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    Subject[PATCH 3/4] Documentation/local_ops.txt: convert to ReST markup
    Date
    ... and move to core-api folder.

    Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>
    ---
    Documentation/core-api/index.rst | 1 +-
    Documentation/core-api/local_ops.rst | 208 ++++++++++++++++++++++++++++-
    Documentation/local_ops.txt | 191 +--------------------------
    3 files changed, 209 insertions(+), 191 deletions(-)
    create mode 100644 Documentation/core-api/local_ops.rst
    delete mode 100644 Documentation/local_ops.txt

    diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
    index f3e5f5e..25b4e4a 100644
    --- a/Documentation/core-api/index.rst
    +++ b/Documentation/core-api/index.rst
    @@ -9,6 +9,7 @@ Kernel and driver related documentation.

    assoc_array
    atomic_ops
    + local_ops
    workqueue

    .. only:: subproject
    diff --git a/Documentation/core-api/local_ops.rst b/Documentation/core-api/local_ops.rst
    new file mode 100644
    index 0000000..01f1880
    --- /dev/null
    +++ b/Documentation/core-api/local_ops.rst
    @@ -0,0 +1,208 @@
    +
    +.. _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
    +==================================
    +
    +.. code-block:: c
    +
    + #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``.
    +
    +.. code-block:: c
    +
    + /* 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");
    diff --git a/Documentation/local_ops.txt b/Documentation/local_ops.txt
    deleted file mode 100644
    index 407576a..0000000
    --- a/Documentation/local_ops.txt
    +++ /dev/null
    @@ -1,191 +0,0 @@
    - Semantics and Behavior of Local Atomic Operations
    -
    - 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 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.
    -
    ---- BEGIN ---
    -/* 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");
    ---- END ---
    --
    git-series 0.9.1
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