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