[PATCH 2/2] c-user: Mention thread pinning

[Sebastian Huber]

Close #3508.
---
 c-user/symmetric_multiprocessing_services.rst | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/c-user/symmetric_multiprocessing_services.rst b/c-user/symmetric_multiprocessing_services.rst
index 4204d3f..4c80cb9 100644
--- a/c-user/symmetric_multiprocessing_services.rst
+++ b/c-user/symmetric_multiprocessing_services.rst
@@ -809,3 +809,21 @@ RTMES provides two means for per-processor data:
    `<rtems/score/percpudata.h> <https://git.rtems.org/rtems/tree/cpukit/include/rtems/score/percpudata.h>`_.
    This API is not intended for general application use.  Please ask on the
    development mailing list in case you want to use it.
+
+Thread Pinning
+--------------
+
+Thread pinning may be used to access per-processor data structures in critical
+sections with enabled thread dispatching, e.g. a pinned thread is allowed to
+block.  The `_Thread_Pin()` operation will pin the executing thread to its
+current processor.  A thread may be pinned recursively, the last unpin request
+via `_Thread_Unpin()` revokes the pinning.
+
+Thread pinning should be used only for short critical sections and not all
+the time.  Thread pinning is a very low overhead operation in case the
+thread is not preempted during the pinning.  A preemption will result in
+scheduler operations to ensure that the thread executes only on its pinned
+processor.  Thread pinning must be used with care, since it prevents help
+through the locking protocols.  This makes the :ref:`OMIP <OMIP>` and
+:ref:`MrsP <MrsP>` locking protocols ineffective if pinned threads are
+involved.
-- 
2.13.7