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 (C) 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
  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 */