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 <paul.mckenney@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(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_kernel() is invoked on one CPU while other CPUs
 161  * are within RCU read-side critical sections, then the
 162  * synchronize_kernel() 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 extern void rcu_init(void);
 260 extern void rcu_check_callbacks(int cpu, int user);
 261 extern void rcu_restart_cpu(int cpu);
 262
 263 /* Exported interfaces */
 264 extern void FASTCALL(call_rcu(struct rcu_head *head,
 265                                 void (*func)(struct rcu_head *head)));
 266 extern void FASTCALL(call_rcu_bh(struct rcu_head *head,
 267                                 void (*func)(struct rcu_head *head)));
 268 extern void synchronize_kernel(void);
 269
 270 #endif /* __KERNEL__ */
 271 #endif /* __LINUX_RCUPDATE_H */