Re: [Linux-kernel-mentees] [PATCH] doc: listRCU: Add some more listRCU patterns in the kernel

From: Jonathan Corbet <corbet@lwn.net>
To: Amol Grover <frextrite@gmail.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>,
	linux-doc@vger.kernel.org, Lai Jiangshan <jiangshanlai@gmail.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	linux-kernel-mentees@lists.linuxfoundation.org
Subject: Re: [Linux-kernel-mentees] [PATCH] doc: listRCU: Add some more listRCU patterns in the kernel
Date: Tue, 3 Dec 2019 06:41:32 -0700	[thread overview]
Message-ID: <20191203064132.38d75348@lwn.net> (raw)
In-Reply-To: <20191203063941.6981-1-frextrite@gmail.com>

On Tue,  3 Dec 2019 12:09:43 +0530
Amol Grover <frextrite@gmail.com> wrote:

> - Add more information about listRCU patterns taking examples
> from audit subsystem in the linux kernel.
> 
> - The initially written audit examples are kept, even though they are
> slightly different in the kernel.
> 
> - Modify inline text for better passage quality.
> 
> - Fix typo in code-blocks and improve code comments.
> 
> - Add text formatting (italics, bold and code) for better emphasis.

Thanks for improving the documentation!  I'll leave the RCU stuff to the
experts, but I do have one request...

[...]

> +When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)`` under
> +``tasklist_lock`` writer lock protection, to remove the task from the list of
> +all tasks. The ``tasklist_lock`` prevents concurrent list additions/removals
> +from corrupting the list. Readers using ``for_each_process()`` are not protected
> +with the ``tasklist_lock``. To prevent readers from noticing changes in the list
> +pointers, the ``task_struct`` object is freed only after one or more grace
> +periods elapse (with the help of ``call_rcu()``). This deferring of destruction
> +ensures that any readers traversing the list will see valid ``p->tasks.next``
> +pointers and deletion/freeing can happen in parallel with traversal of the list.
> +This pattern is also called an **existence lock**, since RCU pins the object in
> +memory until all existing readers finish.

Please don't put function names as literal text.  If you just say
call_rcu(), it will be formatted correctly and cross-linked to the
appropriate kerneldoc entry.  Saying ``call_rcu()`` defeats that and
clutters the plain-text reading experience.

Thanks,

jon
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