.. include:: <mmlalias.txt>


OS Primitives: Spinlocks
========================

.. topic:: Documentation

   * :doc:`../why-os/locking`
   * Zephyr: Spinlock APIs
     <https://docs.zephyrproject.org/latest/doxygen/html/group__spinlock__apis.html>`__

Producer/Consumer Interrupts
----------------------------

* One interrupt (say a sensor) produces some data
* Another (say device-buffer-empty) consumes
* |longrightarrow| Parallelism
* |longrightarrow| No good!

.. literalinclude:: code/do-much-in-isr.cpp
   :caption: :download:`code/do-much-in-isr.cpp`
   :language: c++

**IRQ engine internals**

.. code-block:: console
   :caption: In ``why-shell``

   > pid
   833252
   > irqsig
   60

.. code-block:: console
   :caption: In another terminal

   $ kill -s 60 -q 2 833252

**Or, for short**

.. code-block:: console
   :caption: In ``why-shell``

   > irq 2
   > #elems: 1
   irq 3
   > #elems: 0

Problem: Interrupt Parallelism
------------------------------

* Interrupt nesting
* Awkward in total
* IRQ2 and IRQ3 *are considered parallel*

Bombard it:

.. code-block:: console

   $ while true; do echo irq 2; echo irq 3; done|why-shell ./why-spinlocks-do-much-in-isr
   double free or corruption (fasttop)
   child died: signal 6, core dumped: 128
   

Solution: Spinlock
------------------

jjj check this

* On a multiprocessor: spin until lock can be had

  * In schedulable context, disable interrupts on the local CPU

* On a single processor: 

  * In schedulable context: disable interrupts, be happy with the lock
  * In interrupt context: nothing to do - interrupts not disabled,
    that's enough

Bombard it:

.. code-block:: console

   $ while true; do echo irq 2; echo irq 3; done|why-shell ./why-spinlocks-do-much-in-isr-spinlock
   ...

.. literalinclude:: code/do-much-in-isr-spinlock.cpp
   :caption: :download:`code/do-much-in-isr-spinlock.cpp`
   :language: c++