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c-user: Document thread switch extension changes

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Close #3885.
This commit is contained in:
Sebastian Huber 2020-03-03 07:51:13 +01:00
parent ac61465d2c
commit 8bd4e6a55b
2 changed files with 29 additions and 13 deletions
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2
c-user/symmetric_multiprocessing_services.rst
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@ -683,6 +683,8 @@ The withdraw operation takes away scheduler nodes once the thread is no longer
allowed to use them, e.g. it released a mutex. The availability of scheduler
nodes for a thread is controlled by the thread queues.
.. _SMPThreadDispatchDetails:
Thread Dispatch Details
-----------------------

40
c-user/user_extensions.rst
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@ -261,9 +261,7 @@ destructors and thread-local object destructors run in this context.
Thread Switch Extension
-----------------------
The thread switch extension is invoked before the context switch from the
currently executing thread to the heir thread. The thread switch extension is
defined as follows.
The thread switch extension is defined as follows.
.. code-block:: c
@ -272,18 +270,34 @@ defined as follows.
rtems_tcb *heir
);
The :c:data:`executing` is a pointer to the TCB of the currently executing
thread. The :c:data:`heir` is a pointer to the TCB of the heir thread.
The invocation conditions of the thread switch extension depend on whether RTEMS
was configured for uniprocessor or SMP systems. A user must pay attention to
the differences to correctly implement a thread switch extension.
In uniprocessor configurations, the thread switch extension is invoked before
the context switch from the currently executing thread to the heir thread. The
:c:data:`executing` is a pointer to the TCB of the currently executing thread.
The :c:data:`heir` is a pointer to the TCB of the heir thread. The context
switch initiated through the multitasking start is not covered by the thread
switch extension.
In SMP configurations, the thread switch extension is invoked after the context
switch to the new executing thread (previous heir thread). The
:c:data:`executing` is a pointer to the TCB of the previously executing thread.
Despite the name, this is not the currently executing thread. The
:c:data:`heir` is a pointer to the TCB of the newly executing thread. This is
the currently executing thread. The context switches initiated through the
multitasking start are covered by the thread switch extension. The reason for
the differences to uniprocessor configurations is that the context switch may
update the heir thread of the processor, see :ref:`SMPThreadDispatchDetails`.
The thread switch extensions are invoked with disabled interrupts and with
ownership of a per-processor SMP lock. Thread switch extensions may run in
parallel on multiple processors. It is recommended to use thread-local or
per-processor data structures for thread switch extensions. A global SMP lock
should be avoided for performance reasons.
The thread switch extension is invoked in forward order with thread dispatching
disabled. In SMP configurations, interrupts are disabled and the per-processor
SMP lock is owned. Thread switch extensions may run in parallel on multiple
processors. It is recommended to use thread-local or per-processor data
structures in SMP configurations for thread switch extensions. A global SMP
lock should be avoided for performance reasons.
The context switches initiated through the multitasking start are not covered
by the thread switch extension.
disabled.
.. index:: rtems_task_begin_extension