Subject: Re: [patch V2 08/15] Documentation: Add lock ordering and nesting documentation

From: Jonathan Corbet <[email-masked]>
Date: Thu, 19 Mar 2020 09:04:26 -0600

On Wed, 18 Mar 2020 21:43:10 +0100
Thomas Gleixner <[email-masked]> wrote:

> From: Thomas Gleixner <[email-masked]>
> 
> The kernel provides a variety of locking primitives. The nesting of these
> lock types and the implications of them on RT enabled kernels is nowhere
> documented.
> 
> Add initial documentation.

...time to add a a couple of nits...:)

> Signed-off-by: Thomas Gleixner <[email-masked]>
> ---
> V2: Addressed review comments from Randy
> ---
>  Documentation/locking/index.rst     |    1 
>  Documentation/locking/locktypes.rst |  298 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 299 insertions(+)
>  create mode 100644 Documentation/locking/locktypes.rst
> 
> --- a/Documentation/locking/index.rst
> +++ b/Documentation/locking/index.rst
> @@ -7,6 +7,7 @@ locking
>  .. toctree::
>      :maxdepth: 1
>  
> +    locktypes
>      lockdep-design
>      lockstat
>      locktorture
> --- /dev/null
> +++ b/Documentation/locking/locktypes.rst
> @@ -0,0 +1,298 @@
> +.. _kernel_hacking_locktypes:
> +

So ... I vaguely remember that some Thomas guy added a document saying we
should be putting SPDX tags on our files? :)

> +==========================
> +Lock types and their rules
> +==========================

[...]

> +PREEMPT_RT caveats
> +==================
> +
> +spinlock_t and rwlock_t
> +-----------------------
> +
> +The substitution of spinlock_t and rwlock_t on PREEMPT_RT enabled kernels
> +with RT-mutex based implementations has a few implications.
> +
> +On a non PREEMPT_RT enabled kernel the following code construct is
> +perfectly fine::
> +
> +   local_irq_disable();
> +   spin_lock(&lock);
> +
> +and fully equivalent to::
> +
> +   spin_lock_irq(&lock);
> +
> +Same applies to rwlock_t and the _irqsave() suffix variant.
> +
> +On a PREEMPT_RT enabled kernel this breaks because the RT-mutex
> +substitution expects a fully preemptible context.
> +
> +The preferred solution is to use :c:func:`spin_lock_irq()` or
> +:c:func:`spin_lock_irqsave()` and their unlock counterparts.

We don't need (and shouldn't use) :c:func: anymore; just saying
spin_lock_irq() will cause the Right Things to happen.

Thanks,
jon