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 #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 #include <linux/lockdep.h>
  45
  46 /**
  47  * struct rcu_head - callback structure for use with RCU
  48  * @next: next update requests in a list
  49  * @func: actual update function to call after the grace period.
  50  */
  51 struct rcu_head {
  52         struct rcu_head *next;
  53         void (*func)(struct rcu_head *head);
  54 };
  55
  56 #include <linux/rcuclassic.h>
  57
  58 #define RCU_HEAD_INIT   { .next = NULL, .func = NULL }
  59 #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
  60 #define INIT_RCU_HEAD(ptr) do { \
  61        (ptr)->next = NULL; (ptr)->func = NULL; \
  62 } while (0)
  63
  64 /**
  65  * rcu_read_lock - mark the beginning of an RCU read-side critical section.
  66  *
  67  * When synchronize_rcu() is invoked on one CPU while other CPUs
  68  * are within RCU read-side critical sections, then the
  69  * synchronize_rcu() is guaranteed to block until after all the other
  70  * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
  71  * on one CPU while other CPUs are within RCU read-side critical
  72  * sections, invocation of the corresponding RCU callback is deferred
  73  * until after the all the other CPUs exit their critical sections.
  74  *
  75  * Note, however, that RCU callbacks are permitted to run concurrently
  76  * with RCU read-side critical sections.  One way that this can happen
  77  * is via the following sequence of events: (1) CPU 0 enters an RCU
  78  * read-side critical section, (2) CPU 1 invokes call_rcu() to register
  79  * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
  80  * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
  81  * callback is invoked.  This is legal, because the RCU read-side critical
  82  * section that was running concurrently with the call_rcu() (and which
  83  * therefore might be referencing something that the corresponding RCU
  84  * callback would free up) has completed before the corresponding
  85  * RCU callback is invoked.
  86  *
  87  * RCU read-side critical sections may be nested.  Any deferred actions
  88  * will be deferred until the outermost RCU read-side critical section
  89  * completes.
  90  *
  91  * It is illegal to block while in an RCU read-side critical section.
  92  */
  93 #define rcu_read_lock() __rcu_read_lock()
  94
  95 /**
  96  * rcu_read_unlock - marks the end of an RCU read-side critical section.
  97  *
  98  * See rcu_read_lock() for more information.
  99  */
 100
 101 /*
 102  * So where is rcu_write_lock()?  It does not exist, as there is no
 103  * way for writers to lock out RCU readers.  This is a feature, not
 104  * a bug -- this property is what provides RCU's performance benefits.
 105  * Of course, writers must coordinate with each other.  The normal
 106  * spinlock primitives work well for this, but any other technique may be
 107  * used as well.  RCU does not care how the writers keep out of each
 108  * others' way, as long as they do so.
 109  */
 110 #define rcu_read_unlock() __rcu_read_unlock()
 111
 112 /**
 113  * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section
 114  *
 115  * This is equivalent of rcu_read_lock(), but to be used when updates
 116  * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks
 117  * consider completion of a softirq handler to be a quiescent state,
 118  * a process in RCU read-side critical section must be protected by
 119  * disabling softirqs. Read-side critical sections in interrupt context
 120  * can use just rcu_read_lock().
 121  *
 122  */
 123 #define rcu_read_lock_bh() __rcu_read_lock_bh()
 124
 125 /*
 126  * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
 127  *
 128  * See rcu_read_lock_bh() for more information.
 129  */
 130 #define rcu_read_unlock_bh() __rcu_read_unlock_bh()
 131
 132 /*
 133  * Prevent the compiler from merging or refetching accesses.  The compiler
 134  * is also forbidden from reordering successive instances of ACCESS_ONCE(),
 135  * but only when the compiler is aware of some particular ordering.  One way
 136  * to make the compiler aware of ordering is to put the two invocations of
 137  * ACCESS_ONCE() in different C statements.
 138  *
 139  * This macro does absolutely -nothing- to prevent the CPU from reordering,
 140  * merging, or refetching absolutely anything at any time.
 141  */
 142 #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
 143
 144 /**
 145  * rcu_dereference - fetch an RCU-protected pointer in an
 146  * RCU read-side critical section.  This pointer may later
 147  * be safely dereferenced.
 148  *
 149  * Inserts memory barriers on architectures that require them
 150  * (currently only the Alpha), and, more importantly, documents
 151  * exactly which pointers are protected by RCU.
 152  */
 153
 154 #define rcu_dereference(p)     ({ \
 155                                 typeof(p) _________p1 = ACCESS_ONCE(p); \
 156                                 smp_read_barrier_depends(); \
 157                                 (_________p1); \
 158                                 })
 159
 160 /**
 161  * rcu_assign_pointer - assign (publicize) a pointer to a newly
 162  * initialized structure that will be dereferenced by RCU read-side
 163  * critical sections.  Returns the value assigned.
 164  *
 165  * Inserts memory barriers on architectures that require them
 166  * (pretty much all of them other than x86), and also prevents
 167  * the compiler from reordering the code that initializes the
 168  * structure after the pointer assignment.  More importantly, this
 169  * call documents which pointers will be dereferenced by RCU read-side
 170  * code.
 171  */
 172
 173 #define rcu_assign_pointer(p, v)        ({ \
 174                                                 smp_wmb(); \
 175                                                 (p) = (v); \
 176                                         })
 177
 178 /**
 179  * synchronize_sched - block until all CPUs have exited any non-preemptive
 180  * kernel code sequences.
 181  *
 182  * This means that all preempt_disable code sequences, including NMI and
 183  * hardware-interrupt handlers, in progress on entry will have completed
 184  * before this primitive returns.  However, this does not guarantee that
 185  * softirq handlers will have completed, since in some kernels, these
 186  * handlers can run in process context, and can block.
 187  *
 188  * This primitive provides the guarantees made by the (now removed)
 189  * synchronize_kernel() API.  In contrast, synchronize_rcu() only
 190  * guarantees that rcu_read_lock() sections will have completed.
 191  * In "classic RCU", these two guarantees happen to be one and
 192  * the same, but can differ in realtime RCU implementations.
 193  */
 194 #define synchronize_sched() __synchronize_sched()
 195
 196 /**
 197  * call_rcu - Queue an RCU callback for invocation after a grace period.
 198  * @head: structure to be used for queueing the RCU updates.
 199  * @func: actual update function to be invoked after the grace period
 200  *
 201  * The update function will be invoked some time after a full grace
 202  * period elapses, in other words after all currently executing RCU
 203  * read-side critical sections have completed.  RCU read-side critical
 204  * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 205  * and may be nested.
 206  */
 207 extern void call_rcu(struct rcu_head *head,
 208                               void (*func)(struct rcu_head *head));
 209
 210 /**
 211  * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
 212  * @head: structure to be used for queueing the RCU updates.
 213  * @func: actual update function to be invoked after the grace period
 214  *
 215  * The update function will be invoked some time after a full grace
 216  * period elapses, in other words after all currently executing RCU
 217  * read-side critical sections have completed. call_rcu_bh() assumes
 218  * that the read-side critical sections end on completion of a softirq
 219  * handler. This means that read-side critical sections in process
 220  * context must not be interrupted by softirqs. This interface is to be
 221  * used when most of the read-side critical sections are in softirq context.
 222  * RCU read-side critical sections are delimited by :
 223  *  - rcu_read_lock() and  rcu_read_unlock(), if in interrupt context.
 224  *  OR
 225  *  - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context.
 226  *  These may be nested.
 227  */
 228 extern void call_rcu_bh(struct rcu_head *head,
 229                         void (*func)(struct rcu_head *head));
 230
 231 /* Exported common interfaces */
 232 extern void synchronize_rcu(void);
 233 extern void rcu_barrier(void);
 234
 235 /* Internal to kernel */
 236 extern void rcu_init(void);
 237 extern void rcu_check_callbacks(int cpu, int user);
 238
 239 extern long rcu_batches_completed(void);
 240 extern long rcu_batches_completed_bh(void);
 241
 242 #endif /* __KERNEL__ */
 243 #endif /* __LINUX_RCUPDATE_H */