.. include:: <mmlalias.txt>

Coding: Fix *Sleep While Atomic* (Slideshow)
============================================

.. contents::
   :local:

.. sidebar:: Source

   `See Github
   <https://github.com/jfasch/jf-kernel-course/tree/my_driver_spinlock_atomic/_morph>`__

Simple Spinlock Usage: ``spin_lock()``
--------------------------------------

* Replace ``struct mutex`` with ``spinlock_t``
* Use plain ``spin_lock()``
* Test

  * Two tight loops injecting events via ``ioctl()``
  * Generate interrupts in parallel
  * |longrightarrow| hang sooner or later

Discuss Call Chain, IRQ State
-----------------------------

* Event: spin lock taken from ISR directly, top-level
* What if caller already holds a spinlock?
* |longrightarrow| ``spin_unlock()`` *unconditionally* reenables
  interrupts
* |longrightarrow| conversely, ``spin_lock()`` *unconditionally*
  disables interrupts

Enter ``spin_lock_irqsave()``, ``spin_unlock_irqrestore()``
-----------------------------------------------------------

* Fix
* Test
* Gone

Ah, ``kmalloc()``
-----------------

* ``event.c``: still using ``GFP_KERNEL``

  * *Called from interrupt service routine!*

* ``CONFIG_DEBUG_ATOMIC_SLEEP`` would have complained loudly

  .. image:: menuconfig-lock-debugging.png

**Fix**

* Pass ``gfp_t`` parameter to ``my_event_list_add()``