Commit 9cc07df4 authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab Committed by Jonathan Corbet
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preempt-locking.txt: standardize document format



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Change its representation to follow the adopted standard,
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Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 9a4aa7bf
Proper Locking Under a Preemptible Kernel:
Keeping Kernel Code Preempt-Safe
Robert Love <rml@tech9.net>
Last Updated: 28 Aug 2002
===========================================================================
Proper Locking Under a Preemptible Kernel: Keeping Kernel Code Preempt-Safe
===========================================================================
:Author: Robert Love <rml@tech9.net>
:Last Updated: 28 Aug 2002
INTRODUCTION
Introduction
============
A preemptible kernel creates new locking issues. The issues are the same as
......@@ -17,9 +20,10 @@ requires protecting these situations.
RULE #1: Per-CPU data structures need explicit protection
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Two similar problems arise. An example code snippet:
Two similar problems arise. An example code snippet::
struct this_needs_locking tux[NR_CPUS];
tux[smp_processor_id()] = some_value;
......@@ -35,6 +39,7 @@ You can also use put_cpu() and get_cpu(), which will disable preemption.
RULE #2: CPU state must be protected.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Under preemption, the state of the CPU must be protected. This is arch-
......@@ -52,6 +57,7 @@ However, fpu__restore() must be called with preemption disabled.
RULE #3: Lock acquire and release must be performed by same task
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
A lock acquired in one task must be released by the same task. This
......@@ -61,17 +67,20 @@ like this, acquire and release the task in the same code path and
have the caller wait on an event by the other task.
SOLUTION
Solution
========
Data protection under preemption is achieved by disabling preemption for the
duration of the critical region.
preempt_enable() decrement the preempt counter
preempt_disable() increment the preempt counter
preempt_enable_no_resched() decrement, but do not immediately preempt
preempt_check_resched() if needed, reschedule
preempt_count() return the preempt counter
::
preempt_enable() decrement the preempt counter
preempt_disable() increment the preempt counter
preempt_enable_no_resched() decrement, but do not immediately preempt
preempt_check_resched() if needed, reschedule
preempt_count() return the preempt counter
The functions are nestable. In other words, you can call preempt_disable
n-times in a code path, and preemption will not be reenabled until the n-th
......@@ -89,7 +98,7 @@ So use this implicit preemption-disabling property only if you know that the
affected codepath does not do any of this. Best policy is to use this only for
small, atomic code that you wrote and which calls no complex functions.
Example:
Example::
cpucache_t *cc; /* this is per-CPU */
preempt_disable();
......@@ -102,7 +111,7 @@ Example:
return 0;
Notice how the preemption statements must encompass every reference of the
critical variables. Another example:
critical variables. Another example::
int buf[NR_CPUS];
set_cpu_val(buf);
......@@ -114,7 +123,8 @@ This code is not preempt-safe, but see how easily we can fix it by simply
moving the spin_lock up two lines.
PREVENTING PREEMPTION USING INTERRUPT DISABLING
Preventing preemption using interrupt disabling
===============================================
It is possible to prevent a preemption event using local_irq_disable and
......
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