Documentation/local_ops.txt: convert to ReST markup
... and move to core-api folder. Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com> Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Silvio Fricke
Jonathan Corbet
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c3cbf1a704
Коммит
c232694ec1
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@ -8,6 +8,7 @@ Kernel and driver related documentation.
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:maxdepth: 1
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assoc_array
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local_ops
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workqueue
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.. only:: subproject
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@ -0,0 +1,206 @@
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.. _local_ops:
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=================================================
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Semantics and Behavior of Local Atomic Operations
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=================================================
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:Author: Mathieu Desnoyers
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This document explains the purpose of the local atomic operations, how
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to implement them for any given architecture and shows how they can be used
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properly. It also stresses on the precautions that must be taken when reading
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those local variables across CPUs when the order of memory writes matters.
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.. note::
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Note that ``local_t`` based operations are not recommended for general
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kernel use. Please use the ``this_cpu`` operations instead unless there is
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really a special purpose. Most uses of ``local_t`` in the kernel have been
|
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replaced by ``this_cpu`` operations. ``this_cpu`` operations combine the
|
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relocation with the ``local_t`` like semantics in a single instruction and
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yield more compact and faster executing code.
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Purpose of local atomic operations
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==================================
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Local atomic operations are meant to provide fast and highly reentrant per CPU
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counters. They minimize the performance cost of standard atomic operations by
|
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removing the LOCK prefix and memory barriers normally required to synchronize
|
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across CPUs.
|
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|
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Having fast per CPU atomic counters is interesting in many cases: it does not
|
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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.
|
||||
|
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Local atomic operations only guarantee variable modification atomicity wrt the
|
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CPU which owns the data. Therefore, care must taken to make sure that only one
|
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CPU writes to the ``local_t`` data. This is done by using per cpu data and
|
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making sure that we modify it from within a preemption safe context. It is
|
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however permitted to read ``local_t`` data from any CPU: it will then appear to
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be written out of order wrt other memory writes by the owner CPU.
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Implementation for a given architecture
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=======================================
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It can be done by slightly modifying the standard atomic operations: only
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their UP variant must be kept. It typically means removing LOCK prefix (on
|
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i386 and x86_64) and any SMP synchronization barrier. If the architecture does
|
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not have a different behavior between SMP and UP, including
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``asm-generic/local.h`` in your architecture's ``local.h`` is sufficient.
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The ``local_t`` type is defined as an opaque ``signed long`` by embedding an
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``atomic_long_t`` inside a structure. This is made so a cast from this type to
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a ``long`` fails. The definition looks like::
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typedef struct { atomic_long_t a; } local_t;
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Rules to follow when using local atomic operations
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==================================================
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* Variables touched by local ops must be per cpu variables.
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* *Only* the CPU owner of these variables must write to them.
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* This CPU can use local ops from any context (process, irq, softirq, nmi, ...)
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to update its ``local_t`` variables.
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* Preemption (or interrupts) must be disabled when using local ops in
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process context to make sure the process won't be migrated to a
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different CPU between getting the per-cpu variable and doing the
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actual local op.
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* When using local ops in interrupt context, no special care must be
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taken on a mainline kernel, since they will run on the local CPU with
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preemption already disabled. I suggest, however, to explicitly
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disable preemption anyway to make sure it will still work correctly on
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-rt kernels.
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* Reading the local cpu variable will provide the current copy of the
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variable.
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* Reads of these variables can be done from any CPU, because updates to
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"``long``", aligned, variables are always atomic. Since no memory
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synchronization is done by the writer CPU, an outdated copy of the
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variable can be read when reading some *other* cpu's variables.
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How to use local atomic operations
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==================================
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::
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#include <linux/percpu.h>
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#include <asm/local.h>
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static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
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Counting
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========
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Counting is done on all the bits of a signed long.
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In preemptible context, use ``get_cpu_var()`` and ``put_cpu_var()`` around
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local atomic operations: it makes sure that preemption is disabled around write
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access to the per cpu variable. For instance::
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local_inc(&get_cpu_var(counters));
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put_cpu_var(counters);
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If you are already in a preemption-safe context, you can use
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``this_cpu_ptr()`` instead::
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local_inc(this_cpu_ptr(&counters));
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Reading the counters
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====================
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Those local counters can be read from foreign CPUs to sum the count. Note that
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the data seen by local_read across CPUs must be considered to be out of order
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relatively to other memory writes happening on the CPU that owns the data::
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long sum = 0;
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for_each_online_cpu(cpu)
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sum += local_read(&per_cpu(counters, cpu));
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If you want to use a remote local_read to synchronize access to a resource
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between CPUs, explicit ``smp_wmb()`` and ``smp_rmb()`` memory barriers must be used
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respectively on the writer and the reader CPUs. It would be the case if you use
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the ``local_t`` variable as a counter of bytes written in a buffer: there should
|
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be a ``smp_wmb()`` between the buffer write and the counter increment and also a
|
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``smp_rmb()`` between the counter read and the buffer read.
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Here is a sample module which implements a basic per cpu counter using
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``local.h``::
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/* test-local.c
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*
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* Sample module for local.h usage.
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*/
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#include <asm/local.h>
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#include <linux/module.h>
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#include <linux/timer.h>
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static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
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static struct timer_list test_timer;
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/* IPI called on each CPU. */
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static void test_each(void *info)
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{
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/* Increment the counter from a non preemptible context */
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printk("Increment on cpu %d\n", smp_processor_id());
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local_inc(this_cpu_ptr(&counters));
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/* This is what incrementing the variable would look like within a
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* preemptible context (it disables preemption) :
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*
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* local_inc(&get_cpu_var(counters));
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* put_cpu_var(counters);
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*/
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}
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static void do_test_timer(unsigned long data)
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{
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int cpu;
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/* Increment the counters */
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on_each_cpu(test_each, NULL, 1);
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/* Read all the counters */
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printk("Counters read from CPU %d\n", smp_processor_id());
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for_each_online_cpu(cpu) {
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printk("Read : CPU %d, count %ld\n", cpu,
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local_read(&per_cpu(counters, cpu)));
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}
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del_timer(&test_timer);
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test_timer.expires = jiffies + 1000;
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add_timer(&test_timer);
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}
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static int __init test_init(void)
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{
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/* initialize the timer that will increment the counter */
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init_timer(&test_timer);
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test_timer.function = do_test_timer;
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test_timer.expires = jiffies + 1;
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add_timer(&test_timer);
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return 0;
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}
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static void __exit test_exit(void)
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{
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del_timer_sync(&test_timer);
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}
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module_init(test_init);
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module_exit(test_exit);
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MODULE_LICENSE("GPL");
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MODULE_AUTHOR("Mathieu Desnoyers");
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MODULE_DESCRIPTION("Local Atomic Ops");
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@ -1,191 +0,0 @@
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Semantics and Behavior of Local Atomic Operations
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||||
|
||||
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.
|
||||
|
||||
|
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* Purpose of local atomic operations
|
||||
|
||||
Local atomic operations are meant to provide fast and highly reentrant per CPU
|
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counters. They minimize the performance cost of standard atomic operations by
|
||||
removing the LOCK prefix and memory barriers normally required to synchronize
|
||||
across CPUs.
|
||||
|
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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
|
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out of order wrt other memory writes by the owner CPU.
|
||||
|
||||
|
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* 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
|
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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
|
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long fails. The definition looks like :
|
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|
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typedef struct { atomic_long_t a; } local_t;
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|
||||
|
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* Rules to follow when using local atomic operations
|
||||
|
||||
- Variables touched by local ops must be per cpu variables.
|
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- _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
|
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synchronization is done by the writer CPU, an outdated copy of the
|
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variable can be read when reading some _other_ cpu's variables.
|
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|
||||
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* How to use local atomic operations
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|
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#include <linux/percpu.h>
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#include <asm/local.h>
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static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
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|
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* Counting
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|
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Counting is done on all the bits of a signed long.
|
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|
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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 :
|
||||
|
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local_inc(&get_cpu_var(counters));
|
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put_cpu_var(counters);
|
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|
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If you are already in a preemption-safe context, you can use
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this_cpu_ptr() instead.
|
||||
|
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local_inc(this_cpu_ptr(&counters));
|
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|
||||
|
||||
|
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* 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.
|
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|
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long sum = 0;
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for_each_online_cpu(cpu)
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sum += local_read(&per_cpu(counters, cpu));
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|
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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.
|
||||
|
||||
|
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Here is a sample module which implements a basic per cpu counter using local.h.
|
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--- BEGIN ---
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/* test-local.c
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*
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* Sample module for local.h usage.
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*/
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#include <asm/local.h>
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#include <linux/module.h>
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#include <linux/timer.h>
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static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
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static struct timer_list test_timer;
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/* IPI called on each CPU. */
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static void test_each(void *info)
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{
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/* Increment the counter from a non preemptible context */
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printk("Increment on cpu %d\n", smp_processor_id());
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local_inc(this_cpu_ptr(&counters));
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|
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/* This is what incrementing the variable would look like within a
|
||||
* preemptible context (it disables preemption) :
|
||||
*
|
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* local_inc(&get_cpu_var(counters));
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* put_cpu_var(counters);
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*/
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}
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static void do_test_timer(unsigned long data)
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{
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int cpu;
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/* Increment the counters */
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on_each_cpu(test_each, NULL, 1);
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/* Read all the counters */
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printk("Counters read from CPU %d\n", smp_processor_id());
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for_each_online_cpu(cpu) {
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printk("Read : CPU %d, count %ld\n", cpu,
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local_read(&per_cpu(counters, cpu)));
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}
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del_timer(&test_timer);
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test_timer.expires = jiffies + 1000;
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add_timer(&test_timer);
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||||
}
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||||
static int __init test_init(void)
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{
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||||
/* initialize the timer that will increment the counter */
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init_timer(&test_timer);
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||||
test_timer.function = do_test_timer;
|
||||
test_timer.expires = jiffies + 1;
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||||
add_timer(&test_timer);
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||||
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return 0;
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||||
}
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||||
|
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static void __exit test_exit(void)
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||||
{
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||||
del_timer_sync(&test_timer);
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||||
}
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||||
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||||
module_init(test_init);
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||||
module_exit(test_exit);
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||||
|
||||
MODULE_LICENSE("GPL");
|
||||
MODULE_AUTHOR("Mathieu Desnoyers");
|
||||
MODULE_DESCRIPTION("Local Atomic Ops");
|
||||
--- END ---
|
||||
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