linux-kernel - Re: [PATCH bpf-next 7/8] bpf/docs: Document cpumask kfuncs in a new file

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Date:   Fri, 20 Jan 2023 00:01:12 -0600
From:   David Vernet <void@...ifault.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...a.com, john.fastabend@...il.com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...a.com, tj@...nel.org
Subject: Re: [PATCH bpf-next 7/8] bpf/docs: Document cpumask kfuncs in a new
 file

On Thu, Jan 19, 2023 at 09:59:06PM -0800, Alexei Starovoitov wrote:
> On Thu, Jan 19, 2023 at 05:58:32PM -0600, David Vernet wrote:
> > +
> > +For example:
> > +
> > +.. code-block:: c
> > +
> > +	/**
> > +	 * A trivial example tracepoint program that shows how to
> > +	 * acquire and release a struct bpf_cpumask *.
> > +	 */
> > +	SEC("tp_btf/task_newtask")
> > +	int BPF_PROG(task_acquire_release_example, struct task_struct *task, u64 clone_flags)
> > +	{
> > +		struct bpf_cpumask *cpumask, *acquired;
> > +
> > +		cpumask = bpf_cpumask_create();
> > +		if (!cpumask)
> > +			return 1;
> > +
> > +		acquired = bpf_cpumask_acquire(cpumask);
> > +		bpf_cpumask_release(cpumask);
> > +		bpf_cpumask_acquire(acquired);
> > +
> > +		return 0;
> > +	}
> 
> As the first example in the doc it was... alarming :)
> I've read it as it says that bpf_cpumask_acquire has to be called on
> freshly created cpumask before it can be used.

That's fair, I treated this example like a testcase :-)

> I've started to doubt by code reading skills of the previous patches :)
> A basic example is probably necessary to introduce the concept.
> Or this example should have bpf_cpumask_set_cpu right after create and
> more alu ops after release with comments to demonstrate the point.

Makes sense. I'll update the examples to be more clear and reflect
realistic use cases.