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks
 119  * consider completion of a softirq handler to be a quiescent state,
 120  * a process in RCU read-side critical section must be protected by
 121  * disabling softirqs. Read-side critical sections in interrupt context
 122  * can use just rcu_read_lock().
 123  *
 124  */
 125 #define rcu_read_lock_bh() __rcu_read_lock_bh()
 126
 127 /*
 128  * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
 129  *
 130  * See rcu_read_lock_bh() for more information.
 131  */
 132 #define rcu_read_unlock_bh() __rcu_read_unlock_bh()
 133
 134 /*
 135  * Prevent the compiler from merging or refetching accesses.  The compiler
 136  * is also forbidden from reordering successive instances of ACCESS_ONCE(),
 137  * but only when the compiler is aware of some particular ordering.  One way
 138  * to make the compiler aware of ordering is to put the two invocations of
 139  * ACCESS_ONCE() in different C statements.
 140  *
 141  * This macro does absolutely -nothing- to prevent the CPU from reordering,
 142  * merging, or refetching absolutely anything at any time.
 143  */
 144 #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
 145
 146 /**
 147  * rcu_dereference - fetch an RCU-protected pointer in an
 148  * RCU read-side critical section.  This pointer may later
 149  * be safely dereferenced.
 150  *
 151  * Inserts memory barriers on architectures that require them
 152  * (currently only the Alpha), and, more importantly, documents
 153  * exactly which pointers are protected by RCU.
 154  */
 155
 156 #define rcu_dereference(p)     ({ \
 157                                 typeof(p) _________p1 = ACCESS_ONCE(p); \
 158                                 smp_read_barrier_depends(); \
 159                                 (_________p1); \
 160                                 })
 161
 162 /**
 163  * rcu_assign_pointer - assign (publicize) a pointer to a newly
 164  * initialized structure that will be dereferenced by RCU read-side
 165  * critical sections.  Returns the value assigned.
 166  *
 167  * Inserts memory barriers on architectures that require them
 168  * (pretty much all of them other than x86), and also prevents
 169  * the compiler from reordering the code that initializes the
 170  * structure after the pointer assignment.  More importantly, this
 171  * call documents which pointers will be dereferenced by RCU read-side
 172  * code.
 173  */
 174
 175 #define rcu_assign_pointer(p, v) \
 176         ({ \
 177                 if (!__builtin_constant_p(v) || \
 178                     ((v) != NULL)) \
 179                         smp_wmb(); \
 180                 (p) = (v); \
 181         })
 182
 183 /**
 184  * synchronize_sched - block until all CPUs have exited any non-preemptive
 185  * kernel code sequences.
 186  *
 187  * This means that all preempt_disable code sequences, including NMI and
 188  * hardware-interrupt handlers, in progress on entry will have completed
 189  * before this primitive returns.  However, this does not guarantee that
 190  * softirq handlers will have completed, since in some kernels, these
 191  * handlers can run in process context, and can block.
 192  *
 193  * This primitive provides the guarantees made by the (now removed)
 194  * synchronize_kernel() API.  In contrast, synchronize_rcu() only
 195  * guarantees that rcu_read_lock() sections will have completed.
 196  * In "classic RCU", these two guarantees happen to be one and
 197  * the same, but can differ in realtime RCU implementations.
 198  */
 199 #define synchronize_sched() __synchronize_sched()
 200
 201 /**
 202  * call_rcu - Queue an RCU callback for invocation after a grace period.
 203  * @head: structure to be used for queueing the RCU updates.
 204  * @func: actual update function to be invoked after the grace period
 205  *
 206  * The update function will be invoked some time after a full grace
 207  * period elapses, in other words after all currently executing RCU
 208  * read-side critical sections have completed.  RCU read-side critical
 209  * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 210  * and may be nested.
 211  */
 212 extern void call_rcu(struct rcu_head *head,
 213                               void (*func)(struct rcu_head *head));
 214
 215 /**
 216  * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
 217  * @head: structure to be used for queueing the RCU updates.
 218  * @func: actual update function to be invoked after the grace period
 219  *
 220  * The update function will be invoked some time after a full grace
 221  * period elapses, in other words after all currently executing RCU
 222  * read-side critical sections have completed. call_rcu_bh() assumes
 223  * that the read-side critical sections end on completion of a softirq
 224  * handler. This means that read-side critical sections in process
 225  * context must not be interrupted by softirqs. This interface is to be
 226  * used when most of the read-side critical sections are in softirq context.
 227  * RCU read-side critical sections are delimited by :
 228  *  - rcu_read_lock() and  rcu_read_unlock(), if in interrupt context.
 229  *  OR
 230  *  - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context.
 231  *  These may be nested.
 232  */
 233 extern void call_rcu_bh(struct rcu_head *head,
 234                         void (*func)(struct rcu_head *head));
 235
 236 /* Exported common interfaces */
 237 extern void synchronize_rcu(void);
 238 extern void rcu_barrier(void);
 239 extern long rcu_batches_completed(void);
 240 extern long rcu_batches_completed_bh(void);
 241
 242 /* Internal to kernel */
 243 extern void rcu_init(void);
 244 extern int rcu_needs_cpu(int cpu);
 245
 246 #endif /* __LINUX_RCUPDATE_H */