linux/Documentation/local_ops.txt
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   1             Semantics and Behavior of Local Atomic Operations
   2
   3                            Mathieu Desnoyers
   4
   5
   6        This document explains the purpose of the local atomic operations, how
   7to implement them for any given architecture and shows how they can be used
   8properly. It also stresses on the precautions that must be taken when reading
   9those local variables across CPUs when the order of memory writes matters.
  10
  11
  12
  13* Purpose of local atomic operations
  14
  15Local atomic operations are meant to provide fast and highly reentrant per CPU
  16counters. They minimize the performance cost of standard atomic operations by
  17removing the LOCK prefix and memory barriers normally required to synchronize
  18across CPUs.
  19
  20Having fast per CPU atomic counters is interesting in many cases : it does not
  21require disabling interrupts to protect from interrupt handlers and it permits
  22coherent counters in NMI handlers. It is especially useful for tracing purposes
  23and for various performance monitoring counters.
  24
  25Local atomic operations only guarantee variable modification atomicity wrt the
  26CPU which owns the data. Therefore, care must taken to make sure that only one
  27CPU writes to the local_t data. This is done by using per cpu data and making
  28sure that we modify it from within a preemption safe context. It is however
  29permitted to read local_t data from any CPU : it will then appear to be written
  30out of order wrt other memory writes by the owner CPU.
  31
  32
  33* Implementation for a given architecture
  34
  35It can be done by slightly modifying the standard atomic operations : only
  36their UP variant must be kept. It typically means removing LOCK prefix (on
  37i386 and x86_64) and any SMP synchronization barrier. If the architecture does
  38not have a different behavior between SMP and UP, including asm-generic/local.h
  39in your architecture's local.h is sufficient.
  40
  41The local_t type is defined as an opaque signed long by embedding an
  42atomic_long_t inside a structure. This is made so a cast from this type to a
  43long fails. The definition looks like :
  44
  45typedef struct { atomic_long_t a; } local_t;
  46
  47
  48* Rules to follow when using local atomic operations
  49
  50- Variables touched by local ops must be per cpu variables.
  51- _Only_ the CPU owner of these variables must write to them.
  52- This CPU can use local ops from any context (process, irq, softirq, nmi, ...)
  53  to update its local_t variables.
  54- Preemption (or interrupts) must be disabled when using local ops in
  55  process context to   make sure the process won't be migrated to a
  56  different CPU between getting the per-cpu variable and doing the
  57  actual local op.
  58- When using local ops in interrupt context, no special care must be
  59  taken on a mainline kernel, since they will run on the local CPU with
  60  preemption already disabled. I suggest, however, to explicitly
  61  disable preemption anyway to make sure it will still work correctly on
  62  -rt kernels.
  63- Reading the local cpu variable will provide the current copy of the
  64  variable.
  65- Reads of these variables can be done from any CPU, because updates to
  66  "long", aligned, variables are always atomic. Since no memory
  67  synchronization is done by the writer CPU, an outdated copy of the
  68  variable can be read when reading some _other_ cpu's variables.
  69
  70
  71* How to use local atomic operations
  72
  73#include <linux/percpu.h>
  74#include <asm/local.h>
  75
  76static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
  77
  78
  79* Counting
  80
  81Counting is done on all the bits of a signed long.
  82
  83In preemptible context, use get_cpu_var() and put_cpu_var() around local atomic
  84operations : it makes sure that preemption is disabled around write access to
  85the per cpu variable. For instance :
  86
  87        local_inc(&get_cpu_var(counters));
  88        put_cpu_var(counters);
  89
  90If you are already in a preemption-safe context, you can directly use
  91__get_cpu_var() instead.
  92
  93        local_inc(&__get_cpu_var(counters));
  94
  95
  96
  97* Reading the counters
  98
  99Those local counters can be read from foreign CPUs to sum the count. Note that
 100the data seen by local_read across CPUs must be considered to be out of order
 101relatively to other memory writes happening on the CPU that owns the data.
 102
 103        long sum = 0;
 104        for_each_online_cpu(cpu)
 105                sum += local_read(&per_cpu(counters, cpu));
 106
 107If you want to use a remote local_read to synchronize access to a resource
 108between CPUs, explicit smp_wmb() and smp_rmb() memory barriers must be used
 109respectively on the writer and the reader CPUs. It would be the case if you use
 110the local_t variable as a counter of bytes written in a buffer : there should
 111be a smp_wmb() between the buffer write and the counter increment and also a
 112smp_rmb() between the counter read and the buffer read.
 113
 114
 115Here is a sample module which implements a basic per cpu counter using local.h.
 116
 117--- BEGIN ---
 118/* test-local.c
 119 *
 120 * Sample module for local.h usage.
 121 */
 122
 123
 124#include <asm/local.h>
 125#include <linux/module.h>
 126#include <linux/timer.h>
 127
 128static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
 129
 130static struct timer_list test_timer;
 131
 132/* IPI called on each CPU. */
 133static void test_each(void *info)
 134{
 135        /* Increment the counter from a non preemptible context */
 136        printk("Increment on cpu %d\n", smp_processor_id());
 137        local_inc(&__get_cpu_var(counters));
 138
 139        /* This is what incrementing the variable would look like within a
 140         * preemptible context (it disables preemption) :
 141         *
 142         * local_inc(&get_cpu_var(counters));
 143         * put_cpu_var(counters);
 144         */
 145}
 146
 147static void do_test_timer(unsigned long data)
 148{
 149        int cpu;
 150
 151        /* Increment the counters */
 152        on_each_cpu(test_each, NULL, 1);
 153        /* Read all the counters */
 154        printk("Counters read from CPU %d\n", smp_processor_id());
 155        for_each_online_cpu(cpu) {
 156                printk("Read : CPU %d, count %ld\n", cpu,
 157                        local_read(&per_cpu(counters, cpu)));
 158        }
 159        del_timer(&test_timer);
 160        test_timer.expires = jiffies + 1000;
 161        add_timer(&test_timer);
 162}
 163
 164static int __init test_init(void)
 165{
 166        /* initialize the timer that will increment the counter */
 167        init_timer(&test_timer);
 168        test_timer.function = do_test_timer;
 169        test_timer.expires = jiffies + 1;
 170        add_timer(&test_timer);
 171
 172        return 0;
 173}
 174
 175static void __exit test_exit(void)
 176{
 177        del_timer_sync(&test_timer);
 178}
 179
 180module_init(test_init);
 181module_exit(test_exit);
 182
 183MODULE_LICENSE("GPL");
 184MODULE_AUTHOR("Mathieu Desnoyers");
 185MODULE_DESCRIPTION("Local Atomic Ops");
 186--- END ---
 187
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