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+.. _local_ops:
+
+=================================================
+Semantics and Behavior of Local Atomic Operations
+=================================================
+
+:Author: Mathieu Desnoyers
+
+
+This document explains the purpose of the local atomic operations, how
+to implement them for any given architecture and shows how they can be used
+properly. It also stresses on the precautions that must be taken when reading
+those local variables across CPUs when the order of memory writes matters.
+
+.. note::
+
+    Note that ``local_t`` based operations are not recommended for general
+    kernel use. Please use the ``this_cpu`` operations instead unless there is
+    really a special purpose. Most uses of ``local_t`` in the kernel have been
+    replaced by ``this_cpu`` operations. ``this_cpu`` operations combine the
+    relocation with the ``local_t`` like semantics in a single instruction and
+    yield more compact and faster executing code.
+
+
+Purpose of local atomic operations
+==================================
+
+Local atomic operations are meant to provide fast and highly reentrant per CPU
+counters. They minimize the performance cost of standard atomic operations by
+removing the LOCK prefix and memory barriers normally required to synchronize
+across CPUs.
+
+Having fast per CPU atomic counters is interesting in many cases: it does not
+require disabling interrupts to protect from interrupt handlers and it permits
+coherent counters in NMI handlers. It is especially useful for tracing purposes
+and for various performance monitoring counters.
+
+Local atomic operations only guarantee variable modification atomicity wrt the
+CPU which owns the data. Therefore, care must taken to make sure that only one
+CPU writes to the ``local_t`` data. This is done by using per cpu data and
+making sure that we modify it from within a preemption safe context. It is
+however permitted to read ``local_t`` data from any CPU: it will then appear to
+be written out of order wrt other memory writes by the owner CPU.
+
+
+Implementation for a given architecture
+=======================================
+
+It can be done by slightly modifying the standard atomic operations: only
+their UP variant must be kept. It typically means removing LOCK prefix (on
+i386 and x86_64) and any SMP synchronization barrier. If the architecture does
+not have a different behavior between SMP and UP, including
+``asm-generic/local.h`` in your architecture's ``local.h`` is sufficient.
+
+The ``local_t`` type is defined as an opaque ``signed long`` by embedding an
+``atomic_long_t`` inside a structure. This is made so a cast from this type to
+a ``long`` fails. The definition looks like::
+
+    typedef struct { atomic_long_t a; } local_t;
+
+
+Rules to follow when using local atomic operations
+==================================================
+
+* Variables touched by local ops must be per cpu variables.
+* *Only* the CPU owner of these variables must write to them.
+* This CPU can use local ops from any context (process, irq, softirq, nmi, ...)
+  to update its ``local_t`` variables.
+* Preemption (or interrupts) must be disabled when using local ops in
+  process context to make sure the process won't be migrated to a
+  different CPU between getting the per-cpu variable and doing the
+  actual local op.
+* When using local ops in interrupt context, no special care must be
+  taken on a mainline kernel, since they will run on the local CPU with
+  preemption already disabled. I suggest, however, to explicitly
+  disable preemption anyway to make sure it will still work correctly on
+  -rt kernels.
+* Reading the local cpu variable will provide the current copy of the
+  variable.
+* Reads of these variables can be done from any CPU, because updates to
+  "``long``", aligned, variables are always atomic. Since no memory
+  synchronization is done by the writer CPU, an outdated copy of the
+  variable can be read when reading some *other* cpu's variables.
+
+
+How to use local atomic operations
+==================================
+
+::
+
+    #include <linux/percpu.h>
+    #include <asm/local.h>
+
+    static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
+
+
+Counting
+========
+
+Counting is done on all the bits of a signed long.
+
+In preemptible context, use ``get_cpu_var()`` and ``put_cpu_var()`` around
+local atomic operations: it makes sure that preemption is disabled around write
+access to the per cpu variable. For instance::
+
+    local_inc(&get_cpu_var(counters));
+    put_cpu_var(counters);
+
+If you are already in a preemption-safe context, you can use
+``this_cpu_ptr()`` instead::
+
+    local_inc(this_cpu_ptr(&counters));
+
+
+
+Reading the counters
+====================
+
+Those local counters can be read from foreign CPUs to sum the count. Note that
+the data seen by local_read across CPUs must be considered to be out of order
+relatively to other memory writes happening on the CPU that owns the data::
+
+    long sum = 0;
+    for_each_online_cpu(cpu)
+            sum += local_read(&per_cpu(counters, cpu));
+
+If you want to use a remote local_read to synchronize access to a resource
+between CPUs, explicit ``smp_wmb()`` and ``smp_rmb()`` memory barriers must be used
+respectively on the writer and the reader CPUs. It would be the case if you use
+the ``local_t`` variable as a counter of bytes written in a buffer: there should
+be a ``smp_wmb()`` between the buffer write and the counter increment and also a
+``smp_rmb()`` between the counter read and the buffer read.
+
+
+Here is a sample module which implements a basic per cpu counter using
+``local.h``::
+
+    /* test-local.c
+     *
+     * Sample module for local.h usage.
+     */
+
+
+    #include <asm/local.h>
+    #include <linux/module.h>
+    #include <linux/timer.h>
+
+    static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
+
+    static struct timer_list test_timer;
+
+    /* IPI called on each CPU. */
+    static void test_each(void *info)
+    {
+            /* Increment the counter from a non preemptible context */
+            printk("Increment on cpu %d\n", smp_processor_id());
+            local_inc(this_cpu_ptr(&counters));
+
+            /* This is what incrementing the variable would look like within a
+             * preemptible context (it disables preemption) :
+             *
+             * local_inc(&get_cpu_var(counters));
+             * put_cpu_var(counters);
+             */
+    }
+
+    static void do_test_timer(unsigned long data)
+    {
+            int cpu;
+
+            /* Increment the counters */
+            on_each_cpu(test_each, NULL, 1);
+            /* Read all the counters */
+            printk("Counters read from CPU %d\n", smp_processor_id());
+            for_each_online_cpu(cpu) {
+                    printk("Read : CPU %d, count %ld\n", cpu,
+                            local_read(&per_cpu(counters, cpu)));
+            }
+            del_timer(&test_timer);
+            test_timer.expires = jiffies + 1000;
+            add_timer(&test_timer);
+    }
+
+    static int __init test_init(void)
+    {
+            /* initialize the timer that will increment the counter */
+            init_timer(&test_timer);
+            test_timer.function = do_test_timer;
+            test_timer.expires = jiffies + 1;
+            add_timer(&test_timer);
+
+            return 0;
+    }
+
+    static void __exit test_exit(void)
+    {
+            del_timer_sync(&test_timer);
+    }
+
+    module_init(test_init);
+    module_exit(test_exit);
+
+    MODULE_LICENSE("GPL");
+    MODULE_AUTHOR("Mathieu Desnoyers");
+    MODULE_DESCRIPTION("Local Atomic Ops");